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CHEMISTRY AND CORROSION MECHANISMS OF
STEELS EMBEDDED IN HIGH-DENSITY SLAG
CONCRETE FOR STORAGE OF USED NUCLEAR FUEL
by
Parthiban Nadarajah
A thesis submitted in conformity with the requirements
for the degree of Master of Applied Science
Department of Chemical Engineering and Applied Chemistry
University of Toronto
© Copyright by Parthiban Nadarajah 2011
ii
ABSTRACT
Chemistry and Corrosion Mechanisms of Steels Embedded in
High-Density Slag Concrete for Storage of Used Nuclear Fuel
Parthiban Nadarajah
Master of Applied Science
Department of Chemical Engineering and Applied Chemistry
University of Toronto
2011
The chemistry and corrosion mechanisms associated with reduced sulfur compounds such as
calcium sulfide, present in ground granulated blast-furnace slag (GGBFS), have been studied in
high-density concrete, mortar and simulated pore-water environments. The high-density concrete
and mortar samples were produced to replicate the high-density GGBFS concrete, in the dry
storage containers (DSCs), used for radiation shielding from used nuclear fuel. Electrochemical
measurements on embedded steel electrodes in high-density GGBFS concrete and mortar samples,
showed that sulfide is capable of consuming oxygen to create a stable, reducing environment,
though not in all cases, and the high-frequency electrolyte resistance increases with hydration time.
Ion chromatography on simulated pore-water environments determined that thiosulfate is quite
kinetically stable as a sulfide oxidation product and magnetite is capable of oxidizing sulfide.
Microscopy has also been used to provide visual evidence of GGBFS hydration and elemental
quantification of the hydrating microstructure in different environments.
iii
ACKNOWLEDGEMENTS
My appreciation goes to my supervisor Professor Roger C. Newman who has allowed me to
contribute to this field of study and for his excellent guidance throughout the research project. I
would like to thank Dr. Anatolie G. Carcea, Nick Senior and the Corrosion and Advanced
Materials research group for their assistance throughout the course of my research.
I would like to thank John Balinski, Bruce Cornelius and Barry Shenton from AMEC Earth and
Environmental and Jim Sato from the Ontario Power Generation, for their generous collaboration
and efforts, as the contributing industry partners in the research project. I would like to
acknowledge financial support from the Department of Chemical Engineering and Applied
Chemistry at the University of Toronto, Natural Sciences and Engineering Research Council
(NSERC) Canada and the University Network of Excellence in Nuclear Engineering (UNENE).
Finally, I would like to thank my family and friends for their continuous inspiration, motivation
and support during the course of my Master’s degree studies.
iv
Table of Contents
Abstract ........................................................................................................................................... ii
Acknowledgements ........................................................................................................................ iii
Table of Contents ........................................................................................................................... iv
List of Tables ................................................................................................................................ vii
List of Figures .............................................................................................................................. viii
List of Appendices ......................................................................................................................... xi
CHAPTER 1 - INTRODUCTION ...................................................................................................1
1.1 Objectives and Motivations ....................................................................................................1
CHAPTER 2 - BACKGROUND AND LITERATURE REVIEW .................................................3
2.1 Nuclear Waste Management ..................................................................................................3
2.1.1 Dry Storage Container Processes and Challenges ......................................................3
2.1.2 Radiation and Shielding ..............................................................................................5
2.2 High-Density Concrete and Mortar ........................................................................................6
2.2.1 Ordinary Portland Cement (OPC) ...............................................................................6
2.2.2 Ground Granulated Blast-Furnace Slag (GGBFS) .....................................................8
2.2.3 Coarse and Fine Aggregates .....................................................................................10
2.2.3.1 Aggregate Properties and Test Methods ....................................................11
2.2.4 Concrete and Mortar Microstructure ........................................................................13
2.2.4.1 Hydration Products ....................................................................................14
2.2.4.2 Aggregates and Hydrated Cement Paste ....................................................17
2.2.4.3 Pore Solution ..............................................................................................18
2.2.4.4 Kinetics of Oxygen Diffusion ....................................................................19
2.2.5 Admixtures ................................................................................................................20
2.3 Steel Corrosion in Concrete .................................................................................................21
2.3.1 Corrosion Mechanisms .............................................................................................22
2.3.1.1 Carbonation ................................................................................................24
2.3.1.2 Chloride Attack ..........................................................................................25
2.3.2 Measurement Techniques .........................................................................................27
2.3.3 Reference Electrodes ................................................................................................30
v
CHAPTER 3 - EXPERIMENTAL DETAILS ...............................................................................31
3.1 High-Density Concrete and Mortar Sample Preparation .....................................................31
3.1.1 Cementitious Materials Specifications .....................................................................31
3.1.2 Coarse and Fine Aggregate Specifications and Testing ...........................................32
3.1.3 Admixtures ................................................................................................................34
3.1.4 Mix Designs ..............................................................................................................34
3.1.5 Mixing and Casting Procedures ................................................................................35
3.1.6 Concrete to Mortar Procedure and Theory ...............................................................37
3.1.7 High-Density Concrete and Mortar Sample List Summary ......................................38
3.2 Electrochemistry Experiments .............................................................................................39
3.2.1 Electrochemical Cell .................................................................................................39
3.2.2 Reference Electrode Preparation ...............................................................................40
3.2.3 Corrosion Potential Measurements ...........................................................................40
3.2.4 Electrochemical Impedance Spectroscopy Measurements .......................................40
3.2.5 Cyclic Voltammetry Measurements .........................................................................41
3.2.6 Coarse Aggregate Resistance ....................................................................................41
3.3 Ion Chromatography Experiments .......................................................................................42
3.3.1 Ion Chromatography Column ...................................................................................42
3.3.2 GGBFS in Water .......................................................................................................43
3.3.3 GGBFS in Basic Solutions........................................................................................44
3.3.4 Aggregate and GGBFS in Basic Solutions ...............................................................44
3.3.5 Ion Chromatography Sample List Summary ............................................................45
3.4 Microscopy Experiments ......................................................................................................46
3.4.1 Environmental Scanning Electron Microscope (ESEM) ..........................................46
3.4.2 Microscopy Sample List and Mounting Procedures ................................................47
3.4.3 Grinding and Polishing Procedures ..........................................................................48
CHAPTER 4 - RESULTS AND DISCUSSION ...........................................................................49
4.1 Electrochemical Analysis .....................................................................................................49
4.1.1 Embeddable Reference Electrode Measurements .....................................................49
4.1.2 Open Circuit Potential Analysis................................................................................50
4.1.3 Electrochemical Impedance Spectroscopy Analysis ................................................53
vi
4.1.4 Cyclic Voltammetry Analysis ...................................................................................55
4.1.5 Coarse Aggregate Resistance Measurement .............................................................56
4.2 Ion Chromatography Analysis .............................................................................................56
4.2.1 GGBFS in Water Results ..........................................................................................57
4.2.2 GGBFS in Basic Solutions Results ...........................................................................58
4.2.3 Aggregate and GGBFS in Basic Solutions Results ..................................................60
4.3 Microscopy and Analysis .....................................................................................................61
4.3.1 Dry GGBFS ..............................................................................................................62
4.3.2 GGBFS in Water ......................................................................................................63
4.3.3 GGBFS in Basic Solutions........................................................................................64
4.3.4 OPC and GGBFS Paste .............................................................................................65
4.3.5 High-Density Concrete .............................................................................................66
4.3.6 High-Density Mortar .................................................................................................67
CHAPTER 5 - SUMMARY AND CONCLUSIONS ....................................................................69
CHAPTER 6 – FUTURE WORK .................................................................................................71
REFERENCES ..............................................................................................................................72
vii
LIST OF TABLES
Table 1: Chemical composition of Type GU OPC in Canada .........................................................7
Table 2: Chemical composition of GGBFS in Canada ....................................................................9
Table 3: Chemical composition and physical analysis of OPC used in experimental work..........32
Table 4: Chemical composition and physical analysis of GGBFS used in experimental work ....32
Table 5: Properties of coarse and fine aggregates .........................................................................34
Table 6: Mix design results for high-density GGBFS concrete and mortar samples ....................37
Table 7: Casting results for high-density GGBFS concrete and mortar samples ..........................37
Table 8: High-density concrete and mortar sample list summary .................................................38
Table 9: Ion chromatography sample list summary .......................................................................45
Table 10: Microscopy sample list summary ..................................................................................48
Table A1: Specific gravity and absorption calculations ................................................................78
Table A2: Surface moisture content calculations ..........................................................................79
Table A3: Mix design for 50% OPC-50% GGBFS mortar with fine hematite sand .....................82
Table A4: Mix design for 100% OPC mortar with fine hematite sand .........................................83
Table A5: Mix design for 50% OPC-50% GGBFS mortar with fine silica sand ..........................84
Table A6: Mix design for 100% OPC mortar with fine silica sand ...............................................85
Table A7: Mix design for 50% OPC-50% GGBFS concrete with iron oxide aggregates .............86
Table A8: Mix design for 100% OPC concrete with iron oxide aggregates .................................87
Table A9: Concrete to mortar data and calculations ......................................................................91
Table A10: Sulfur mass balance calculations for ion chromatography analysis ..........................97
viii
LIST OF FIGURES
Figure 1: Row of DSCs at the Pickering waste management facility ..............................................3
Figure 2: Graphic representation of OPC grain cross-section .........................................................8
Figure 3: Moisture conditions of aggregates .................................................................................12
Figure 4: OPC hydration products .................................................................................................16
Figure 5: Early age, rosette-shaped AFm phases in GGBFS HCP ................................................16
Figure 6: Interfacial transition zone between GGBFS HCP and crushed basalt rock aggregate ...17
Figure 7: Three-stage corrosion damage model for reinforced concrete .......................................21
Figure 8: Schematic flowchart of the corrosion of steel in concrete .............................................23
Figure 9: Corrosion of steel in the presence of chloride ions ........................................................26
Figure 10: Randles circuit model ...................................................................................................29
Figure 11: Warburg impedance circuit model ...............................................................................29
Figure 12: Coarse aggregate grading chart ....................................................................................33
Figure 13: Fine aggregate grading chart ........................................................................................33
Figure 14: Schematic of high-density GGBFS concrete and mortar samples ...............................36
Figure 15: Cross-sections of concrete samples ((A) 50% OPC-50% GGBFS (B) 100% OPC) ....36
Figure 16: Experimental MnO2 reference electrode potentials versus time...................................49
Figure 17: Corrosion potentials of embedded carbon and stainless steels in high-density
GGBFS concrete samples with iron oxide aggregates .................................................51
Figure 18: Corrosion potentials of embedded carbon and stainless steels in high-density
GGBFS mortar samples with iron oxide aggregate .....................................................51
Figure 19: High frequency electrolyte resistance of embedded carbon and stainless steels in high-
density GGBFS concrete samples with iron oxide aggregates ....................................54
Figure 20: High frequency electrolyte resistance of embedded carbon and stainless steels in high-
density GGBFS mortar samples with iron oxide aggregate.........................................54
Figure 21: Cyclic voltammogram for silver in 100% OPC high-density mortar ...........................55
Figure 22: Cyclic voltammogram for platinum in 100% OPC high-density mortar .................... 55
Figure 23: Thiosulfate concentration versus hydration time for GGBFS in water ........................57
Figure 24: Sulfate concentration versus hydration time for GGBFS in water .............................. 57
Figure 25: Thiosulfate concentration versus hydration time for GGBFS in basic solutions .........58
ix
Figure 26: Sulfate concentration versus hydration time for GGBFS in basic solutions ............... 58
Figure 27: Thiosulfate concentration versus hydration time for aggregates and GGBFS in basic
solutions .......................................................................................................................61
Figure 28: Sulfate concentration versus hydration time for aggregates and GGBFS in basic
solutions .......................................................................................................................61
Figure 29: Typical EDX spectrum of GGBFS grains ....................................................................62
Figure 30: ESEM image of dry GGBFS grains mixed in epoxy ...................................................63
Figure 31: ESEM image of dry GGBFS grains mixed in epoxy ...................................................63
Figure 32: ESEM image of single dry GGBFS grain ....................................................................63
Figure 33: EDX quantification table for dry GGBFS ....................................................................63
Figure 34: ESEM image of GGBFS grains in aerated water .........................................................64
Figure 35: ESEM image of GGBFS grain in aerated water ...........................................................64
Figure 36: ESEM image of GGBFS grains in deaerated water .....................................................64
Figure 37: ESEM image of GGBFS grain in deaerated water .......................................................64
Figure 38: EDX quantification table for GGBFS in aerated water ................................................64
Figure 39: EDX quantification table for GGBFS in deaerated water ............................................64
Figure 40: ESEM image of GGBFS in NaOH ...............................................................................65
Figure 41: EDX sulfur mapping of GGBFS in NaOH ...................................................................65
Figure 42: ESEM image of GGBFS in Ca(OH)2 + NaOH ............................................................65
Figure 43: EDX sulfur mapping of GGBFS in Ca(OH)2 + NaOH ................................................65
Figure 44: EDX quantification table for GGBFS in NaOH ...........................................................65
Figure 45: EDX quantification table for GGBFS in Ca(OH)2 + NaOH ........................................65
Figure 46: ESEM image of OPC and GGBFS paste......................................................................66
Figure 47: ESEM image of OPC and GGBFS paste......................................................................66
Figure 48: EDX sulfur mapping of GGBFS grains .......................................................................66
Figure 49: ESEM image of GGBFS grain .....................................................................................66
Figure 50: EDX quantification table for GGBFS grain in OPC and GGBFS paste ......................66
Figure 51: ESEM image of high-density GGBFS concrete ...........................................................67
Figure 52: ESEM image of high-density GGBFS concrete ...........................................................67
Figure 53: EDX iron mapping of coarse aggregates ......................................................................67
Figure 54: ESEM image of high-density 100% OPC concrete ......................................................67
x
Figure 55: EDX quantification table for GGBFS grains in high-density GGBFS concrete ..........67
Figure 56: EDX quantification table for coarse aggregates ..........................................................67
Figure 57: ESEM image of high-density GGBFS mortar ..............................................................68
Figure 58: ESEM image of high-density GGBFS mortar ..............................................................68
Figure 59: ESEM image of silica GGBFS mortar .........................................................................68
Figure 60: ESEM image of silica GGBFS mortar .........................................................................68
Figure 61: EDX quantification table for GGBFS grain in high-density GGBFS mortar ..............68
Figure 62: EDX quantification table for GGBFS grain in silica GGBFS mortar ..........................68
Figure A1: XRD Spectrum of fine hematite sand ..........................................................................79
Figure A2: Corrosion potential of embedded carbon and stainless steels in mortar sample
type 2 ...........................................................................................................................93
Figure A3: Corrosion potential of embedded carbon and stainless steels in mortar sample
type 3 ...........................................................................................................................93
Figure A4: Corrosion potential of embedded carbon and stainless steels in mortar sample
type 4 ...........................................................................................................................93
Figure A5: Corrosion potential of embedded carbon and stainless steels in high-density concrete
sample type 6 ..............................................................................................................93
Figure A6: Pourbaix diagram for iron-water system at 298 K ......................................................95
Figure A7: Pourbaix diagram for chromium-water system at 298 K ............................................95
Figure A8: Pourbaix diagram for silver .........................................................................................95
Figure A9: Pourbaix diagram for platinum ....................................................................................95
Figure A10: High frequency electrolyte resistance of embedded carbon and stainless steels in
mortar sample type 2 .................................................................................................96
Figure A11: High frequency electrolyte resistance of embedded carbon and stainless steels in
mortar sample type 3 .................................................................................................96
Figure A12: High frequency electrolyte resistance of embedded carbon and stainless steels in
mortar sample type 4 .................................................................................................96
Figure A13: High frequency electrolyte resistance of embedded carbon and stainless steels in
high-density concrete sample type 6 .........................................................................96
xi
LIST OF APPENDICES
A-1: Coarse and Fine Aggregate Property Calculations ................................................................78
A-2: X-ray Diffraction (XRD) Spectrum of Fine Hematite Sand..................................................79
A-3: Mix Design Calculations .......................................................................................................80
A-4: High-Density Concrete and Mortar Mixing Procedures ........................................................88
A-5: Mortar Air Content Calculation .............................................................................................90
A-6: Concrete to Mortar Calculations............................................................................................91
A-7: OCP Measurements for Mortar Sample Types 2, 3, 4 and Concrete Sample Type 6 ...........93
A-8: Sulfide to Oxygen Molar Ratio Calculation .........................................................................94
A-9: Pourbaix Diagrams for Metals ...............................................................................................95
A-10: EIS Measurements for Mortar Sample Types 2, 3, 4 and Concrete Sample Type 6 ...........96
A-11: Sulfur Mass Balance Calculations for Ion Chromatography Analysis ................................97
1
CHAPTER 1 - INTRODUCTION
1.1 Objectives and Motivations
The resurgence of nuclear power in Ontario as a safe, affordable and reliable energy source for
electricity production will involve an increased production of used nuclear fuel from the nuclear
fission process. The safe and responsible management of this high-level nuclear waste concerns
several organizations, such as the Ontario Power Generation and the Nuclear Waste Management
Organization, who must effectively plan the adequate storage and disposal of the waste and its
potential future interactions within the environment. Currently, the decommissioning process of the
used fuel involves water storage to remove heat and radioactivity for approximately 10 years and
then interim storage within dry storage containers (DSCs). The 70 tonne, double-walled steel DSCs
made of high-density reinforced concrete are of interest and motivation for this study, due to the
requirement of scientific evidence for the DSCs to withstand corrosion within their lifetime. Since
it is likely that the used fuel will spend more than 50 years within the DSCs, it is of much
significance to study the science and technology of the DSC materials to address any potential
concerns that may affect the storage of the used fuel.
The DSC materials include reinforced high-density concrete, which is a proportioned mixture of
ordinary Portland cement (OPC) and ground granulated blast-furnace slag (GGBFS), as
cementitious material, as well as iron oxide aggregates and water. The varying proportions of
coarse and fine iron oxide aggregates in the form of magnetite (Fe3O4) and hematite (α-Fe2O3) are
fundamental for creating the high-density of the concrete and provide an effective barrier against
any gamma radiation effects from the used fuel. The GGBFS is of particular interest since it
2
contains up to 2% by mass of calcium sulfide (CaS), which represents a stoichiometric excess over
the oxygen present in the concrete, and should in principle create a reducing environment; however
there is a theoretical possibility of steel corrosion due to the creation of oxidation products, such as
thiosulfate. Therefore, the objectives of the experimental work involve:
1. Developing an understanding of the chemistry of the cementitious materials, aggregates
and chemical admixtures in concrete, mortar and simulated pore water environments.
2. Investigating corrosion mechanisms and kinetic stability of reduced sulfur species, such
as thiosulfate.
3. Monitoring the electrochemical behaviour of embedded steels in high-density concrete
and mortar environments.
This project was a part of a collaborative research interest with the Ontario Power Generation and
AMEC Earth and Environmental, with the major objectives to be addressed by experimental
research in the areas of electrochemistry, ion chromatography and microscopy. The significance of
this study is to develop knowledge on the underlying science of the DSC materials at a laboratory
scale, in order to obtain a broader understanding of their application and usage in industry.
In the forthcoming sections, relevant literature on nuclear waste management, high-density
concrete, mortar materials and steel corrosion will be presented in Chapter 2. The sample
preparation of high-density concrete and mortar and the experimental procedures related to the key
research areas mentioned above are presented in Chapter 3. The analysis of the results and
discussion in the experimental areas are presented in Chapter 4. The main conclusions and
summary of the study are drawn in Chapter 5. Finally, implications for future work are presented in
Chapter 6, followed by References and Appendices sections.
3
CHAPTER 2 - BACKGROUND AND
LITERATURE REVIEW
2.1 Nuclear Waste Management
The current, interim management of high-level nuclear waste in Canada involves the dry storage
process, as a safe and regulated technology for containing and shielding the used fuel in a dry state.
Currently seven licensed facilities across Canada, with four in Ontario, manage the used fuel with a
DSC processing building, indoor DSC storage warehouse (as depicted in Figure 1) and an
amenities area (1).
Figure 1 - Row of DSCs at the Pickering waste management facility (1)
2.1.1 Dry Storage Container Processes and Challenges
In order to investigate background information on the construction of the DSCs, a site visit was
conducted to Niagara Energy Products (NEP), one of the manufacturers of the DSCs. In essence,
the DSC manufacturing process involves detailed and controlled welding operations to create the
rectangular carbon steel container and lid sub-assemblies, loading of 12 batches of high-density
concrete, drying stage and a helium leak detection test. Since the DSC is designed to contain up to
384 used fuel bundles it needs to be ensured that the requirements of a total radiation shielding of
4
21 inches be met, with the reinforced high-density concrete accounting for 20 inches and both the
inner and outer carbon steel lining accounting for approximately 0.5 inches each. Upon placement
of the used fuel bundles in the DSCs within the water storage bays, the DSCs need to be drained,
decontaminated, vacuum dried and back-filled with helium gas to create an inert atmosphere for
the used fuel (1). Currently, the DSC drain is the only stainless steel component of the DSC. The
meticulous DSC fabrication and used fuel loading processes are carefully regulated at all important
stages of the operation by the Canadian Nuclear Safety Commission (CNSC).
The relevant challenge identified in the DSC fabrication process involves maintaining uniformity
in the high-density concrete loads. Industrial concrete batching is greatly affected by seasonal
weather conditions, since the aggregate stockpiles either increase or decrease in temperature which
affects their moisture content and absorption properties. The air content and slump can also be
affected if the high-density concrete is not mixed properly from load to load. Since the main
function is to serve as a radiation shielding material, the interaction of radiation with improperly
batched high-density concrete can be detrimental to the safe and secure containment of the used
fuel. However, if the consistency of the loads is maintained, experience has shown that high-
density concrete is an effective, versatile and economical material for usage in radiation shielding
applications (2). High-density concrete is already widely used as a containment material for large
stationary installations such as nuclear power plants and particle accelerators, which makes it an
applicable option for shielding high-level nuclear waste in the stationary DSCs.
5
2.1.2 Radiation and Shielding
During the interim storage of the high level nuclear waste, the attenuation of photon (gamma and
X-rays) radiation within the high-density concrete can occur due to the concrete’s varying
proportions of light and heavy elements. The main interaction processes of photons with matter are
the photoelectrical effect, pair production and Compton scattering, but actual interaction patterns
within concrete are dependent on the density and proportion of elements within the concrete (3).
Since photon radiation is considered to be of great concern, natural, high-density mineral
aggregates such as magnetite and hematite are useful for creating high-density concrete that can
attenuate photons. An increase in density must result with the usage of these aggregates; however
the thickness of concrete radiation shields can be effectively reduced to compensate (4).
Furthermore, since used fuel can contain up to 3% fission products with variable radioactive decay
time and energy levels, high-density concrete needs to be able to withstand heat generation effects
from gamma radiation over the period of storage (3, 5). Since energy captured from the photon
radiation is deposited directly into the high-density concrete and liberated as heat, the thermal
stresses can be signifigant to cause physical release of chemically bound water from the hydrated
cement paste (HCP) (6). This ultimately leads to release of hydrogen and creates additional safety
issues, such as the deteroriation of the high-density concrete by cracking, due to the decrease in
compression and tensile strength of the concrete from long-term exposure to high-temperature (6).
Furthermore, the poor thermal properties, such as a low thermal conductivity, of most concretes are
generally viewed as a disadvantage since high temperature gradients and thermal stresses may be
created within the concrete and are important design considerations for maintaining the structural
integrity (3).
6
2.2 High Density Concrete and Mortar
High-density concrete is defined as a proportioned mixture of hydraulic cement and water paste
with embedded coarse and fine heavyweight aggregates, combined to weigh generally more than
3200 kg/m3 (7). High-density concrete is usually required for a dual compliance of strength and
density and is generally more expensive than normal concrete. Since high-density concrete is more
dependent on the aggregate properties than the cementitious material properties, it can be more
prone to segregation or bleeding phenomena during hydration (2). Other important concrete
properties such as the compressive and tensile strength, elasticity behaviour, shrinkage and
workablility may be affected as well, but any expected changes can be accounted for in proper mix
designing. High-density mortar is a proportioned mixture of hydraulic cement and water paste with
embedded fine heavyweight aggregates, combined to weigh anywhere between 2900-4300 kg/m3
(2). It is usually produced for specialized applications and tends to have higher workability than
concrete, due to its rheology properties. Flow testing is an important way to assess the consistenty
and workability of a mortar, while slump testing is essential for freshly batched concrete. However,
despite testing procedures, it is the chemistry of the materials that have the most significant effect
on the final properties of high-density concrete and mortar.
2.2.1 Ordinary Portland Cement (OPC)
OPC is the most widely used cement in the world and there are various types manufactured to meet
different physical and chemical requirements. Type General Use (GU, Type I or CSA Type 10)
OPC is of particular interest and upon manufacturing becomes a porous clinker that is ground to
fine, dark-gray granular powder that has a typical composition as shown in Table 1. The standard
requirements for OPC are detailed in ASTM 150 (Standard Specification for Portland Cement),
7
long with reference to testing properties such as strength, setting time and fineness. The fineness
averages about 370 m2/kg (Blaine) and is an important parameter, which affects the water demand
and rate of hydration of cement (9). Similarly, the particle size distribution is equally as important
and can be estimated from the Rosin-Rammler function, used for analyzing particle size data, with
typically 7-9% of OPC being finer than 2 µm and 0-4% being coarser than 90 µm (8, 10).
Chemical
Compounds
Mass Composition
(%)
Mineral
Compounds
Mass Composition
(%)
CaO (Lime) 67 Tricalcium silicate (alite)
CaO3SiO5 (C3S)
55
SiO2 (Silica) 22 Dicalcium silicate (belite)
CaO2SiO4 (C2S)
19
Al2O3
(Alumina)
5 Tricalcium aluminate
(aluminate)
Ca3Al2O6 (C3A)
10
Fe2O3
(Iron Oxide)
3 Tetracalcium
aluminoferrite (ferrite)
Ca2AlFeO5 (C4AF)
7
Other (CaSO4,
CaO or MgO)
3 Other (alkali sulfates,
calcium oxide or
magnesium oxide)
9
Table 1 – Chemical composition of Type GU OPC in Canada (8,9)
OPC grains are homogenous in composition and their microstructure is usually angular alite and
rounded belite crystals embedded in an interstitial matrix of dendritic ferrite and aluminate, as
depicted in Figure 2. The alite crystals range from 15-20 µm, the belite crystals range from 25-40
µm, while the ferrite and aluminate are variable due to their presence as a solid solution (8). Since
the mineral compounds are all present as oxides, their cementing ability is highly dependent on
oxygen’s ability to acquire electrons during chemical reaction with water (11). The actual phase
equilibrium composition within the crystalline structure of the compounds also determines
reactivity. A study performed on the synthesis of the pure OPC mineral compounds by Wesselsky,
suggests that it is practically impossible to separate them into individual components due to the
complexity of their crystal systems (12). Therefore, it is likely that the overall reactivity of OPC is
8
determined by a stable phase assemblage of the mineral compounds, rather than their individual
hydraulicity.
Figure 2 - Graphic representation of OPC grain cross-section (13)
The sulfur chemistry in OPC is present as sulfate (SO42-
) in the form of calcium sulfate as gypsum,
hemihydrate and/or anhydrite, respectively, to help control early setting properties. The total level
is normally reported as a SO3 equivalent and is limited in Type GU cement at 3% by mass, with
the majority of the sulfate being soluble within the crystal structure of the mineral compounds (14).
Elemental sulfur is almost never found in OPC, except in trace amounts, since cement clinker is
produced in an oxidizing rotary kiln environment (15).
2.2.2 Ground Granulated Blast-Furnace Slag (GGBFS)
GGBFS is a whitish, glassy and granular cementitious material obtained from quenching and
grinding molten slag, which is a by-product from iron production. Due to its amorphous structure,
GGBFS is not considered to be strongly self-cementing and is usually blended as a partial
replacement to OPC at dosages between 5-70% by mass, depending on the required chemical and
physical characteristics (9). Known as a green cementitious material, GGBFS can effectively lower
the cost of cement when used as a partial replacement, by saving energy in the production process
9
and reducing CO2 emissions from cement manufacturing. The most important GGBFS properties
that influence reactivity are the fineness, glass content, activity index and chemical constituents.
The standard requirements for GGBFS and GGBFS-blended cements are detailed in ASTM C989
(Standard Specification for Slag Cement for Use in Concrete and Mortars) and ASTM C595
(Standard Specification for Blended Hydraulic Cements), respectively. The fineness averages
approximately 450 m2/kg (Blaine) for GGBFS produced in Canada and the fineness of GGBFS
must usually be greater than that of OPC for acceptable performance upon hydration (9). Current
research has shown that there is no correlation between glass content and hydraulicity, but it has
been generally reported that the glass content should be in excess of 90% to show satisfactory
properties (16). The chemical composition of Canadian GGBFS is shown in Table 2.
Chemical Composition
CaO
(Lime)
SiO2
(Silica)
Al2O3
(Alumina)
MgO
(Magnesia)
Fe2O3
(Iron
Oxide)
MnO
(Manganese
Oxide)
S
(Sulfur)
Mass
Composition
(%)
32-45 32-42 7-16 5-15 0.1-1.5 0.2-1 0.7-2.2
Table 2 - Chemical composition of GGBFS in Canada (9)
The glass microstructure of individual GGBFS grains consists of a continuous anionic network of
oxygen and silicon in charge balance with calcium, magnesium and other electropositive elements,
within the cavities of the network (8). The grains tend to be quite angular and studies have shown
evidence of phase separation within the grains, due to compositional variation between glass and
crystalline interfaces (17). Any crystalline phases present are usually in the form of unevenly
distributed dendritic merwinite, melilite, calcite and quartz (11).
The sulfur chemistry in GGBFS exists primarily as CaS, with trace amounts of iron and manganese
sulfides, and is speculated to exist homogeneously as a colloidal state throughout the glass
10
network. X-ray absorption spectroscopy in a study by Roy, indicated that most of the sulfur in
GGBFS is frozen in amorphous form, with only a minor amount appearing as oldhamite
(mineralogical CaS) (18). Research by Scott et al., reported that oldhamite occurred both as
independent dendritic crystals and inclusions within melilite (19). Radwan also indicates that
sulfide (S2-
) is the main species of sulfur in GGBFS and if GGBFS is not properly stored, the
sulfide can oxidize into sulfate, making it difficult to monitor the amount of initial sulfide (20).
2.2.3 Coarse and Fine Aggregates
Aggregates are natural or artificial particulate matter that are chemically inert and used in the
production of composite materials such as concrete or mortar. Aggregates account for
approximately 60-75% of the volume of concrete mixtures, and their properties are able to
influence the workability of plastic concrete and the durability, strength, thermal properties and
density of hardened concrete (21). The fine and coarse aggregates of interest are heavyweight,
natural hematite (α-Fe2O3) and magnetite (Fe3O4) respectively, as mentioned for their radiation
shielding properties and high specific gravity in the range of 4.6-5.2 (22). Magnetite is a
ferromagnetic material with a mixed-valence Fe2+
-Fe3+
structure and at room temperature very
slowly oxidizes to ferromagnetic maghemite (γ-Fe2O3) and at higher temperatures to anti-
ferromagnetic hematite (23). Research has shown that the oxidation of magnetite depends on the
outward diffusion of iron cations from its center to the surface, but the kinetics are expected to be
slow in alkaline conditions such as concrete, where iron is insoluble (24). The oxidation process is
also complicated by the ratio of Fe2+
to Fe3+
, oxygen fugacity and topotactic reorganization of the
iron oxide crystal structures (25). Aggregate properties, such as hardness, may be affected by the
compositional variation of oxidized magnetite, which also affects the abrasion resistance of the
11
aggregates when mixed with cementitious material and water. The hardness of both aggregates in
their pure mineral state is approximately 5.5-6.5 on the Moh’s hardness scale, which means they
are relatively scratch resistive (3). The electrical resistivity of magnetite at room temperature has
been reported to be as low as 5 Ω·cm, which is much lower than hematite’s reported resistivity of
2000 kΩ·cm (8,25).
Heavyweight iron oxide aggregates are often found in weathered soils, clays and sedimentary rock
with iron band formations (11). Due to their source variability, the heavyweight aggregates are
processed differently depending on where they are quarried. However, the general procedure for
aggregate processing is to extract, crush, screen and sort, in order to eliminate any undesirable
constituents. Upon processing completion, the coarse aggregate tends to be angular in particle
shape with a rough surface texture, while the fine aggregate is relatively granular. Test methods
can also be performed after processing completion to determine necessary aggregate properties for
concrete and mortar production.
2.2.3.1 Aggregate Properties and Test Methods
It is important to classify aggregates by performing test methods to determine their grading
specific gravity, absorption and surface moisture content properties. Grading refers to the
distribution of particle sizes present in an aggregate (21). Outlined in ASTM C136 (Sieve
Analysis of Fine and Coarse Aggregates) grading is performed by passing a sample of
aggregates through a series of square-wired sieves that decrease in opening size, to obtain the
mass percent passing each successive sieve. Coarse and fine aggregates are usually sieved
separately with the coarse aggregates being sieved through sieve sizes decreasing from 28 to 5
12
mm and the fine aggregates being sieved through sieve sizes decreasing from 2.5 to 0.08 mm.
The individual percent of aggregates retained and the total percent passing between successive
sieves are the key parameters of interest in evaluating the acceptability of a grading test.
Implications of a grading test can determine if there is well-distributed size range of aggregates,
which is important for cement paste coverage on aggregates during mixing. The specific gravity
of an aggregate is needed to properly account for the aggregate’s yield and mix density in a well-
proportioned mix of concrete or mortar. It is fundamentally defined as the mass of the aggregate
in air divided by the mass of equal volume of water and refers to the space occupied by the
aggregate particles alone (i.e.-the volume of solid aggregate and internal aggregate pores),
excluding the voids between particles (21). The specific gravity is determined at fixed moisture
content and the four possible moisture conditions are shown in Figure 3. The damp or wet
condition occurs when the aggregates pores are filled with water and free water exists on the
surface. Saturated surface-dry (SSD) aggregates contain no free water on their surface and
aggregates are usually brought to this condition for specific gravity determination. Air-dry refers
to aggregates that contain some water in the pores, but have a dry surface. Oven or bone dry is
the extreme condition, where the aggregates have no water in their pores or surface. Specific
gravity determination involves measurement of an aggregate’s apparent mass in water, as well as
saturated surface-dry and oven-dry masses in air.
Figure 3 - Moisture conditions of aggregates (21)
13
Absorption is a measure of the total pore volume accessible to water and can determined from
saturated surface-dry and oven-dry masses, similarly to the specific gravity, as described in
ASTM C127 and ASTM C128 (Specific Gravity and Absorption of Coarse Aggregate) and
(Specific Gravity and Absorption of Fine Aggregate), respectively. Moisture content
determination is summarized in ASTM C566 (Total Moisture Content of Aggregate by Drying)
and is a calculation of the water evaporated from the aggregates’ natural to oven-dry condition.
Since natural aggregates are rarely at saturated surface-dry condition, their existing specific
gravity, surface moisture content and absorption are necessary to determine, to account for the
amount of aggregates and mixing water in concrete or mortar mix design.
2.2.4 Concrete and Mortar Microstructure
The hydration of cementitious material in the presence of water develops hydration products in a
cement paste microstructure that changes from a plastic to hardened state. It is accomplished
through stages of progressive strength development, such as stiffening and hardening. Stiffening
refers to the initial setting of the HCP, as it changes from a fluid to rigid stage and usually occurs
within 1 to 4 hours after mixing the paste (26). As the paste is able to withstand a prescribed
pressure, usually after 3 to 6 hours, the final set is achieved and hardening is taking place due to the
continuous strength development of the HCP (27). Furthermore, in order to achieve desired paste
properties, the drying and hardening can be slowed down by curing processes to ensure optimal
strength development. Abnormal setting behaviour can also occur, depending on the liberation of
heat, in the form of false or flash set and is detrimental to the HCP if unobserved (9). As hydration
progresses, the relative humidity decreases within the HCP, due to the volume of water changing
as it is consumed by the cementitious material. The porosity of the HCP is also affected and
14
depends on the water to cementitious material ratio (w/cm), degree of hydration and ultimately
affects strength, permeability and drying shrinkage (27). A high w/cm ratio indicates a more
permeable HCP, while a low w/cm ratio signifies a much denser HCP. The main types of pores
present are large interconnected capillary pores and ultra-fine gel pores that exist within the
hydrated products. The capillary pores tend to hold free water which is lost upon drying, while the
hydrated products contain chemically bound water that is not lost (26). The gel pores contain water
in both states, and as hydration progresses, the gel pore volume generally increases while the
capillary pore volume decreases (26). Spherical air voids, which are larger than the capillary voids,
can also be present in the HCP at 2-6% by volume, due to mixing entrainment. The voids tend to
be usually uniformly distributed throughout the HCP and do not affect the HCP permeability (26).
2.2.4.1 Hydration Products
The main hydration products that are formed when OPC reacts with water are calcium silicate
hydrate (3CaO∙2SiO2∙2H2O or C-S-H), calcium hydroxide (Ca(OH)2), AFm phases such as
monosulfoaluminate (C3A∙CaSO4∙12H2O), AFt phases such as ettringite (C3A∙3CaSO4∙32H2O) and
hydrogarnet (C3A∙6H2O) (8). The C-S-H gel makes up 50-60% of the HCP and varies from poorly
formed crystalline fibers to a reticular network of small particles (28). The calcium hydroxide
exists as large hexagonal crystals and occupies up to 20-25% of the HCP (28). Calcium
sulfoaluminates (AFm and AFt) consist of 15-20% of the HCP and initially favour formation of
rod-like ettringite crystals, while rosette-shaped monosulfoaluminate forms later during hardening
(8). Eventually continued hydration releases aluminate and the ettringite is slowly converted to
monosulfate hydrate. Aluminate and ferrite contribute to setting and early strength gain, as
aluminate reacts early to form ettringite, while alite and belite contribute to hardening and long-
15
term strength gain, as they react to form C-S-H and Ca(OH)2. The release of SO42-
and OH- into
the HCP is important for activating the reaction of alite and belite. Equations [1] to [3] describe the
formation of OPC hydration products from the OPC mineral compounds (3):
Ca3Al2O6 + 6H2O → 3CaO∙Ca3Al2O6∙6H2O + Ca(OH)2 [1]
2CaO3SiO5 + 6H2O → 3CaO∙2SiO2∙3H2O + 3Ca(OH)2 [2]
2CaO2SiO4 + 4H2O → 3CaO∙2SiO2∙3H2O + Ca(OH)2 [3]
GGBFS does not hydrate or harden in pure water in the same way as OPC, due to a different
compositional and morphological structure. Hydration of GGBFS is very slow in water, since it is
likely that hydration is delayed by an impermeable aluminosilicate coating that is deficient in Ca2+
and forms on the surface of GGBFS grains (29). The hydraulic reactivity of glassy GGBFS grains
is dependent on the activation of the glass and water is not at a high enough pH to attack the glass
(8). However, upon contact with water, it has been observed that the initial slag hydration appears
to be an incongruent dissolution and limited precipitation of C-S-H occurs at a pH below 9.5 (11).
The amorphous, foil-like C-S-H gel has high aluminum content and a lower CaO/SiO2 ratio, which
defers from the fibrillar C-S-H gel in OPC (29). GGBFS is usually blended with OPC to increase
its hydraulic reactivity and the hydration products formed are similar to that of OPC. The Ca(OH)2
produced from OPC hydration serves as an activator for GGBFS hydration and is consumed by
GGBFS grains to increase the amount of C-S-H gel in hydrated OPC-GGBFS pastes (11). Other
differences from the OPC hydration products, include the formation of different AFm phases
(C4A∙13H2O and C2A∙8H2O) and a magnesium and aluminum rich hydrotalcite phase (30).
GGBFS hydration is relatively faster as a blended cement with OPC and during hydration the
crystalline structure of GGBFS remains intact and is inert (31). The progressive strength gain of
the blended cement is slower due to postponed setting, but higher strengths are achieved at later
ages. GGBFS allows more capillary pores to be filled with C-S-H gel and has a greater gel pore
16
volume, thus creating a denser microstructure of lower porosity. The workability and slump of the
hydrated blended paste are also improved with the addition of GGBFS, however bleeding may
occur due to the greater fineness of GGBFS compared with that of OPC (32). Figures 4 and 5 show
SEM imagery of the hydration products and AFm phases in GGBFS HCP.
Figure 4 (left) – OPC hydration products (33)
Figure 5 (right) – Early age, rosette-shaped AFm phases in GGBFS HCP (29)
Hydrated cements can contain mixtures of AFm phases and the fate of sulfur compounds is
speculated to be linked with them. The SO42-
and SO32-
from OPC are accommodated as interlayer
anions associated with AFm phases, but as hydration progresses the AFm phases become poorer in
SO42-
. The thermodynamic stability of the AFm phases is a key issue in retaining distinctive SO42-
matter, as research has shown that monosulfoaluminate is calculated to be stable only above 40°C,
since decomposition to AFt and hydrogarnet occurs at lower temperatures (34). Odler has shown
that OPC, in the absence of Al3+
or Fe3+
, had Ca2+
and SO42-
ions adsorbed by C-S-H (35). It was
also shown that after hydration, ettringite and monosulfoaluminate combined to contain 32% of the
sulfate content, while gypsum retained 46.5% of the sulfate content (34). GGBFS upon hydration
develops a transient green colour that becomes darker over time, which suggests that polysulfides
are formed from the S2-
present in glass GGBFS grains. The S2-
released from GGBFS has been
assumed to enter AFm phases by many researchers and the possibility of interactions between
17
SO42-
and sulfur in lower oxidation states cannot be overlooked (30, 36). Vernet found that in
hydrated GGBFS cements the AFm phase, disulfur aluminate (C3A∙2CaS∙10H2O) was formed and
consumed S2-
, while decreasing sulfur ions in the pore solution (11). A decrease in SO42-
concentration can also transform ettringite into monosulfoaluminate, which suggests that SO42-
is
key anion in stabilizing the formation of AFm (34). Furthermore, GGBFS that is activated by high
pH solutions, have shown decrease in S2-
and increase in crystalline SO42-
, which suggests that the
sulfur is exsolving from the glass to form distinct crystalline phases (18).
2.2.4.2 Aggregates and Hydrated Cement Paste
The aggregate phase within a concrete or mortar microstructure is predominantly responsible for
the unit weight, elastic modulus and dimensional stability. These properties are dependent on an
interfacial transition zone, which represents an interfacial region between aggregates and the HCP
and is typically 10-50 µm thick around large aggregates (28). The aggregates and the HCP interact
by Van der Waals forces of attraction and an example of the interfacial transition zone is shown in
Figure 6. The hydration product within the transition zone that is mostly in contact with the
aggregates is C-S-H as found by Javelas, whereas Ca(OH)2 possesses less adhesion (37).
Figure 6 – Interfacial transition zone between GGBFS HCP and crushed basalt rock aggregate,
(right side is the HCP and left side is the aggregate) (31)
18
As the surface area increases, more HCP is needed to coat the additional surface of aggregates or
else the resulting concrete or mortar would be too stiff. Similarly, the reverse problem also exists,
when a decrease in surface area occurs due to a more fluid HCP, which contributes to a decrease in
strength and durability. Cement is more expensive than aggregates and it is recommended to have
a uniform gradation of aggregates to ensure that the right amount of cement is used for aggregate
coverage, workability and low void content (21). Phenomena such as microcracking or internal
bleeding can also occur when there is poor adhesion, stress concentrations and water film
accumulation next to the surface of aggregates, thus weakening the transition zone (26).
2.2.4.3 Pore Solution
The amount of water required for complete hydration of cementitious material is generally less
than the calculated w/cm ratio and any excess water that is not consumed by the aggregates exists
as an aqueous phase. The excess aqueous phase, termed the pore fluid, deposits itself within the
pore structure of the HCP and has a varying composition depending on the pH and cementitious
material used. Analysis of OPC pastes and mortars at a 0.5 w/c ratio for 6 months had shown that
the pore solution is essentially an alkali hydroxide solution with dissolved ions of sodium,
potassium and hydroxide and a solution pH ranging from 13.4-14.0 (11). Research has shown that
typically 40-60 per cent of Na+
ions and 50-70 of K+ ions in OPC ended up in the pore solution
(38). The solubilities of silica, calcium, aluminum and magnesium in pore solutions from GGBFS
pastes have been observed to be strong functions of the solution pH (8). GGBFS hydration is
activated at high pH and studies by Song have shown that a high pH increases the concentration of
silica and aluminum in the pore fluid, but reduces the amount of calcium and magnesium (39). The
decrease in calcium is due to the low aqueous solubility of Ca(OH)2 in the pore fluid and
19
favourable thermodynamic stability of solid Ca(OH)2 at high pH (40). Similar results were
observed by Longuet, with a decrease in the alkalinity of GGBFS pore solutions and increase in
concentration of chemically reduced sulfur species over time (11). The pore solution of hydrated
GGBFS activated with gypsum also showed a high total reduced sulfur concentration for up to 56
days (41). Furthermore, reduced sulfur species, such as thiosulfate, were found to have stably
formed in lixiviation water prepared from GGBFS pastes over time (42). Studies on OPC-GGBFS
pore solutions have shown intermediate results, with an insignificant decrease in the alkalinity of
the pore fluid and maintenance of high pH over time (11).
2.2.4.4 Kinetics of Oxygen Diffusion
The kinetics of oxygen diffusion in concrete or mortar microstructure affect the electrochemical
behaviour of embedded steels. Oxygen diffusion has been shown to be a strong function of the
moisture content of HCPs (43). A moisture content of 100% indicates a low diffusion coefficient,
since oxygen slowly diffuses through capillary pores filled with water. A moisture content of 0%
indicates a high diffusion coefficient, since oxygen quickly diffuses through capillary pores filled
with air. Page and Lambert measured the oxygen diffusivity for hydrated OPC pastes at different
w/cm ratios and temperatures, with the primary finding indicating that they were of similar
magnitude to chloride ion diffusivities in HCPs (44). At 25°C and at w/cm ratio of 0.5, the oxygen
diffusivity was measured to be 1.52 x 10-8
cm2/sec (44). This differs greatly from Kobayashi’s
reported value of 10-3
cm2/sec for 0.6 w/cm OPC curing in air, but is within the range of 10
-3-10
-9
cm2/sec found in Hunkeler’s work (45, 46). OPC-GGBFS blended mortars were found to have a
lower diffusivity than OPC, as the GGBFS content increased, most likely due to the slower
transport of oxygen through silicate glasses (45, 47). Furthermore, the oxygen diffusivity was
20
found to also increase when the w/cm ratio and temperature of the HCP both increased (44). At
lower w/cm ratios, the capillary pores would be filled with C-S-H which acts as a barrier for
diffusion. The kinetics of oxygen diffusion are also affected by the path tortuosity and activation
energy of the cementitious material-water adsorption isotherm, which are favourable at high w/cm
ratios (43).
2.2.5 Admixtures
Chemical admixtures are defined as materials other than hydraulic cements, water or aggregates
that are added before or during mixing to improve the properties of concrete or mortar in its plastic
or hardened state (48). The two types of admixtures that are of primary interest are air-entraining
agents (AEAs) and superplasticizers. AEAs are liquid admixtures that have the ability to trap or
entrain tiny air bubbles during mixing, while improving workability and plasticity. AEAs tend to
consist of long chain organic molecules with polar groups at one end, allowing them to lower
surface tension and stabilize hydrophobic air molecule interactions with water (8). Along with
AEAs, other important factors that affect the air content are the fineness of cement, an increase in
slump and efficiency of the mixer. Superplasticizer, also known as a high-range water-reducing
(HRWR) admixture, is used to reduce the water requirement of cementitious material, by more
than 30% without retardation side effects and essentially produce normal workability concrete or
mortar at a lower w/cm ratio (48). By controlling the rheology of the HCP, superplasticizers can
improve strength development, finishability and surface appearance. The polycarboxylate
chemistry of superplasticizers can also disperse the cementitious material, which helps reduce
permeability and shrinkage (11).
21
2.3 Steel Corrosion in Concrete
The corrosion of steel in concrete is one of the main causes of damage and failure of reinforced
concrete structures. The high alkalinity of concrete pore water allows embedded steel to passivate
and remain protected, unless the passive film is destroyed by a corrosion mechanism. Under
aerobic conditions, the passive film is a hard, non-reactive surface film that is self-maintaining and
consists primarily of metal oxides. The development of the corrosion mechanism and its ensuing
level of damage can be described in three stages as shown in Figure 7 (49). The first stage is an
initiation period before corrosion activation, where contaminants, such as carbon dioxide and
chloride, ingress into the concrete matrix. This is followed by propagation period where corrosion
products and propagation of micro-cracking develops. The third stage, which is most detrimental,
is the acceleration stage where the corrosion rate of the steel increases, due to the low permeability
of concrete to corrosion agents, such as oxygen and water. Well-known corrosion mechanisms that
are applicable to this model include carbonation-induced corrosion and chloride attack, both which
have been widely studied; in both cases oxygen reduction being the cathodic reaction.
Figure 7 - Three-stage corrosion damage model for reinforced concrete (49)
22
2.3.1 Corrosion Mechanisms
The breakdown of the steel’s passive layer will cause rust formation as lepidocricite (γ-FeOOH).
The chemical reactions for the formation of rust, shown in equations [4] to [7], and most notably
involve the coupled reactions of anodic oxidation of the steel’s metallic iron and cathodic reduction
of oxygen, in equations [4] and [5]:
Fe → Fe2+
+ 2e- [4]
2H2O + O2 + 4e- → 4OH
- [5]
Fe2+
+ 2OH- → Fe(OH)2 [6]
2Fe(OH)2 +
O2 → 2 γ-FeOOH + H2O [7]
If a reduction in concrete alkalinity occurs, as the iron is being removed by anodic oxidation, the
ferrous ions will dissolve into the surrounding pore water solution of the concrete and the steel
loses mass, due to its cross-sectional area becoming smaller (50). The ferrous ions then react with
hydroxide ions to form ferrous hydroxide, which reacts with oxygen to ultimately produce
lepidocricite. The corrosion process is dependent on the flow of electrons between the anodic and
cathodic sites on the steel, as well as the flow of ions through the capillary pores of the concrete
(50). If the pores are dried out, the flow becomes difficult and the corrosion process slows down. It
is also dependent on the oxygen rate of diffusion to the steel surface and availability within the
concrete. The stability of the steel’s passivation film is also a key issue and different metals can
help improve the stability. Stainless steels can form passivation layers that consist of iron,
chromium and rarely molybdenum oxides, while carbon steels are stable at high pH, due to the
formation of iron (III) oxide (51). The general properties of a stable passivation layer include
having a low ionic conductivity, low chemical solubility, good adhesion to the steel, considerable
electron conductivity and a large range of potential thermodynamic stability (52). Both carbonation
and chloride attack undergo fundamentally different corrosion mechanisms, as shown in Figure 8,
and to be discussed in the following sections.
23
Figure 8 – Schematic flowchart of the corrosion of steel in concrete (53)
The usage of GGBFS as a cementitious material in concrete creates another corrosion mechanism
of interest, related to the reducing characteristics of sulfide. Sulfides can affect steels in the
following two ways:
(1) Oxidation of sulfides to sulfate by the available oxygen present in concrete, which
depletes the oxygen concentration near the steel, to create a beneficial, reducing
environment. Oxidation to thiosulfate can result in pitting corrosion on stainless steel, if
there is a decrease in alkalinity.
(2) Precipitation as iron (II) sulfide on the steel surface or sulfide incorporation into the
oxide layer, which limits the formation of the passivation layer on steel (54,55).
The cathodic reduction of oxygen is affected by both mechanisms and if passivation is insufficient,
the embedded steel may become more susceptible to carbonation and chloride attack.
24
2.3.1.1 Carbonation
Carbonation is a neutralizing chemical reaction, where carbon dioxide gas in the atmosphere can
react with solid calcium hydroxide, C-S-H gel, alkali and calcium ions in the pore water solution
(50). This can result in a reduction in the pH of the HCP from above 12 to below 9 (56). Carbon
dioxide dissolves in the pore water solution of concrete to form carbonic acid, which neutralizes
the alkali hydroxides and reacts to form calcium carbonate, as shown in equations [8] and [9].
CO2 + H2O → H2CO3 [8]
H2CO3 + Ca(OH)2 → CaCO3 + 2H2O [9]
The decrease in pH by carbonation causes depassivation of the steel and subsequently the
corrosion mechanism responsible for rust formation. Carbonation is primarily dependent on the
diffusion of carbon dioxide and the length of carbonation can be measured from the surface of the
concrete to the depth to which it occurs at in the concrete matrix (52). This is known as the
carbonation depth and its relationship is approximately parabolic with exposure time (57). Other
factors that affect carbonation are the moisture content of the concrete, temperature, w/cm ratio and
concrete composition. Carbonation does not occur if the relative humidity is very low, since
moisture is required to form carbonic acid, or very high, since carbon dioxide diffusivity becomes
low. Numerous studies have shown that OPC-GGBFS concrete mixtures that contained an
increasing amount of GGBFS, generally showed an increase in carbonation depth with time (58).
Song and Saraswathy suggested that in environmental situations, where there is a risk of excessive
carbonation over a long period of time, the GGBFS levels should be restricted to 50%, since higher
GGBFS levels increase carbonation significantly (59). Similarly, in a study on the long term
behaviour of concrete in nuclear waste repositories, it was found that the depth of carbonation was
greater for specimens containing GGBFS rather than OPC only, after 28 years of storage at
approximately 60% relative humidity (60). GGBFS increases the carbonation depth, since the
25
addition of GGBFS with OPC leads to less calcium hydroxide content upon hydration. This limits
the extent of reaction [9] and as a result less calcium carbonate is formed in the pore space.
Although there is no standard method to measure carbonation, many studies reference Parrott’s
approach to determining carbonation rates from empirical equations involving air permeability data
in concrete (51). ASTM C856 (Standard Practice for Petrographic Examination of Hardened
Concrete) presents a qualitative approach to determine carbonation, based on colour indication of
differential pH areas using 1-2% phenolphthalein solution.
2.3.1.2 Chloride Attack
Chloride attack is a locally-induced pitting corrosion phenomenon, which requires hydrolysis of
Fe2+
and surrounding of chloride ions at anodic sites of the steel surface at a significant
concentration to form hydrochloric acid (50). This subsequently decreases the pH of the HCP and
breaks down the steel passivation layer. The depassivation of steel has been suggested to be a
competing process between hydroxide, as it stabilizes the passivation layer, and chloride ions, as it
disrupts the passivation layer (50). The locally activated anodes are small areas, while the large
steel surface area is the cathode for reactions between Fe2+
and Cl- to form chloride or oxychloride
compounds (52). The process is self-sustaining due to the hydrolysis of these compounds which
leads to recycling of chlorine ions, increased acidity of the anodic areas and continuous oxidation
of iron (52). Chloride attack can occur internally within the concrete if the source of chlorides is
from using seawater, calcium chloride as an admixture or aggregates that contain chlorides (51). It
can also occur externally from the environment by sources such as seawater spray or the deicing
salt (51). However, regardless of the method of attack, chloride movement is dependent on
diffusion inside the capillary pores of the concrete, similarly to oxygen and carbon dioxide. In
26
Figure 9, the corrosion of steel in the presence of chloride ions is depicted.
Figure 9 – Corrosion of steel in the presence of chloride ions (52)
Concrete that consists primarily of OPC is considered at high risk to corrosion when the chloride
contents (by weight of the cement) are greater than 1% and therefore GGBFS is generally added to
the concrete to reduce this occurrence (57). The corrosion resistance of reinforced steel in OPC-
GGBFS concrete has been widely researched with an indication that the chloride diffusion
coefficients and permeability decrease, as an increasing amount of GGBFS is used. Yeau and Song
were both able to independently prove that the free chloride content was much lower in OPC-
GGBFS concrete, due to a finer pore structure that reduced the mobility of chloride ions (59, 61).
Furthermore, the high alumina content in the pore fluid of OPC-GGBFS concrete can contribute to
binding chloride ions, which also leads to a decrease in mobility (62). Therefore the corrosion risk
of chloride attack is primarily dependent on free chloride ions, rather than bound ones, but if bound
ions are released, due to the reversible nature of binding reactions, then they may pose a similar
risk (52). The total chloride ion concentration necessary to depassivate the steel, which is known as
the chloride threshold level and is usually described as a Cl-/OH
- ratio, is useful for predicting the
extent of corrosion caused by chloride attack (51).
27
2.3.2 Measurement Techniques
The corrosion behaviour of steel in concrete can be measured using electrochemical techniques
that are suitable for the alkaline, solidifying nature of reinforced concrete. Techniques are useful if
they do not involve the removal of the steel in concrete for separate study and account for the high
ionic resistivity of concrete compared with normal aqueous environments (50). Since the steel is
permanently embedded in concrete, over time as it passivates it will come to an steady state
condition where the anodic and cathodic reaction rates are equal. The potential at which this occurs
is known as the corrosion potential, Ecorr, and is determined by measurement of the potential
difference between the steel and a reference electrode that is within the same environment (50).
The measurement of the corrosion potential is identical to the open-circuit potential (OCP), under
the conditions of no external current flow between the two electrodes, and is useful for assessing
whether an oxidizing or reducing environment exists within the concrete. Several studies have
shown from corrosion potential measurements that a reducing environment exists with OPC-
GGBFS mortars, where it is speculated that oxygen consumption is occurring by the sulfide. The
study performed by Benjamin had revealed negative drifting potentials as low as -750 mV vs.
saturated calomel electrode (SCE) after 30 days, when the silica sand to cementitious material ratio
was 1:1 (62). However, when the ratio was 3:1 (which is more representative of a mortar), higher
potentials were observed at -200 mV vs. SCE with no negative drift observed (62). The negative
drift in potentials was also observed by Yeau, after only 20 days, in OPC-GGBFS concrete with
limestone as coarse aggregate (61). In OPC-GGBFS blends with greater than 70% percent
GGBFS, Angus observed potentials of -400 mV (63). Furthermore, Pal was able to determine
similar behaviour in the influence of reducing characteristics of GGBFS on OPC-GGBFS pore
solutions (64). Despite GGBFS’s capability to create a reducing environment, due to the reducing
28
effects of sulfur species, there is no established standard for the potential at which it can become a
severe corrosion risk. ASTM C876-91 (Test Method for Half Cell Potentials of Uncoated
Reinforcing Steel in Concrete) indicates that an OCP measurement less than -0.5 V vs. CSE means
there is a risk of severe corrosion in air-exposed concrete.
The overall concrete resistivity is one factor that can also be measured to determine the risk of
corrosion. A concrete resistivity less than 5000 ohm·cm means there is risk of severe corrosion, but
the risk is variable since the resistivity is dependent on the w/cm ratio, porosity, relative humidity,
as well as the chloride content and carbonation depth (65). Electrochemical impedance
spectroscopy (EIS) is a useful technique in determining the resistance of concrete by applying an
alternating potential, with respect to Ecorr of the embedded steel, at varying frequencies and
measuring the resulting alternating current (AC) (66). The output impedance (Z) is usually plotted
as a function of frequency and is a measure of the ability of a circuit to resist the flow of electrical
current, at relative amplitudes and phase angles of the current and voltage (50). The impedance is a
sum of both real and imaginary parts and can also be represented by Nyquist (real Z versus
imaginary Z) and Bode plots (|Z| and phase angle versus frequency) (67). Regardless of the data
representation, the data can be fit accordingly to electrical equivalent circuits, with elements such
as resistors, capacitors and inductors, to determine useful electrochemical properties about the
reinforced concrete. The series ionic resistance is a property that can be obtained in this manner
and is a measure of the permeability of its pore structure. It is dependent on the conductivity of the
HCP, geometry of the reinforced steel and is indicated by the high-frequency intercept on the
Nyquist plot (65). One of the most common cell models that is used as a starting point to fit
impedance data in reinforced concrete is the Randles circuit, which is shown in Figure 10 (67). The
29
Randles circuit has a series ionic resistance, double layer capacitor or constant phase element and a
polarization resistance, that is charge transfer resistance in the diagrams shown below (67).
Figure 10 (left) – Randles circuit model (67)
Figure 11 (right) - Warburg impedance circuit model (69)
OPC-GGBFS concretes have a denser microstructure with lower porosity than OPC concrete,
which leads to a greater resistance and reduction in corrosion rate of the embedded steels. Song
and Saraswathy reported that the resistivity of concrete is increased, as an increasing amount of
GGBFS up to 60% is used (59). Furthermore, Macphee and Cao indicated that GGBFS has a
drastic alteration of the pore structure and reported increases in concrete resistivity and reduction in
the corrosion rate of the steel, due to good protection by the GGBFS microstructure (68). In the
study performed by Benjamin, the polarization resistance of steel in OPC-GGBFS mortars had a
tendency to decrease after 10 days, which suggests that the corrosion rate is affected most likely by
the reducing characteristics of sulfide in GGBFS (62). Although the polarization resistance is
affected by fitting the impedance data, Sanchez has shown that certain equivalent circuits are more
accurate at modeling the formation of a passive layer on steel in concrete. The investigated circuits
had constant phase elements and a Warburg impedance component, which represents diffusion
affected impedance at low frequencies, as shown in Figure 11 (69).
30
2.3.3 Reference Electrodes
Reference electrodes are used in the monitoring of the corrosion potential of reinforced steel by
either being permanently embedded in the concrete or externally contacting the concrete surface.
They are generally fabricated from material with behaviour that is independent of the environment
and are non-polarizable with a stable electrode potential (50). Embeddable reference electrodes are
useful for remote monitoring and are advantageous due to their ability to conduct long-term
measurements without risk of oxygen exposure to the concrete (71). However, unlike non-
embeddable reference electrodes, they cannot be periodically tested for accuracy and can be
affected by carbonation and moisture content within concrete. The embeddable manganese dioxide
(MnO2) pseudo-reference electrode is of interest, due to the MnO2 and Mn2O3 reaction in alkaline
environments. It is designed as a double junction electrode with an interface between the metal and
inner electrolyte, as well as an interface between the inner electrolyte and concrete (70). The inner
electrolyte is typically a NaOH solution, at pH 13.5 (71). The MnO2 electrode also contains a
concrete porous plug, shaped to give contact with the concrete specimen (71). At 25°C, the MnO2
electrode in a saturated Ca(OH)2 solution is +0.150 V vs. SCE or +0.396 V vs. SHE (70). In theory
however, the MnO2 electrode is not a true reversible reference electrode and the possibility of a
liquid junction potential error across its porous concrete plug is an issue, due to difference in
mobilities of Na+ and OH
- in the inner electrolyte solution (70). In experiments performed by
Muralidharan, MnO2 sensors showed long-term stability and little variations in potential, in
concrete samples under laboratory conditions (72). The non-embeddable mercury-mercury oxide
(Hg/HgO) reference electrode can also be used in alkaline environments and at 25°C, the standard
electrode potential is +0.098 V vs. SHE in 20% potassium hydroxide (KOH) (73). Its inner
electrolyte consists of 20% KOH and it is also designed as a double junction electrode (73).
31
CHAPTER 3 – EXPERIMENTAL DETAILS
3.1 High-Density Concrete and Mortar Sample Preparation
The basis for the experimental work was to design and replicate the reinforced high-density
concrete, used in the DSCs, in a laboratory environment. In order to effectively accomplish this,
the concrete engineers at AMEC Earth and Environmental were consulted for the usage of their
facilities and materials to establish the mix design and prepare samples. The mix designs were
created for both high-density concrete and mortar, with the purpose of calculating the amount of
components needed to successfully batch correctly proportioned concrete or mortar mixtures. The
mix design incorporates yield and batch calculations and accounts for the air content, w/cm ratio
and specific gravity of all components. Mix designs were prepared for producing the high-density
concrete and mortar samples with a 50% OPC and 50% GGBFS mix, as well as 100% OPC only.
The fine aggregate of the high-density mortar samples was also varied between fine hematite (iron
oxide) sand and silica sand. Therefore in total, two types of high-density concrete samples and four
types of high-density mortar samples were prepared, as indicated in Table 8.
3.1.1 Cementitious Materials Specifications
OPC and GGBFS were used as the cementitious materials in producing the high-density concrete
and mortar. Prior to usage, the cementitious materials were verified with the OPC certification
CSA A3001-08 (Type 10 OPC) and the GGBFS certification CSA A3001 (Type S GGBFS). The
chemical compositions and physical analysis of the OPC and GGBFS used are specified in Tables
3 and 4, respectively. It should be noted that only the sulfur content was determined for the
GGBFS and the remaining composition is expected to be similar to Table 2.
32
Chemical
Compounds
Mass
Composition
(%)
Mineral
Compounds
Mass
Composition
(%)
Physical
Analysis
CaO (Lime) 62.50 Tricalcium
silicate (alite)
CaO3SiO5 (C3S)
57.33 Blaine
Fineness
(m2/kg)
385
SiO2 (Silica) 19.20 Dicalcium
silicate (belite)
CaO2SiO4 (C2S)
11.79 Air Content
(%)
6.22
Al2O3
(Alumina)
5.43 Tricalcium
aluminate
(aluminate)
Ca3Al2O6 (C3A)
10.88 Initial
Setting Time
(minutes)
115
Fe2O3
(Iron Oxide)
2.08 Tetracalcium
aluminoferrite
(ferrite)
Ca2AlFeO5
(C4AF)
6.32 Compressive
Strength,
7 days
(MPa)
33.09
MgO
(Magnesia)
2.32
Compressive
Strength,
28 days
(MPa)
40.70
SO3 (Sulfur
trioxide)
4.12
Total Alkali 0.99 Loss on
Ignition (%)
2.49
Free Lime 1.18 Table 3 – Chemical composition and physical analysis of OPC used in experimental work
Chemical
Composition
Physical
Analysis
S (Sulfide
Sulfur)
Blaine
Fineness
(m2/kg)
Air
Content
(%)
Compressive Strength
(50% OPC-50% GGBFS)
Slag Activity
Index (%)
7 days
(MPa)
28 days
(MPa)
7
days
28
days
Mass
Composition
(%)
1.14 717 6.80 22.81 38.37 80.1 110.8
Table 4 - Chemical composition and physical analysis of GGBFS used in experimental work
3.1.2 Coarse and Fine Aggregate Specifications and Testing
Magnetite (Fe3O4) and hematite (α-Fe2O3) were used as the coarse and fine aggregates,
respectively and their grading, specific gravity, absorption and surface moisture content properties
were determined by the ASTM specification testing procedures mentioned in section 2.2.3.1.
33
Samples of the aggregates from the raw stockpile were taken, washed, dried and immersed in water
for 24 hours, prior to testing. They were then removed and dried to a saturated-surface dry state
before being evenly split into two equal batch sizes for duplicate testing. For the grading analysis,
the aggregates were shaken through successive CSA sieves in an 8 inch shaker chamber with a
shaking time of 10 minutes. The particle sizes present in the coarse and fine aggregates were well
distributed and for the most part within the allowable limits, as shown in Figures 12 and 13, the
grading charts which are used to graphically represent the sieve analysis results.
Figure 12 (left) – Coarse aggregate grading chart (Red lines indicate maximum and minimum limits)
Figure 13 (right) – Fine aggregate grading chart (Red lines indicate maximum and minimum limits)
The specific gravity and absorption properties were determined together with the usage of
calibrated equipment, such a wire basket suspended in water for the coarse aggregate and
volumetric flask filled to capacity with water for the fine aggregate, to determine the apparent
mass in water. Along with the determination of the saturated surface-dry and oven-dry masses,
the properties were then experimentally calculated as shown in Appendix A-1. The surface
moisture content of the aggregates was also determined similarly, however the aggregate
samples that were tested did not need to be washed or dried, as the actual wet mass of sampled
raw stockpile aggregates was needed to find their existing moisture content. Table 5 summarizes
the average properties of the coarse and fine aggregates from the duplicate testing results.
34
Specific gravity
(SSD) (kg/m3)
Absorption
(%)
Moisture content
(%)
Coarse aggregate 4310 0.183 0.563
Fine aggregate 4754 0.140 0.120 Table 5- Properties of coarse and fine aggregates
Compositional testing was performed using X-ray diffraction (XRD) analysis on the crushed
hematite aggregate to determine its purity existing as hematite. The XRD analysis was performed
at the University of Toronto, Department of Chemistry, X-ray Powder Diffraction Lab using a
Siemens D5000 conventional θ/2θ diffractometer. Although the fine aggregate sand is considered
to consist entirely of hematite, the XRD analysis showed that it consisted of 89.6% hematite, 7.9%
magnetite and 2.5% impurities. The XRD spectrum is shown in Appendix A-2.
3.1.3 Admixtures
The AEA and superplasticizer materials that were used were the BASF brand Micro Air and PS
1466. Micro Air offers excellent stability of air-entrainment and the recommended dosage was 2
mL/kg of cementitious material (74). PS 1466 is useful for concrete mixtures containing additional
cementitious materials other than OPC and the recommended dosage was 3.06 mL/kg of
cementitious material (75). Although these dosages were suggested by the manufacturer, in
practice, it was found that the recommended dosages were high and the actual dosages used had to
be experimentally determined depending on the mixing and batching conditions. Furthermore,
since the air content is affected by the AEA and superplasticizer dosage rate, experimentally
verifying the correct dosage rates to obtain the required air content is important.
3.1.4 Mix Designs
The first part of the mix design involved a yield calculation that ensured that the components were
35
properly proportioned at theoretical masses to create a concrete volume of 1 m3
at the desired
density, if they were mixed together. Concrete is normally produced with the components being
specified and measured on a mass basis; however it is sold on a volume basis, since the mass
required will vary depending on the specific gravity of the concrete’s components. The calculation
involved adding the volumes (m3) of the cementitious material, aggregates and water, determined
from their masses and specific gravities, to total 1 m3
of concrete. The air content was subtracted
from the 1 m3
basis to give the total theoretical yield, while the volumes of the AEA and
superplasticizer were not accounted for since they were an insignificant amount. The second part
of the mix design involved a batch calculation, which was a scale-down of the yield calculation, to
calculate the actual component masses that would be mixed together to create a desired amount of
concrete or mortar. The batch calculation took into the account the non-SSD state of the raw
stockpile aggregates, by incorporating their absorption and existing surface moisture content
properties, which corrected the aggregates’ water demand and thus the amount of mix water to be
added. The mixing water needs to be accounted for correctly, since excess water will cause
sedimentation of the aggregates or deficient water will affect the HCP formation. Mix design
calculations for both the high-density concrete and mortar samples are shown in Appendix A-3.
3.1.5 Mixing and Casting Procedures
Upon creation of the mix design, the cementitious material, aggregates and water were then
weighed accurately to three decimal places, in kg, on an electronic balance and kept in dry,
separate containers. An industrial sized, flat-pan mixer was used to prepare 80 kg concrete batches,
while a Hobart N-50 Quart mixer was used to prepare 4.5 kg mortar batches. Concrete and mortar
batches have different mixing times, but the essential procedure is to mix, rest and then mix again.
36
The detailed mixing procedures were documented and are presented in Appendix A-4. The fluidity,
wetness and workability of the batches were monitored to ensure adequate mixing and uniformity
of the batch in the mixer. Upon completion of mixing, the slump (for the concrete batches only)
and air test were performed on the freshly mixed concrete or mortar to ensure they were within
specifications. The slump specification was determined, from the NEP visit, to be 110-135 mm,
while the air content was 5.5% +/- 1.5%. A Humboldt concrete air meter was used as per ASTM
C231 (Standard Test Method for Air Content of Freshly Mixed Concrete by the Pressure Method)
to test the air content of the concrete. For the mortar, the ASTM C185 (Standard Test Method for
Air Content of Hydraulic Cement Mortar) mathematical derivation and formula was modified to
calculate the air content as shown in Appendix A-5. Samples were cast in cylindrical,
propylene/ethylene copolymer blend plastic molds. The concrete samples were cast in 4” diameter
(D) by 8” length (L) molds and the mortar samples were cast in smaller 2” D by 4” L molds.
Figure 14 presents a schematic of the prepared samples and the compositional difference between
high-density concrete and mortar, while Figure 15 shows the two types of high-density concrete
samples after 7 days of setting.
Figure 14 (left) - Schematic of high-density GGBFS concrete and mortar samples
Figure 15 (right) - Cross-sections of concrete samples ((A) 50% OPC-50% GGBFS (B) 100% OPC)
The concrete and mortars were tapped and rodded to ensure that the HCP and aggregates were
consolidated within the molds and not segregated. Carbon and stainless steel (CS and SS) electrode
37
embedment into the samples was performed using a plastic ring support apparatus to hold the
electrodes in place during the setting stages. The carbon steel was specified to be of a 99% pure
iron basis and the stainless steel was grade 308L welding rod (76). The steels were approximately
2 mm D by 200 mm L in the concrete and 2 mm D by 100 mm L in the mortar, with 3 cm length
shrink tubing around their middle. After embedment, the molds were capped, sealed with water
resistant silicone, electrically taped and transported to a deaerated glove box for storage and
electrochemistry experimentation, as indicated in section 3.2. The glove box was purged via
vacuum pump to eliminate oxygen and filled with nitrogen at a high flow rate, as per the
manufacturer’s recommended procedure (77). Tables 6 and 7 summarize the mix design and
casting results for the high-density GGBFS concrete and mortar samples with iron oxide
aggregates. Appendix A-3 presents the detailed results for all the samples.
Sample
Mass Percentages (%) AEA
(mL)
Superplasticizer
(mL)
W/CM
Ratio
Density
(kg/m3) OPC GGBFS Coarse
Aggregate
Fine
Aggregate
Mix
Water
Concrete 5.09 5.09 49.09 36.59 4.12 7.50 7.50 0.42 3535.20
Mortar 8.90 8.90 - 75.14 7.06 1.60 2.45 0.42 3471.76
Table 6 – Mix design results for high-density GGBFS
concrete and mortar samples (with iron oxide aggregates)
Sample Air
Content (%)
Slump
(mm)
Compressive
Strength
7 days
(MPa)
28 days
(MPa)
Concrete 7 110 28 42
Mortar 6.1-23.7 - - - Table 7 – Casting results for high-density GGBFS
concrete and mortar samples (with iron oxide aggregates)
3.1.6 Concrete to Mortar Procedure and Theory
The high-density mortar samples were made to replicate the high-density concrete, with the major
assumption that the coarse aggregate in the concrete does not react with the HCP. In order to have
38
replication of the concrete’s mortar reaction chemistry in mortars themselves, the concrete batches
that were not cast, were taken after mixing and the coarse aggregate was sieved out using a 4.75
inch sieve. The fine aggregate that was coated on the coarse aggregate was determined by washing
the coarse aggregate and collecting the remains in a 200 mm sieve. The cementitious material and
water that were coated on the stone were then determined to be the remaining amount. A
theoretical mortar batch calculation was then performed from these lost amounts to obtain mass
percentages of the cementitious material, fine aggregate and water in a correctly proportioned
mixture with no coarse aggregate. Appendix A-6 indicates in detail the concrete to mortar
calculations. By using these calculated mass percentages, the mortar mix design was then
developed similar to the concrete mix design. Other assumptions involved in this calculation were
that the HCP contains all the water and the sieved coarse aggregate was at SSD condition.
3.1.7 High-Density Concrete and Mortar Sample List Summary
The following table presents a summary of the high-density concrete and mortar samples that were
produced, with an indication of the different reference and working electrodes used.
Sample Type Mortar Concrete
Sample
Number and
Letter
1 2 3 4 5 6
a b c d e f g a b c a b c a b c a b c a b
Cementitious
Material
50% OPC
50% GGBFS
100%
OPC
50% OPC
50% GGBFS
100%
OPC
50% OPC
50% GGBFS
100%
OPC
Coarse
Aggregate - Magnetite Stone
Fine
Aggregate Hematite Sand Silica Sand Hematite Sand
Reference
Electrodes Hg/HgO MnO2 Hg/HgO MnO2 Hg/HgO MnO2 Hg/HgO
Working
Electrodes CS and SS
Table 8 - High-density concrete and mortar sample list summary
39
3.2 Electrochemistry Experiments
The corrosion mechanism that is of interest was mentioned in section 2.3.1 and involves a
relatively beneficial, reducing environment being created in the high-density GGBFS, reinforced
concrete or mortar environment, due to oxygen consumption by CaS. It was hypothesized that
this reducing environment could be created in the prepared samples, outlined in section 3.1,
where the only oxygen availability is within the HCP and not from the external environment. In
order to understand the effect of this sulfur corrosion mechanism on the embedded steels,
electrochemistry experiments involving corrosion potential and EIS measurements were
performed. Additionally, voltammetry scans on embedded noble metals were performed to try to
detect dissolved species including oxygen, chloride, sulphur compounds, etc.
3.2.1 Electrochemical Cell
The electrochemical cell was set-up using a PARSTAT® 2263 potentiostat as the electronic
hardware to control the three-electrode system. The cell consisted of the reinforced high-density
concrete or mortar molded samples with designated carbon and stainless steel working electrodes
to be tested, a designated carbon steel counter electrode, similar to Figure 14, and a 1 cm D
drilled hole for the reference electrode/sample surface contact. The available surface area of the
embedded electrodes, neglecting the shrink tubing surface area, was approximately 2.23 cm2 in
the mortars and 5.37 cm2 in the concrete. The reference electrodes used were both embeddable
(MnO2 with 0.1 M NaOH as the inner electrolyte) and non-embeddable (Hg/HgO with 20%
KOH as the inner electrolyte) types, with electrolytic contact between the sample surface and the
non-embeddable reference electrodes being created by a sponge soaked in 20% KOH. During
non-measurement periods, the samples were stored in the deaerated, nitrogen-filled glove box.
40
3.2.2 Reference Electrode Preparation
The embeddable (MnO2) pseudo-reference electrodes were prepared using stainless steel 304
tubing with silver-soldered copper wire to provide the external electrical connection. The internal
design of the reference electrode consisted of packed manganese (IV) oxide powder, an inner 0.l
M NaOH gel at pH 13.5, an OPC plug at 0.5 w/c ratio at one end and epoxy resin plug at the
other. The NaOH gel was prepared by adding 2% granulated agar by mass in water and heating
the mixture to a dissolving point of 90°C. The mixture was then allowed to cool to 55°C and 0.1
M NaOH was added to achieve a pH of 13.5. Prior to embedment, the four electrodes were
stored in a saturated Ca(OH)2 solution at 25°C, with their potential being periodically monitored
for about 6 weeks with a commercial Force Technology MnO2 reference electrode, using a
voltmeter. The non-embeddable (Hg/HgO) reference electrode did not require any preparation,
however it was periodically calibrated against other reference electrodes and during non-
measurement periods it was stored in a 20% KOH solution at 25°C.
3.2.3 Corrosion Potential Measurements
Corrosion potential measurements were performed on the embedded steels for approximately 10
weeks. The PARSTAT®
2263 potentiostat measured the open circuit potential, between the
working electrode and reference electrode, in volts (V) versus time, relative to the type of
reference electrode used. The total measurement time was approximately 5 minutes and the
collected data points for the last 50 seconds were averaged and taken as the potential value.
3.2.4 Electrochemical Impedance Spectroscopy Measurements
EIS measurements were performed on the embedded steels for approximately 10 weeks. The
41
measurements consisted of the PARSTAT® 2263 potentiostat applying an alternating sinusoidal
potential of 10 mV with amplitude (rms) respect to Ecorr of the embedded steel and measuring the
resulting alternating current, at varying frequencies between 100,000 to 0.1 Hz. The output
impedance measurements were represented as Bode and Nyquist plots and the total measurement
time was approximately 5 minutes.
3.2.5 Cyclic Voltammetry Measurements
Cyclic voltammetry scans were performed on metals embedded in mortar samples made from mix
design 2, as indicated in Table 8. The measurements consisted of the PARSTAT® 2263 applying a
scan rate of 10 mv between potentials of -0.15-1.1 V for silver and -0.75-0.2 V for platinum. The
available surface area of the silver and platinum electrodes was approximately 0.434 and 0.347
cm2, respectively. It was theorized that silver can be used to detect free chloride and be used as a
practical reference electrode, due to its double oxidation state chemistry of Ag2O and AgO, while
platinum voltammetry can imply evidence about the effect of redox reactions involving sulfide and
oxygen in the OPC-GGBFS samples.
3.2.6 Coarse Aggregate Resistance
The resistance of the coarse aggregate was determined by a wet resistivity measurement. One
magnetite stone was taken and two holes were drilled into the stone. The stone was then ground
to rectangular geometry with 4 cm length and total surface area of 66 cm2. Granulated agar salt
gel, prepared from dissolving 2% agar by mass in salt water, was set into the holes and was the
contact medium for copper wire to magnetite. The magnetite was abraded to ensure surface
removal of maghemite, which is more resistive. The resistance was measured by a voltmeter.
42
3.3 Ion Chromatography Experiments
The possible reducing chemistry of the CaS in GGBFS, used in the high-density concrete and
mortar samples, was investigated by ion chromatography experiments to provide complementary
information relevant to the sulfur corrosion mechanism, as discussed in section 3.2. CaS is
capable of consuming oxygen in the concrete and mortar samples, which may cause conversion
into other sulfide forms such polysulfide(s), soluble sulfide and most notably thiosulfate.
Thiosulfate is of particular interest since it is kinetically stable in alkaline environments such as
concrete, and capable (in principle) of corrosively attacking the embedded carbon and stainless
steels, were the pH to drop locally. Ion chromatography was used to determine the concentration
of these anions produced in simulated pore water solutions. Simulated pore water solution
environments were preferred, rather than concrete or mortar environments, since it was easier to
extract solution from liquefied cementitious material at a high solution to GGBFS ratio, rather
than the pore fluid of concrete or mortar. The advantages of ion chromatography as an analytical
technique over other wet-chemical methods are its speed, sensitivity, selectivity and stability of
the separation column (79). It is a favourable option for solutions with ppm concentrations of
thiosulfate (retention time ~11 minutes) and sulfate (retention time ~5 minutes).
3.3.1 Ion Chromatography Column
The ion chromatography column at the University of Toronto, Department of Chemistry,
ANALEST lab was used for performing the experimental work. The system consists of a
PerkinElmer Series 200 binary pump, Alltech ERIS 1000HP Autosupressor and Alltech 550
Conductivity Detector. Essentially the system utilizes temperature compensated conductivity
detection, which can eliminate thermally induced background noise, as well as autosupression of
43
the background conductivity of the eluent passing through the column (79). This allows an
increased signal detection of the analyte. The basic operation of the column is to inject the
sample solutions through the pressurized chromatograph column, which contains a resin with
covalently bonded charged functional groups (79). The ions then become adsorbed onto the
column, while an ionic extraction eluent, 7.2 mol/L of sodium carbonate at a flowrate of 1.5
mL/min, runs through the column causing the ions to separate based on size and type. Prior to
operation, the pump feed line is purged at a flowrate of 10 mL/min. The sample run time is
approximately 16 minutes. The output data is a chromatograph with distinctive peaks that
represent ionic species at different retention times. By testing standard solutions of known
thiosulfate and sulfate concentrations, the corresponding peak areas that are produced and
displayed on the chromatograph can then be used to create a correlation between area and
concentration. This correlation then can be used to approximate unknown solution concentrations
from outputted chromatograph peak areas.
3.3.2 GGBFS in Water
The hydration of GGBFS in deionized water was examined at 10:1 water to GGBFS ratio. It was
hypothesized that the reaction would be slow, as indicated in Juenger’s work, and the amounts of
thiosulfate and sulfate produced would be relatively low (29). An experimental comparison
between a controlled, limited oxygen environment and unlimited oxygen environment was made
by creating two duplicate GGBFS in water samples within cylindrical, plastic molds. The
controlled case was sealed from the external atmosphere, similarly to how the high-density
concrete and mortar samples were sealed as explained in section 3.1.5. The controlled case was
also made carbon dioxide free, by being sealed in a CO2 free glove box containing limewater as a
44
CO2 absorbing medium. The unlimited oxygen samples were designated as non-CO2 free
samples. The purpose of creating CO2 free and non-CO2 free samples was to investigate and
compare the effect of carbonation on the solution pH. The purpose of the limited oxygen
environment solution was to have it replicate a concrete or mortar environment, where the
oxygen content is low. The solutions were allowed to hydrate within the moulds and filtered
samples were periodically extracted from the moulds on a weekly basis for ion chromatography
testing.
3.3.3 GGBFS in Basic Solutions
The hydration of GGBFS in basic solutions of 0.1 M NaOH and saturated Ca(OH)2 in NaOH
(produced to create a total 0.1 M OH- concentration) at a 10:1 solution to GGBFS ratio were also
examined. It was anticipated that the presence of calcium would slow down slag dissolution and
sulfide release for oxidation. The latter experiment was also performed with the hypothesis that an
aqueous calcium equilibrium would effectively be established in the solution, due to saturation
with Ca2+
, which would affect the dissolution of sulfide from GGBFS into the solution. If no
calcium equilibrium is established then the high pH of the solution should be able to attack the
glass GGBFS particles to release any frozen sulfide for oxidation, as theorized in the NaOH
solution experiment. Duplicate samples of both types of the basic solutions were prepared, with
CO2/non-CO2 free and limited oxygen/unlimited oxygen environment comparison.
3.3.4 Aggregate and GGBFS in Basic Solutions
The hydration of GGBFS in a saturated Ca(OH)2 + NaOH (total 0.1 M OH- concentration) at a
10:1 solution to GGBFS ratio, with the addition of 1 g of cleansed, fine aggregate, was investigated
45
to determine if hematite is capable of reacting with reduced sulfur compounds. Duplicate samples
were prepared, with CO2/non-CO2 free and limited oxygen/unlimited oxygen comparison,
similarly to the previous experiments. Another sample set was prepared, with 300 ppm of
thiosulfate and 1 g of cleansed, hematite added to the basic solutions to determine if dissolved
thiosulfate in solution reacts differently with hematite, in the absence of GGBFS. Furthermore, a
third sample set of the basic solution was prepared, with 300 ppm of thiosulfate and 1 g of
synthetic magnetite, to investigate if magnetite reacts with thiosulfate. It was expected that the iron
oxide aggregates and thiosulfate would react in some manner, however the reaction chemistry is
theorized to be complex and the results may not be representative of the reaction chemistry in the
high-density concrete and mortar samples. A comparison between the first two sets of experiments,
involving aggregate in the absence of GGBFS, would also help understand the limitations a
GGBFS environment may introduce on the iron oxide aggregates’ capability to react with CaS.
3.3.5 Ion Chromatography Sample List Summary
The following table presents a summary of the ion chromatography samples that were produced,
with the amount and type of sample medium and components indicated.
Sample
Number
Sample
Medium
Amount
of
Medium
(mL)
Sample
Components
Amount of
Components
(g, unless stated)
1 Deionized Water
100
GGBFS 10
2 0.1 M NaOH GGBFS 10
3 Saturated Ca(OH)2 + NaOH GGBFS 10
4 Saturated Ca(OH)2 + NaOH GGBFS, Hematite 10, 1
5 Saturated Ca(OH)2 + NaOH Thiosulfate,
Hematite
300 ppm, 1
6 Saturated Ca(OH)2 + NaOH Thiosulfate,
Magnetite
300 ppm, 1
Table 9 - Ion chromatography sample list summary
46
3.4 Microscopy Experiments
The microstructures of the cementitious materials, high-density concrete and mortar samples were
investigated by microscopy and analysis. Environmental scanning electron microscopy (SEM) and
energy-dispersive X-ray spectroscopy (EDX) techniques, such as high-resolution imaging,
quantification and mapping, were used to view and analyze the materials and samples. The primary
objective was to understand the composition and elemental distribution of GGBFS environments
upon hydration, as well as any visual evidence of the origin and transport of sulfide from GGBFS.
The general composition of the microstructure was desired to be practically studied rather than a
time-dependent one, since it is difficult to track the hydration products in their unsteady state of
growth and the fate of sulfide during GGBFS hydration has been widely speculated, as discussed
in section 2.2.4. Furthermore, due to the fact that the GGBFS contained only 1.14% sulfide by
mass, it was expected that the sulfur quantification may be unrealistic due to the low mass % and
EDX signal detection limitation. Other challenges that were considered during the microscopy
experiments were relief and morphology effects due to the porous, uneven hydrated
microstructure, as well as the superimposition of the sample geometry during EDX mapping.
3.4.1 Environmental Scanning Electron Microscope (ESEM)
The Hitachi SU6600 analytical variable pressure (environmental) scanning electron microscope
(ESEM) at the University of Toronto, Department of Materials Science and Engineering was used
for microscopy. The ESEM instrument parameters used with the backscatter electron (BSE)
detector and Bruker Quantax EDX detector, were an electron accelerating voltage of 20 kV,
variable chamber pressure of 20 Pa, working analysis distance of 10 mm, medium probe current
intensity and both mechanical apertures set at a focus level of 3. At these parameters, the Schottky-
47
field emission electron source of the microscope provided good beam current stability with a small
probe diameter. The beam stability is affected by the voltage of 20 kV, as charging tends to
become significant as the voltage increases, however the variable pressure in the chamber tends to
dissipate this. Prior to analysis of each sample, aperture, beam and stigma alignment were
performed to stabilize the beam current and any oscillations in the x and y directions. The coarse
and fine adjustment of the resolution was also modified at the appropriate brightness, contrast and
scan refresh rate. The majority of experimentation was performed using the Bruker EDX detector,
however occasionally an Oxford Instruments EDX detector was used in its place. The EDX
quantification for the ESEM was also capable of quantifying lighter elements than sodium’s atomic
weight, such as oxygen. It should also be noted that the samples did not have to be conductively
carbon coated prior to inserting them into the ESEM chamber with a 2 inch diameter stage, as the
accumulation of electric charge on the specimen surface is avoided with an ESEM (80).
3.4.2 Microscopy Sample List and Mounting Procedures
Microscopy experiments were performed on a variety of different samples, as summarized in Table
10. The mounting procedure performed for the wet GGBFS samples in different environments was
identical. The procedure involved initially preparing the sample at a 0.5 solution to GGBFS ratio in
a disposable mounting cup and allowing the sample to hydrate for 1 week. Following hydration,
the sample would then be placed in a larger plastic mounting cup and a mixture of epoxy resin and
hardener would be cast into the cup to permanently embed the sample. The epoxy resin and
hardener were mixed by mass at a 100 to 14 g ratio for approximately 7 minutes. After epoxy
placement into the cup, the embedded sample was then placed in a desiccator and ran under a
vacuum, for approximately 10 minutes, to ensure removal of all air bubbles from the epoxy
48
mixture. The sample was then allowed to set for approximately 1 day, so the epoxy mixture could
permanently harden around the embedded specimen. The mounting procedure performed for the
dry GGBFS sample was similar, however the GGBFS was directly mixed with the epoxy mixture
at an equal volume amount. The mounting procedure performed for the high-density concrete and
mortar samples was also similar, however there was no initial sample preparation, as small, cubic
sections were cut from the high-density concrete and mortar sample types, indicated in table 8.
Sample
Types
Dry
GGBFS
Wet GGBFS in
Different Environments
High-
Density
Mortar
High-
Density
Concrete
Sample
Components
GGBFS GGBFS
GGBFS
+ OPC
1 2 3 4 5 6 Sample
Medium
- Aerated
Water
Deaerated
Water
NaOH Saturated
Ca(OH)2
+ NaOH
Aerated
Water
Table 10 - Microscopy sample list summary
(The high-density concrete and mortar samples are the six types of samples listed in Table 8)
3.4.3 Grinding and Polishing Procedures
The grinding and polishing procedures were identical for all the different samples. The samples
were first cut with a diamond saw to expose the surface of the embedded specimen. They were
then ground from a 78 to 15.3 µm finish with successively finer silicon carbide grinding paper and
polished from a 9 to 1 µm finish on polishing discs, both using a rotating grinding/polishing wheel.
Water was used as the lubricant during the grinding stages, while diamond paste suspensions from
9 to 1 µm were used as the polishing media. During successive grinding and polishing stages, the
samples were ultra-sonically cleaned in deionized water to ensure removal of any debris and
examined under an optical microscope to ensure a satisfactory surface appearance.
49
CHAPTER 4 – RESULTS AND DISCUSSION
4.1 Electrochemical Analysis
Electrochemical analysis of the embedded steels has important implications for understanding their
corrosion behaviour in high-density concrete and mortar samples. Due to the high alkalinity of the
samples, the embedded steels are likely to passivate and remain protected, unless the total available
oxygen in the samples is significantly depleted by CaS oxidation (only in the high-density GGBFS
concrete and mortar samples) leaving little or no oxygen available for cathodic reduction on the
steels. The electrochemistry experiments, involving OCP and EIS measurements on the embedded
steels, were subsequently analyzed to confirm whether this type of reducing environment exists
within the samples. The voltammetry scans on the noble metals also provide complementary
information relevant to this issue.
4.1.1 Embeddable Reference Electrode Measurements
To ensure accurate OCP and EIS electrochemical measurements with the embeddable MnO2
reference electrodes, their potentials were periodically monitored against a Force MnO2 reference
electrode to ensure a known, stable potential value prior to embedment, as shown in Figure 16.
Figure 16 – Experimental MnO2 reference electrode potentials versus time
50
The results indicate that the electrodes reached a relatively steady state potential after 6 weeks of
measurement. A deviation in the saturated Ca(OH)2 solution pH by +/- 0.5 pH units is likely
responsible for the potential variation by approximately +/- 10 mV in the electrodes for samples
1d, 1e and 3c, since changes in alkalinity can affect the MnO2-Mn2O3 reaction (71) . Furthermore,
the electrode for sample 5a was constructed first, using the procedure described in section 3.2.2,
and the potential variation from the other electrodes, may be due to the fact that it was constructed
individually and not concurrently with the other electrodes. An average steady state potential value
of -0.192 V versus the Force MnO2 reference electrode was taken from the results and calculated to
be approximately +0.108 V versus the Hg/HgO reference electrode, based on the literature
potential values presented in section 2.3.3. The large difference in potential from the Force
electrode can be attributed to the experimental electrodes’ stainless steel tubing, which can adopt
its own potential, and is a different type of housing than used in the Force electrode.
4.1.2 Open Circuit Potential Analysis
The OCP measurements of the embedded steels were analyzed to determine the existence of a
reducing environment within the high-density GGBFS concrete and mortar samples, by
observing a negative drift in the OCP with time. A negative drift indicates that oxygen is being
consumed by CaS oxidation, whereas a positive drift would mean that an oxidizing environment
exists, despite the fact that CaS is present in stoichiometric excess to the oxygen. The OCP
measurements for the high-density GGBFS concrete and mortar with iron oxide aggregates,
sample types 1 and 5 indicated in Table 8, are presented in Figures 17 and 18. The OCP
measurements for the remaining concrete and mortar samples are shown in Appendix A-7. After
approximately 10 weeks, in the high-density concrete samples, there are some low potentials
51
suggesting oxygen consumption by the sulfide, however there is also positive to negative
potential variation, which may be influenced by the hematite-magnetite equilibrium potential.
Since magnetite is present at approximately 49% by mass, the contact it has with the embedded
steels is at an unpredictable extent, due to its relatively larger surface area. The lowest potential
observed is approximately -0.4 V versus SHE, which is slightly higher than the potential of the
hematite-magnetite equilibrium at pH 13, shown in the iron Pourbaix diagram, attached in
Appendix A-9. It is reasonable to suggest that if the sulfide can consume the oxygen to very low
levels then the embedded steels will not passivate, and if there is sufficient contact with the
insulating hematite, the potential can rise from near the hematite-magnetite equilibrium potential
to a higher oxidizing potential, as observed on the carbon steel in sample 5c. For the stainless
steel in sample 5c, the potential appears to remain reducing and hematite may not be in sufficient
contact to cause positive potential variation. However, the large amount of hematite in the
samples, approximately 37%, ensures that the potential will not go lower than the magnetite-
hematite equilibrium potential and that this potential dominates the direction that the steel’s
potential takes, rather than the sulfide-oxygen redox potential.
Figures 17 (left) and 18 (right) – Corrosion potentials of embedded carbon and stainless steels in high-density
GGBFS concrete and mortar samples with iron oxide aggregates (samples type 5 and 1)
52
The high-density mortar samples, 1a-1e, show no negative drift towards reducing potentials, but
rather a stable negative potential that is slowly oxidizing, most likely due to the diffusion of
external oxygen into the samples over the 10 weeks of measurement. The high air content
(23.7%), within these samples and lower CaS content, due to the smaller sample size, are likely
the reasons that a negative drift was not achieved. Since these samples were created with the
recommended dosages of AEA and superplasticizer, as shown in mix design 1 in Appendix A-3,
their high air content was not initially expected and accounted for only after the electrochemical
measurements, by a digital point-count method on their hardened state. This led to the
development of the mortar air content formula, as described in Appendix A-6, to ensure that the
air content, of any successive samples produced, could be measured in the mortar’s plastic state.
Successive samples were produced with a lower air content, 1f and 1g at 6.1%, similar to the
concrete air content results. A slightly negative drift in the OCP for approximately 4 weeks was
observed, before eventual oxidizing potentials occurred similarly to samples 1a-1e. The lowest
potentials observed in both the high-density concrete and mortar samples were around -0.5 V
versus Hg/HgO, which is similar to the results of Benjamin and Angus (62, 63).
The variation in the mortar’s OCP results prompted stoichiometric analysis to determine criteria
for the negative drift observance from CaS oxidation. It was found that the molar ratio between
sulfide and oxygen is approximately 4 times greater in samples 1f and 1g than 1a to 1e. The ratio
for samples 1f and 1g was twice as great as for the concrete samples, due to the higher mass
percentage of GGBFS, rather than their similar oxygen content. Since the air content is desired to
be fixed at 5.5% +/- 1.5%, it is reasonable to establish a minimum molar ratio, where a reducing
environment has been shown not to occur. Based on this analysis and the worst-case scenario
53
assumption that all the total available oxygen will react with sulfide and not be consumed in
other HCP reactions, the molar ratio of sulfide to oxygen is approximately 55. The detailed
stoichiometric calculations are shown in Appendix A-8.
The OCP results for the high-density 100% OPC concrete and mortar were similar, as generally
higher oxidizing potentials were observed due to the absence of GGBFS. The OCP results for the
GGBFS mortar with silica sand show no signs of reducing potentials, most likely due to the high
air content of 17.9% and an approximate molar ratio of sulfide to oxygen of 50. However, the
potentials observed were more negative than the 100% OPC mortar with silica sand. The
difference between silica and hematite sand on the OCP results is not evident, however silica
sand seems to have an effect on creating lower stainless steel potentials than carbon steel
potentials. In general, the OCP results of the carbon steels were higher than the stainless steels,
as expected, since the presence of 19-21% chromium lowers the oxygen reduction kinetics
responsible for the formation of the chromium (III) oxide, passive film. Furthermore, Pourbaix
diagrams for both iron and chromium at pH 13, attached in Appendix A-9, indicate that both
types of metals form stable surface phases and corrosion is relatively unlikely.
4.1.3 Electrochemical Impedance Spectroscopy Analysis
The EIS measurements of the embedded steels were analyzed to determine the value of the series
electrolyte resistance between the working and counter electrodes in the high-density concrete and
mortar samples. Although the electrolyte resistance does not directly reveal information about the
existence of a reducing environment, it gives information on the continuity of conductive pore
water throughout the HCP, and also (for the concrete samples) on the possible shorting-out effect
54
of the large magnetite aggregate. The electrolyte resistance for the high-density GGBFS concrete
and mortar with iron oxide aggregates, sample types 1 and 5 indicated in Table 8, are presented in
Figures 19 and 20. The electrolyte resistance measurements for the remaining concrete and mortar
samples are shown in Appendix A-10. From the results, it can be seen that for both types of
samples, the electrolyte resistance has a tendency to increase with time, due to the increase in
strength and decrease in permeability of the HCP. As less free water becomes available, due to the
consumption by the HCP, the total porosity will decrease which creates a denser paste structure.
Figures 19 (left) and 20 (right) – High frequency electrolyte resistance of embedded carbon and stainless
steels in high-density GGBFS concrete and mortar samples with iron oxide aggregates (samples type 5 and 1)
The carbon steel in sample 5c initially shows an increasing resistance with time, however there is a
decrease after approximately 50 days. The decrease is due to the short circuit effect of the
conductive magnetite, which also causes an increase in the double-layer capacitance and correlates
with the potential rise observed in the OCP results. The high resistance results of the stainless steel
in sample 5c also correlate to its reducing OCP results, as there is no potential variation or decrease
in resistance over time, due to the lack of conductive magnetite contact with the stainless steel. In
general, considering the results of all the samples, the stainless steels did not show a trend of
having higher resistances than the carbon steel, however this may be a geometrical effect since the
geometrical environment for current flow may vary for the electrodes. Furthermore, the mortar
55
samples containing silica sand had a much higher resistance than the mortar samples with hematite
sand, most likely since the silica sand is finer and has a higher water demand. Both the concrete
and mortar samples with GGBFS were observed to have a higher resistance than the samples with
100% OPC, due to the effect that the GGBFS has on the HCP. Nevertheless, the concrete had a
lower resistance than the mortar samples, due to the presence of conductive magnetite.
4.1.4 Cyclic Voltammetry Analysis
Cyclic voltammetry measurements of the silver electrode, as shown in Figure 21, confirm the
presence of free chloride within the HCP, which makes it difficult to evaluate the effectiveness of
pure silver as a reference electrode. The broad peak originating around 0.22 V is near the Ag/AgCl
equilibrium potential and the OPC measurement prior to voltammetry measurement was
approximately 0.25 V, suggesting that the present free chloride concentration is sufficient enough
to create a stable potential consistent within the Ag/AgCl potential range, rather than the silver
oxide potentials. The Ag/Ag2O and Ag2O/AgO oxidation peaks were observed at higher potentials,
as expected from their potential-pH curves on the silver Pourbaix diagram in Appendix A-9,
however the second oxidation peak was sufficiently close to the oxygen equilibrium potential.
Figures 21 (left) and 22 (right) – Cyclic voltammograms for silver and platinum
in 100% OPC high-density mortar
56
Analysis of the platinum voltammogram indicates a charging effect related to the double layer
capacitance, as observed by the loop at positive potentials above 0 V, in Figure 22. The observed
cathodic current densities are likely too high for oxygen reduction to be occurring and the cycled
potential range is too positive for hydrogen evolution. Furthermore, the large IR drop observed
suggests that ohmic control may be a factor in contributing to the total impedance of the cell, as the
overall resistance of the cell is observed to be greater than the solution resistance.
4.1.5 Coarse Aggregate Resistance Measurement
The wet resistance (R) measurement of the magnetite, was approximately 0.65 Ω and based on the
aggregate’s geometry (A), length (l) and from the resistivity equation,
, the aggregate’s
resistivity is 10.7 Ω·cm. The low resistivity is comparable to the literature values discussed in
section 2.2.3 and suggests that magnetite can be fairly conductive in the high-density concrete
samples, if sufficient abrasion of surface maghemite occurs by the HCP.
4.2 Ion Chromatography Analysis
The oxidation of CaS from GGBFS into thiosulfate could potentially affect the corrosion
tendency of the embedded steels. The kinetic stability of thiosulfate is an important factor in
determining its ability to cause corrosion – equation 10 – rather than oxidizing to relatively non-
corrosive sulfate (equation 11). Both anodic reactions occur alongside oxygen reduction.
2CaS + 2O2 + H2O → S2O32-
+ 2Ca2+
+ 2OH-
[10]
S2O32-
+ H2O + 2O2 → 2SO42-
+ 2H+
[11]
The concentrations of thiosulfate and sulfate formed are important for determining the total mass
percentage of sulfur that was oxidized. Since a standard of 10 g of GGBFS was used in all the
experiments, the maximum concentration of thiosulfate that could potentially form from 100%
57
conversion of the CaS (equation 10), regardless of the GGBFS environment, would be
approximately 2250 ppm. If this amount of thiosulfate was to be completely converted into
sulfate or CaS oxidizes directly to sulfate without partially forming any thiosulfate, then the
maximum concentration of sulfate formed would be approximately 3850 ppm. Appendix A-11
presents these calculations in detail, which are an important basis, for the experimental
concentrations of the sulfur anions formed in different GGBFS environments, since they are
likely to be lower than these theoretical concentrations, due to partial conversion.
4.2.1 GGBFS in Water Results
The hydration of GGBFS in deionized water yielded relatively low amounts of thiosulfate and
sulfate, compared to the maximum theoretical concentrations calculated. As shown in Figures 23
and 24, the amounts of thiosulfate and sulfate produced from CaS oxidation in water were
monitored as a function of hydration time, for both the CO2 free and non-CO2 free samples. The
pH of both types of the samples was found to be slightly basic, ranging from 9-11.
Figures 23 (left) and 24 (right) –Thiosulfate and sulfate concentrations versus
hydration time for GGBFS in water
The hydration was expected to be very slow in water, since the low pH is insufficient to attack the
glass GGBFS grains and release sulfide. The thiosulfate and sulfate concentrations were higher in
58
the CO2 free sample than in the carbonated sample, suggesting that there was more release of
sulfide from the GGBFS grains, due to the higher pH level in the CO2 free sample. The sulfate
concentrations were generally higher than the thiosulfate concentrations; however it was not much
of a difference to suggest that thiosulfate is not kinetically stable in water. Furthermore, based on
the maximum amounts of thiosulfate and sulfate formed (comparing both samples), the mass
percentage of sulfur oxidized to form thiosulfate was 10.6%, while the mass percentage of sulfide
oxidized to form sulfate was 19.7%. The calculations are detailed in Appendix A-11 and it can be
understood that a fair amount of sulfide has been left unoxidized, possibly remaining in the grains.
4.2.2 GGBFS in Basic Solutions Results
The formation of thiosulfate and sulfate from GGBFS in basic solutions varied, as CaS oxidation is
primarily dependent on the solution pH to attack the glass GGBFS grains to release sulfide. As
shown in Figures 25 and 26, the amounts of thiosulfate and sulfate produced in basic solutions
were monitored as a function of hydration time, for both the CO2 free and non-CO2 free samples.
The pH of the Ca(OH)2 + NaOH samples was monitored to be basic over time, as it ranged from
13-13.5, while the pH of the NaOH samples was slightly less basic in the range of 11-13.
Figures 25 (left) and 26 (right) – Thiosulfate and sulfate concentrations versus
hydration time for GGBFS in basic solutions
59
The results indicate that the thiosulfate and sulfate concentrations are lower in the Ca(OH)2 +
NaOH experiment than the NaOH experiment. Since the simulated GGBFS and Ca(OH)2 + NaOH
pore solution is saturated with Ca2+
, there is more formation of C-S-H gel within the HCP, which
may become a barrier in preventing sulfide from diffusing out into the pore solution for oxidation.
Furthermore, the forward rate of thiosulfate formation may also be affected by an aqueous
equilibrium being reached by the reaction, in equation 10, due to the reservoir of OH- and Ca
2+
ions.
In general, for the GGBFS and Ca(OH)2 + NaOH pore solution that was CO2 free, the thiosulfate
concentration was higher and the sulfate concentration was lower, when compared to the
carbonated sample. The CO2 free sample was monitored to be at a higher pH than the carbonated
sample, which suggests that more sulfide was released and oxidized into thiosulfate, however
further oxidation into sulfate may have been inhibited due to the C-S-H barrier. The carbonated
sample was at slightly lower pH, however most likely resistant to carbonation, due to the reservoir
of OH- ions. Therefore, the thiosulfate concentration was observed to be lower, which also implies
that GGBFS grains may have a certain oxidation depth that is sensitive to pH. Therefore, a lower
pH may only be sufficient to attack the outer glass structure of a GGBFS grain, where the sulfide
may already be present in a higher oxidation state, hence the slightly higher sulfate levels observed
in the carbonated sample.
The high concentrations of thiosulfate and sulfate observed in the NaOH experiment, signify the
absence of a saturated Ca2+
equilibrium, since the sulfur oxidation products are easily able to
diffuse out into the simulated pore solution. The sulfate concentration is approximately 2.5 times
60
greater than the thiosulfate concentration, suggesting that rapid thiosulfate oxidation is occurring
with no maximum thiosulfate concentration observed, as theoretically anticipated. However, due to
the slow nature of the HCP reactions, a decline in thiosulfate concentration and complete oxidation
to sulfate may take longer to occur. Similarly to the GGBFS in water analysis, the maximum mass
percentage of sulfur oxidized to form thiosulfate was 15.9% and 21.3% in the Ca(OH)2 + NaOH
and NaOH experiments, respectively. The maximum mass percentage of sulfur oxidized to form
sulfate was 4.1% and 29.8% in the Ca(OH)2 + NaOH and NaOH experiments, respectively. The
calculations are shown in detail in Appendix A-11. From these results, it can be understood that
thiosulfate is quite kinetically stable in basic solutions and oxidation into non-corrosive sulfate is
pH and time dependent.
4.2.3 Aggregate and GGBFS in Basic Solutions Results
The addition of hematite to GGBFS in basic solutions provides a substrate for surface reactions to
take place between sulfur compounds and oxygen, since the aggregate becomes coated with HCP.
As shown in Figures 27 and 28, the concentrations of thiosulfate and sulfate were observed to be
higher in the carbonated, unlimited oxygen sample than both the CO2 free, limited oxygen sample
and GGBFS in basic solutions samples. This was anticipated, since the unlimited oxygen may
allow hematite to oxidize thiosulfate into higher oxidation state sulfur compounds, as theorized in
equations 13 and 14. Although, similarly to the GGBFS in basic solutions samples, the sulfate
concentrations were lower than the thiosulfate concentrations, due to the aqueous Ca2+
equilibrium
and C-S-H barrier.
2S2O32-
+ Fe2O3 + 6H+ → S4O6
2- + 2Fe
2+ + 3H2O
[13]
3Fe2O3 + S2O32-
+
O2 + H2O → 2Fe3O4 + 2SO4
2- + 2H
+ [14]
61
Figures 27 (left) and 28 (right) - Thiosulfate and sulfate concentrations versus
hydration time for aggregates and GGBFS in basic solutions
There was little sulfate oxidation observed when thiosulfate was dissolved in solution, as the
thiosulfate concentrations remained fairly constant in both sample types, suggesting that hematite
cannot oxidize thiosulfate when there is no surface reaction mechanism. Furthermore, synthetic
magnetite was observed to react with thiosulfate and oxidize it into sulfate in the CO2 free sample,
however this was not the case in the carbonated sample, which signifies that magnetite may
possibly be reacting with sulfide in the high-density GGBFS concrete.
4.3 Microscopy and Analysis
Microscopy and analysis was performed on the samples listed in Table 8, with the majority of the
high-resolution imaging, quantification and mapping focusing on the microstructure of GGBFS
environments upon hydration. The visual analysis is consistent with literature theory and the ion
chromatography results, which suggests that GGBFS grains are hydrated from their outer glass
structure to their internal core. If this is the case, then it is likely that the core of the grains contains
most of the sulfide. However contrary to this theory, it may appear that the sulfur could be evenly
distributed throughout the particle making it difficult to assess if it exists as core inclusions or
particulate phases. The elements of interest at their characteristic peak positions in GGBFS grains
62
are shown in Figure 29, an EDX spectrum. It is evident from the spectrum that calcium, silicon,
magnesium, aluminum and oxygen are the dominating elements, as expected from the chemical
composition of GGBFS, shown in Table 2 in section 2.2.2. EDX quantification tables indicating
the weight and atomic mass percentages of elements in areas of interest will be presented in
subsequent analysis of the individual microscopy samples, rather than EDX spectra.
Figure 29 – Typical EDX spectrum of GGBFS grains
4.3.1 Dry GGBFS
Dry GGBFS was analyzed to determine the elemental composition and average particle size of
GGBFS grains, as a reference, prior to any particle hydration. As shown in Figures 30 and 31, the
size of the GGBFS particles varies anywhere between 5-40 µm, but the average particle size is
quite small, as expected due to the GGBFS’s high Blaine fineness of 717 m2/kg. Furthermore, the
grains tend to appear quite angular and jagged in nature, which is understandable due to their
amorphous structure. The EDX quantification is detailed in Figure 33, on a selected area in Figure
32, where it is apparent that calcium and silicon are present in abundance, evidently in the form of
calcium and silicon oxides. An approximate 1% sulfur weight percentage was observed in the
0 1 2 3 4 5 6 7
keV
0
2
4
6
8
10
12
14
16
18
20
cps/eV
Mg K K Ca
Ca
Ca
Mn Mn Fe Fe
Al Si S O
Na
63
GGBFS grains and was expected due to their unreactive nature when dry and unhydrated. The
remaining elements are within the composition ranges of their chemical constituents, listed in
Table 2.
Figures 30 (left) and 31 (right) – ESEM images of dry GGBFS grains mixed in epoxy
Figures 32 (left) and 33 (right) – ESEM image of single GGBFS grain and
EDX quantification table for dry GGBFS
4.3.2 GGBFS in Water
GGBFS mixtures with aerated and deaerated, deionized water were analyzed, respectively. The
imaging analysis showed that GGBFS hydration in water appears to be relatively slow, which is
consistent with the ion chromatography results for GGBFS in water. The comparison between both
the aerated and deaerated sample, shows that a lack of oxygen in water has an effect on GGBFS
hydration. It is apparent from Figures 34 and 36, that the average GGBFS grain size appears to be
larger in the deaerated sample than in the aerated sample. This suggests that due to the lack of
oxygen, the formation of hydration products such as C-S-H gel, AFm and hydrotalcite phases,
which are essential for the supporting microstructure, may be inhibited. The rate of hydration in
64
both types of samples also appears to be delayed, which is possible due to the aluminosilicate
coating that has been observed to form on the surface of GGBFS grains (29).
Figures 34 (left) and 35 (right) – Figures 36 (left) and 37 (right) -
ESEM images of GGBFS grain(s) in aerated water ESEM images of GGBFS grain(s) in deaerated water
The EDX quantification results were similar, shown in Figures 38 and 39, for the GGBFS grains in
Figures 35 and 37 respectively, indicating a decrease in calcium and silicon content, compared to
the dry GGBFS grains, which implies that hydration is indeed occurring, regardless how slow.
Figures 38 (left) and 39 (right) – EDX quantification tables for GGBFS in aerated and deaerated water
4.3.3 GGBFS in Basic Solutions
The hydration of GGBFS in basic solutions was analyzed to yield similar trends to the ion
chromatography experiments involving basic solutions. In the GGBFS and NaOH sample, it was
visually understood that high pH is effective at attacking the glass GGBFS grains, as there appears
to be more dispersion and hydration of the grains, as shown in Figure 40. In Figure 41, the sulfur
EDX mapping, there were similar dispersion effects suggesting that the sulfide may be reacting in
the basic environment. Furthermore, a lower sulfur weight percentage of 0.59%, was observed in
GGBFS grains as indicated in Figure 44, the EDX quantification table corresponding to the grain
65
area in Figure 40. In the GGBFS and Ca(OH)2 + NaOH sample, the sulfur weight percentage is
higher at approximately 1%, shown in Figure 45, which may be due to the Ca2+
equilibrium and
additional C-S-H gel barrier that is preventing oxidized sulfur compounds from being released into
the surrounding microstructure. There also appears to be calcite formation on the surface of the
grains, which may be precipitation favoured, due to the high dissolved Ca2+
concentration.
Figures 40 (left) and 41 (right) – ESEM image and Figures 42 (left) and 43 (right) – ESEM image and
EDX sulfur mapping of GGBFS in NaOH EDX sulfur mapping of GGBFS in Ca(OH)2 + NaOH
Figures 44 (left) and 45 (right) - EDX quantification tables for GGBFS in NaOH and Ca(OH)2 + NaOH
4.3.4 OPC and GGBFS Paste
The hydration of OPC and GGBFS mixed with aerated water was fairly active, as the GGBFS
grain sizes were relatively smaller compared to the dry GGBFS grain sizes, as observed in Figures
46 and 47. OPC is an excellent activator for GGBFS and the amount of C-S-H gel appears to have
increased as the cementitious materials hydrate together. In Figure 47, the formation of calcium
hydroxide crystals is somewhat evident, which is also important for activating GGBFS hydration.
66
Figures 46 (left) and 47 (right) – Figures 48 (left) and 49 (right) –
ESEM images of OPC and GGBFS paste EDX sulfur mapping and ESEM image of GGBFS grain
The EDX quantification results, shown in Figure 50 for the GGBFS grain in Figure 49, were
similar to the previous samples. It was observed that the topography of the samples, tended to yield
comparable results, despite the possibility of nonuniform absorption of X-rays, due to the porous,
uneven microstructure. The sulfur weight percentage of 0.7% was higher than the GGBFS in
NaOH result, although lower than the dry GGBFS result, suggesting that there may be release of
sulfide from the GGBFS grains. However, despite the high pH of the OPC-GGBFS paste, the
increase in C-S-H gel production can inhibit sulfide transport, as observed in the GGBFS in
Ca(OH)2 + NaOH sample. The majority of the sulfide appears to remain within the GGBFS grains
as indicated by Figure 48, the EDX sulfur mapping of several GGBFS grains within the paste.
Figure 50 - EDX quantification table for GGBFS grain in OPC and GGBFS paste
4.3.5 High-Density Concrete
The microstructure of the high-density concrete was investigated, with the high-resolution imaging
and EDX quantification indicating that the internal concrete structure is quite complex, due to the
presence of coarse and fine aggregate of varying proportions and sizes. In Figure 51, it is apparent
67
that there are partially hydrated or unhydrated GGBFS grains in the high-density GGBFS concrete,
similar to the OPC and GGBFS paste, which suggests that hydration may be slow or delayed. The
complicated aggregate structure is shown in Figure 52, where the bright, large particles represent
magnetite and some of the dark, small particles signify hematite. The internal composition of
magnetite also has some variation, due to the different iron oxidation states present.
Figures 51 (left) and 52 (right) - Figures 53 (left) and 54 (right) – EDX iron mapping
ESEM images of high-density GGBFS concrete and ESEM image of high-density 100% OPC concrete
The EDX quantification results, shown in Figure 55 for the GGBFS grains in Figure 51, were
similar to the previous samples, although there was a higher weight percentage of iron and lower
weight percentage of calcium observed. The coarse aggregates in Figure 53 were found to be
composed of approximately 64% iron and 34% oxygen by weight percentage.
Figures 55 (left) and 56 (right) – EDX quantification tables for GGBFS grains in
high-density GGBFS concrete and coarse aggregates
4.3.6 High-Density Mortar
The hydration of high-density GGBFS mortar was observed to be comparable to the high-density
GGBFS concrete, as most of the GGBFS grains appeared to be partially hydrated or unhydrated, as
68
shown in Figure 57. In the silica GGBFS mortar, the GGBFS grain size varies considerably, as
observed in Figure 59, suggesting that the larger grains may be undergoing very slow hydration.
The significance of this observation is that the finer silica sand has a greater effect on interacting
with the HCP and hydrating most of the GGBFS grains. In Figure 60, the silica GGBFS mortar
appears to be denser around the GGBFS grains, as compared to the hematite microstructure
observed in Figure 58. The hematite aggregate generally appears to be segregated from the
GGBFS grains in a mortar, as compared to concrete, possibly due to variations in the mixing
procedures, which affects the surface area coverage of the aggregates by the HCP.
Figures 57 (left) and 58 (right) - Figure 59 (left) and 60 (right) –
ESEM images of high-density GGBFS mortar ESEM images of silica GGBFS mortar
EDX quantification results, shown in Figure 59 and 60 for the GGBFS grains in Figures 55 and 57,
yielded similar results. The sulfur content was also similar to the high-density GGBFS concrete.
Figures 61 (left) and 62 (right) – EDX quantification tables for GGBFS grains in
high-density GGBFS mortar and silica GGBFS mortar
69
CHAPTER 5 – SUMMARY AND CONCLUSIONS
Electrochemistry, ion chromatography and microscopy have been used to investigate the chemistry
and potential corrosion mechanisms associated with reduced sulfur compounds, such as calcium
sulfide, in high density GGBFS concrete, mortar and simulated pore-water solution environments.
Calcium sulfide is capable of consuming oxygen in high-density GGBFS concrete and mortar
environments, which may cause conversion into reduced sulfur states that can affect the potential
of embedded steels.
High-density concrete is important for shielding photon radiation from used nuclear fuel in dry
storage containers. Through extensive testing and understanding of the physical and chemical
properties of cementitious materials, aggregates and chemical admixtures, replication of the high-
density concrete was performed from mix design calculations. The exact chemistry of the high-
density concrete was also proven to be reproducible in high-density mortars, by sieving out the
coarse aggregate and accounting for the lost cementitious material, fine aggregate and water.
Corrosion potential and electrochemical impedance spectroscopy analyses indicate that a reducing
environment exists within some of the high-density GGBFS concrete samples, where some of the
oxygen is being consumed by the calcium sulfide from GGBFS. However, if hematite is in
sufficient contact with the embedded steel, there can be potential variation and a decrease in the
high-frequency electrolyte resistance, due to magnetite’s short-circuiting ability. Some of the high-
density GGBFS mortars showed a reducing environment as well and the molar ratio of sulfide to
oxygen was determined to be one of the key factors in the creation of this corrosion mechanism.
70
Furthermore, the high-frequency electrolyte resistance was observed to increase with hydration
time, due to the increase in strength, decrease in permeability and loss of moisture within the
hydrated cementitious paste. The addition of GGBFS, type of embedded steel and fine aggregate
used, also had effects on the corrosion potential and high-frequency electrolyte resistance results.
Ion chromatography analysis on simulated pore water solutions has shown than thiosulfate is quite
kinetically stable in neutral and basic GGBFS solutions with or without aggregate. The theoretical
decrease in thiosulfate concentration was not observed over several weeks of monitoring, however
at any given time there was no more than 25% of the initial sulfur oxidizing to thiosulfate. Other
conclusions suggest that high pH is effective at attacking the glass GGBFS grains to release sulfide
for oxidation, saturated Ca2+
pore solutions are able to inhibit the mass transport of sulfide and that
magnetite is capable of oxidizing sulfide into higher oxidation state sulfur compounds.
Microscopy has provided visual evidence of the particle size distribution in dry GGBFS and
GGBFS hydration in a variety of different environments such as water, basic solutions, OPC paste,
high-density concrete and mortar. The majority of the analysis suggests that GGBFS grains
decrease in particle size, as they are hydrated from their outer glass structure to their internal core.
The high pH of the basic solutions was found to hydrate GGBFS the most effectively, while
hydration appeared to be the slowest in deoxygenated water. Additionally, the EDX quantification
results showed that the sulfur composition within GGBFS grains decreased after hydration.
71
CHAPTER 6 – FUTURE WORK
Future work in this field of study should take into account the following considerations to improve
the experimental details, as well as further investigating relevant areas of interest:
1. It may be beneficial to extract pore solutions from GGBFS mortar samples during the
initial hydration stages and perform ion chromatography analysis. The concentration of
reduced sulfur species in the actual pore solutions could then be compared to their
concentrations in the simulated pore solutions. Computer simulation of the ion
chromatography experiments using aqueous equilibrium modeling software would also
provide relevant details about the expected concentration of reduced sulfur species.
2. Time dependent or wet microscopy experiments with a staining or marking chemical
should be performed to better determine the growth of the hydration products. This type of
experimentation would be especially valuable during the first few days of hydration.
3. An experimental rate law that empirically fits the oxygen concentration over time in the
high-density GGBFS concrete and mortar samples would be beneficial to develop to better
understand the kinetics of oxygen consumption by calcium sulfide.
72
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78
APPENDICES
A-1 Coarse and Fine Aggregate Property Calculations
The specific gravity and absorption properties discussed in section 3.1.2 were experimentally
calculated from the measurements and formulae listed in Table A-1. The surface moisture content
calculation is shown in Table A-2. The formulae were obtained from ASTM C127 and C128, as
indicated in section 2.2.3.1.
Measurements
and Properties
Symbols and
Formulae
Fine
Aggregate
Measurements
and Properties
Symbols and
Formulae
Coarse
Aggregate
1 2 1 2
Saturated
Surface-dry
Mass in Air (g)
504.3 500.3 Saturated
Surface-dry
Mass in Air (g)
4458 5243.2
Oven-dry Mass
in Air (g) 503.6 499.6 Oven-dry Mass
in Air (g) 4445.6 5238.6
Mass of Flask
with Specimen
and Water to
Fill Mark (After
1 hr) (g)
1071.8 1062 Saturated Mass
in Water (g)
3429 4024.6
Mass of Flask
with Water to
Fill Mark (g)
673.3 666.9
Bulk Specific
Gravity (g/cm3)
4.760 4.749 Bulk Specific
Gravity (g/cm3)
4.320 4.299
AVERAGE =
4.754
AVERAGE =
4.310
Bulk Specific
Gravity SSD
(g/cm3)
4.767 4.756 Bulk Specific
Gravity SSD
(g/cm3)
4.332 4.303
AVERAGE =
4.762
AVERAGE =
4.318
Absorption %
0.139 0.140 Absorption %
0.279 0.088
AVERAGE =
0.140
AVERAGE =
0.183 Table A1 – Specific gravity and absorption calculations
79
Measurements
and Properties
Symbols and
Formulae
Fine
Aggregate
Measurements
and Properties
Symbols and
Formulae
Coarse
Aggregate
Wet Mass (g) 500.1 Wet Mass (g) 500.4
Oven-dry
Mass in Air (g)
497.3 Oven-dry
Mass in Air (g)
499.8
Moisture
Content (%)
0.563 Moisture
Content (%)
0.120
Table A2 – Surface moisture content calculations
A-2 X-ray Diffraction (XRD) Spectrum of Fine Hematite Sand
Figure A1 – XRD spectrum of fine hematite sand
quote
from the
document
or the
summary
of an
80
A-3 Mix Design Calculations
The mix design calculations discussed in section 3.1.4 were experimentally determined for the six
different types of samples (two high-density concrete and four mortar, as listed in Table 8 in
section 3.1.7) and the results are presented here in Tables A3 to A8. The following points should
be noted about the calculations:
The yield and batch calculations for the mortars (mix designs 1-4) were combined into one
calculation, since the mortar mass percentages, obtained from the concrete to mortar
calculations, discussed in section 3.1.6 and shown in Appendix A-6, were used to calculate
the actual batch masses (defined below).
Yield (m3) of each component is calculated by the following formula:
SSD mass (kg) of each component is calculated by the following formula:
The actual batch mass refers to the mass of the components that were used in the mix,
since the aggregates were not at SSD condition. Depending on whether the surface
moisture content is greater or less than the absorption, for the aggregates, the overall
amount of water in the mix is affected and mix water needs to be removed or added. Based
on the coarse and fine aggregate property calculations discussed and calculated in sections
3.1.2 and A-1, only the fine aggregate’s surface moisture content is significantly greater
than its absorption and needs to be accounted for. The resulting formulae calculate how
81
much additional fine aggregate need to be added and how much mix water needs to be
removed.
In mix designs 3 and 4, for the mortars containing silica sand, the total yield was kept at the
same value as the total yield for the mortars containing hematite sand. This was done in
order to reverse calculate the actual batch mass of silica sand required, from its specific
gravity and the same yield (volume) of hematite sand.
In mix design 5, the total yield of 0.945 m3 accounts for 5.5% air content, hence a total of
1 m3. In mix design 6, the total yield is similarly calculated; however there is a slight
discrepancy due to the OPC and GGBFS specific gravity variation.
In mix designs 5 and 6, the AEA and superplasticizer amounts indicated in the yield
calculation are calculated from the recommended dosage amounts specified in section
3.1.3. AEA and superplasticizer are not accounted for in the total yield, because of their
insignificant amount. The actual amounts that were used are indicated in the batch
calculation and have been experimentally determined on a trial and error basis to give the
desired air content. For mix designs 1 to 4, the recommended dosage amounts were used.
Mix design 1 was specifically of interest and the dosages were also experimentally varied
to reduce the air content in successive samples after the initial samples, 1a-1e, were made.
82
Table A3 – Mix design for 50% OPC-50% GGBFS mortar with fine hematite sand
Mix Design 1
Mass
Percentages
(%)
Actual
Batch
Mass (kg)
(On a per
1 m3
basis)
Specific
Gravity
(kg/m3)
Yield
(m3)
Required Bach
Mass (kg)
4.5
W/CM ratio 0.42
Mortar
Density
(Total mix kg/
per m3 of
mortar)
3471.76
OPC 8.90 0.4 3150 0.00013 Air Content
Measured (%)
1a-1e 23.7
1f and
1g
6.1
GGBFS 8.90 0.4 2860 0.00014
Fine
Aggregate
(Hematite
Sand)
75.14 3.381 4754 0.00071
Mix Water 7.06 0.318 1000 0.00032
Total ~100 4.5 - 0.00130
AEA 1a-1e 1.60 mL
1f and 1g 0.3 mL
Super
Plasticizer
1a-1e 2.45 mL
1f and 1g 0.5 mL
83
Table A4 – Mix design for 100% OPC mortar with fine hematite sand
Mix Design 2
Mass
Percentages
(%)
Actual
Batch
Mass (kg)
(On a per
1 m3
basis)
Specific
Gravity
(kg/m3)
Yield
(m3)
Required Bach
Mass (kg)
4.5
W/CM ratio 0.42
Mortar
Density
(Total mix kg/
per m3 of
mortar)
3487.87
OPC 18.05 0.812 3150 0.00026 Air Content
Measured (%)
2a-2e 18.9
Fine
Aggregate
(Hematite
Sand)
74.73 3.363 4754 0.00071
Mix Water 7.22 0.325 1000 0.00032
Total ~100 4.5 - 0.00129
AEA 1.62 mL
Super
Plasticizer
2.48 mL
84
Table A5 – Mix design for 50% OPC-50% GGBFS mortar with fine silica sand
Mix Design 3
Mass
Percentages
(%)
Actual
Batch
Mass (kg)
(On a per
1 m3
basis)
Specific
Gravity
(kg/m3)
Yield
(m3)
Required Bach
Mass (kg)
3.014
W/CM ratio 0.42
Mortar
Density
(Total mix kg/
per m3 of
mortar)
2332.60
OPC 8.90 0.4 3150 0.00013 Air Content
Measured (%)
3a-3c 17.9
GGBFS 8.90 0.4 2860 0.00014
Fine
Aggregate
(Silica
Sand)
75.14 1.896 2680 0.00071
Mix Water 7.06 0.318 1000 0.00032
Total ~100 3.014 - 0.00130
AEA 1.60 mL
Super
Plasticizer
2.45 mL
85
Table A6 – Mix design for 100% OPC mortar with fine silica sand
Mix Design 4
Mass
Percentages
(%)
Actual
Batch
Mass (kg)
(On a per
1 m3
basis)
Specific
Gravity
(kg/m3)
Yield
(m3)
Required Bach
Mass (kg)
3.024
W/CM ratio 0.42
Mortar
Density
(Total mix kg/
per m3 of
mortar)
2349.92
OPC 18.05 0.812 3150 0.00026 Air Content
Measured (%)
4a-4c 21.9
Fine
Aggregate
(Silica
Sand)
74.73 1.887 2680 0.00070
Mix Water 7.22 0.325 1000 0.00032
Total ~100 3.024 - 0.00129
AEA 1.62 mL
Super
Plasticizer
2.48 mL
86
Table A7 – Mix design for 50% OPC-50% GGBFS concrete with iron oxide aggregates
Mix Design 5
Component
Yield Calculation Batch Calculation Casting Specifications
and Results
Component
Mass (kg)
(On a per 1
m3 basis)
Specific
Gravity
(kg/m3)
Yield
(m3)
SSD
Mass
(kg)
Actual
Batch
Mass
(kg)
Mass
Percentages
(%)
Required Bach
Mass (kg)
80
Required Bach
Volume (m3)
0.023
W/CM ratio 0.42
OPC 180 3150 0.057 4.073 4.073 5.09 Concrete
Density
(Total mix kg/
per m3 of
concrete)
3535.2
GGBFS 180 2860 0.063 4.073 4.073 5.09 Slump
Range (mm)
110-135
Coarse
Aggregate
(Magnetite
Stone)
1735.776 4310 0.403 39.280 39.280 49.09 Slump
Measured
(mm)
120
Fine
Aggregate
(Hematite
Sand)
1288.224 4754 0.271 29.152 29.275 36.59 Air Content
Range (%)
5.5 +/-
1.5
Mix Water 151.2 1000 0.151 3.422 3.298 4.12 Air Content
Measured (%)
7
Total 3535.2 - 0.945 80.041 80.015 ~100 Compressive
Strength (MPa)
7
days
28
28
days
42
AEA 0.72 L
7.5 mL Temperature
of Concrete
(°C)
21.3
Super
Plasticizer
1.01 L
7.5 mL Temperature
of Air (°C)
17.3
87
Table A8 – Mix design for 100% OPC concrete with iron oxide aggregates
Mix Design 6
Component
Yield Calculation Batch Calculation Casting Specifications
and Results
Component
Mass (kg)
(On a per 1
m3 basis)
Specific
Gravity
(kg/m3)
Yield
(m3)
SSD
Mass
(kg)
Actual
Batch
Mass
(kg)
Mass
Percentages
(%)
Required
Bach Mass
(kg)
80
Required
Bach Volume
(m3)
0.023
W/CM ratio 0.42
OPC 360 3150 0.114 8.147 4.073 10.18 Concrete
Density
(Total mix kg/
per m3 of
concrete)
3535.2
Coarse
Aggregate
(Magnetite
Stone)
1735.776 4310 0.403 39.280 39.280 49.09 Slump
Measured
(mm)
150
Fine
Aggregate
(Hematite
Sand)
1288.224 4754 0.271 29.152 29.275 36.59 Air Content
Measured
(%)
7.2
Mix Water 151.2 1000 0.151 3.422 3.298 4.12 Compressive
Strength
(MPa)
7
days
30.7
28
days
37.4
Total 3535.2 - 0.939 80.041 80.015 ~100 Temperature
of Concrete
(°C)
21.5
Temperature
of Air (°C)
21.3
AEA 0.72 L
7.5 mL
Super
Plasticizer
1.01 L 7.5 mL
88
A-4 High-Density Concrete and Mortar Mixing Procedures
The mixing procedures briefly discussed in section 3.1.5 for producing the high-density concrete
and mortar samples are sequentially detailed here.
Procedure for Mixing Concrete
1. Ensure mixer is completely clean and dry.
2. Add coarse and fine aggregate into the mixer together.
3. Turn mixer on.
4. Add cementitious material into the mixer. Pour approximately 75% of the mix water at the
same time into the mixer, while holding back approximately 25% of the mix water. This is
performed depending on the mix appearance. If it appears too dry, add the remaining mix
water. If it appears too wet, hold back the mix water. Add the AEA by syringe injection.
5. Start the timer and mix for 3 minutes. Add the superplasticizer by syringe injection, holding
back if necessary depending on the appearance of the mix fluidity, wetness and workability.
6. Off the mixer and rest for 2 minutes. Cover the mixer with a plastic sheet to ensure water
does not evaporate during the rest period.
7. Remove plastic sheet, turn the mixer back on and mix for 3 minutes.
8. Off the mixer. Measure the temperature of the concrete. Perform slump and air test.
Procedure for Mixing Mortar
1. Pour mix water into the mixer. Add AEA by syringe injection.
2. Set mixer on low speed (speed 1) and turn mixer on, start the timer and add cementitious
material into the mixer. Mix for 30 seconds.
3. At 30 seconds, add fine aggregate while mixing.
89
4. At 60 seconds (1 minute), add superplasticizer by syringe injection. Turn off mixer and set
mixer at medium speed (speed 2). Turn mixer on. Mix to 90 seconds (1.5 minutes).
5. At 90 seconds (1.5 minutes), turn off mixer and rest for 90 additional seconds (1.5 minutes).
Scrape down sides of mixer bowl with spatula.
6. At 180 seconds (3 minutes), turn mixer on. Mixer should be at medium speed (speed 2). Mix
for additional 60 seconds (1 min). At 240 seconds (4 minutes) total, turn mixer off. Perform air
test.
90
A-5 Mortar Air Content Calculation
The air content measurement of a mortar, as discussed in section 3.1.5, can only be performed
when the mortar remains in a plastic state, thus upon the completion of mixing and before
stiffening occurs. ASTM C185 outlines the test procedure and mathematical formula used to
calculate the air content of a mortar. However, modification of the formula is necessary to ensure
that the calculated air content is consistent with the actual batch masses of the mortar mix design.
The following derivation uses the data in Mix Design 1, Table A3, to determine an explicit formula
for the air content. Similar derivations were performed to calculate the air content for the other
types of mortars (Mix Designs 2-4, Tables A4-A6).
Therefore based on the experimentally measured mass of the mortar in a specified mould volume,
the air content can be calculated.
91
A-6 Concrete to Mortar Calculations
The mortar mix designs in Appendix A-3 were created after determining the mass percentages of
the components required to replicate the high-density mortar chemistry from the concrete, in
mortars themselves. Section 3.1.6 outlines the procedure used to obtain the relevant data needed
to create a mortar that replicates the mortar chemistry of a concrete. The theoretical calculation
performed to obtain the mortar mass percentages from high-density OPC-GGBFS concrete, mix
design 5, is shown here. A similar calculation was performed for the high-density OPC concrete,
mix design 6.
Sieving Data
Sieved Coarse Aggregate Mixture Mass
from Concrete Batch (Before Wash) (g)
(contains Coarse Aggregate, Fine Aggregate,
Cementitious Material and Water)
6409
Sieved Coarse Aggregate Mass from Sieved
Coarse Aggregate Mixture (After Wash) (g)
(Any Coarse Aggregate retained
on the 4.75 inch sieve)
5046.20
Fine Aggregate, Water and Cementitious
Material Mass coated on the Sieved Coarse
Aggregate (g) (Remaining mixture after Coarse
Aggregate was sieved out)
= 6409-5046.20 1362.80
Sieved Fine Aggregate Mass from Sieved
Coarse Aggregate Mixture (After Wash) (g)
(Any Fine Aggregate passing 4.75 inch sieve
and retained on a 200 mm sieve)
853.90
Water and Cementitious Material Mass coated
on Sieved Coarse Aggregate (g) (remaining
mixture after Fine Aggregate sieved out)
= 1362.80-853.90 508.90
Water and Cementitious Material Calculation
W/CM ratio 0.42
Cementitious Material Mass (x)
coated on Sieved Coarse Aggregate (g)
358.38
Water (1-x) Mass coated on Sieved Coarse
Aggregate (g)
150.52
92
Concrete Mass Percentages (%) (from Mix Design 5)
OPC 5.09
GGBFS 5.09
Coarse Aggregate 49.09
Fine Aggregate 36.59
Mix Water 4.12
Total ~100
Theoretical Concrete Batch Calculation from the Sieved Coarse Aggregate
in the Sieved Coarse Mixture (Using Concrete Mass Percentages)
Sieved Coarse Aggregate Mass from Sieved
Coarse Aggregate Mixture (After Wash) (g)
5046.20
Total Batch Mass (g) = 5046.20/49.09% 10279.36
Fine Aggregate Mass (g) = 10279.36*36.59% 3760.93
OPC Mass (g) = 10279.36*5.09% 523.39
GGBFS Mass (g) = 10279.36*5.09% 523.39
Water Mass (g) = 10279.36*4.12% 423.72
Theoretical Mortar Batch Calculation
(Accounting for lost Fine Aggregate, Cementitious Material and Water that was coated on the
Sieved Coarse Aggregate from the Sieved Coarse Aggregate Mixture)
Fine Aggregate Mass (g) = 3760.93 – 853.90 2907.03
OPC Mass (g) = 523.39 – 0.5*358.38 344.2
GGBFS Mass (g) = 523.39 – 0.5*358.38 344.2
Water Mass (g) = 423.72 – 150.52 273.2
Total Mass (g) = 3868.63
Mortar Mass Percentages (%)
Fine Aggregate = (2907.03/3868.63)*100 75.14%
OPC = (344.2/3868.63)*100 8.90%
GGBFS = (344.2/3868.63)*100 8.90%
Water = (273.2/3868.63)*100 7.06%
Total - ~100% Table A9 – Concrete to mortar data and calculations
93
A-7 OCP Measurements for Mortar Sample Types 2, 3, 4 and Concrete Sample Type 6
The OCP measurements for the remaining high-density concrete and mortar samples, not presented
in section 4.1.2, are attached in this appendix.
Figures A2 (left) and A3 (right) – Corrosion potentials of embedded carbon and
stainless steels in mortar samples types 2 and 3
Figures A4 (left) and A5 (right) – Corrosion potentials of embedded carbon and stainless steels
in mortar sample type 4 and high-density concrete sample type 6
94
A-8 Sulfide to Oxygen Molar Ratio Calculation
The molar ratio of sulfide, existing as CaS, to oxygen in the high-density GGBFS concrete and
mortar samples, as discussed in section 4.1.2, is determined by the following calculation.
Stoichiometrically, under ideal reaction conditions where the reactants have direct access to each
other to chemically combine and react, one mole of oxygen is needed to oxidize one mole of
calcium sulfide, as shown in equation 10, in section 4.2. However, the sulfide is present in excess
to the oxygen in the high-density concrete and mortar samples and that excess amount is important
in determining whether the sulfide is consuming oxygen, under non-ideal reaction conditions in the
samples. The ratio calculation for the high-density GGBFS mortar samples, 1a-1e, is shown below,
with the ratio for the other samples being similarly calculated:
95
A-9 Pourbaix Diagrams for Metals
Pourbaix diagrams used in the analysis of equilibrium phases for the embedded steels and noble
metals are presented here, with the dashed line signifying the approximate alkaline pH of the high-
density concrete and mortar samples. The potential-pH equilibrium diagrams are for iron,
chromium, silver and platinum as metal-water systems, at 25°C.
Figures A6 (left) and A7 (right) – Pourbaix diagrams for iron-water and
chromium-water systems at 298 K (81, 82)
Figures A8 (left) and A9 (right) – Pourbaix diagrams for silver and platinum (83, 84)
96
A-10 EIS Measurements for Mortar Sample Types 2, 3, 4 and Concrete Sample Type 6
The EIS measurements for the remaining high-density concrete and mortar samples, not presented
in section 4.1.3, are attached in this appendix.
,
Figures A10 (left) and A11 (right) – High frequency electrolyte resistance of embedded carbon and
stainless steels in mortar samples types 2 and 3
Figures A12 (left) and A13 (right) – High frequency electrolyte resistance of embedded carbon and stainless
steels in mortar sample type 4 and high-density concrete sample type 6
97
A-11 Sulfur Mass Balance Calculations for Ion Chromatography Analysis
In section 4.2, the maximum amounts of thiosulfate and sulfate that can be formed from CaS
oxidation and thiosulfate oxidation are discussed, respectively. The stoichiometric calculations
based on the reactions described in equations A1 to A3, are detailed as follows:
2CaS + 2O2 + H2O → S2O32-
+ 2Ca2+
+ 2OH-
[A1]
CaS + 2O2 → SO42-
+ Ca2+
[A2]
S2O32-
+ H2O + 2O2 → 2SO42-
+ 2H+
[A3]
The mass of sulfur in thiosulfate and sulfate are calculated from the following relationships:
The mass of sulfur from CaS oxidized to form thiosulfate and sulfate are calculated from the
following relationship (with the exception of the experiments involving no GGBFS):
Sulfur Mass Balance Calculations
Theoretical Maximum Amount of Sulfur in GGBFS
CaS in GGBFS (Weight %) 1.14
Mass of GGBFS used (g) 10
Mass of CaS in GGBFS used (mg) 114
Mass of S (sulfide) in GGBFS used (mg) 114
Volume of Water used (L) 0.1
Max Concentration of S (mg/L, ppm) 1140
Molecular Weight of S (g/mol) 32.07
Moles of S in GGBFS 0.004
Theoretical Maximum Thiosulfate Concentration Produced (from [A1])
Moles of Thiosulfate Stoichiometrically Produced 0.002
Molecular Weight of Thiosulfate (g/mol) 112.13
Mass of Thiosulfate Produced (mg) 224.26
Max Concentration of Thiosulfate Produced (mg/L, ppm)
(if 100% conversion of S to S2O32-
) 2242.6
98
Theoretical Maximum Sulfate Concentration Produced (from [A2] or [A3])
Moles of Sulfate Stoichiometrically Produced 0.004
Molecular Weight of Sulfate (g/mol) 96.06
Mass of Sulfate Produced (mg) 384.24
Max Concentration of Sulfate Produced (mg/L, ppm)
(if 100% conversion of CaS or S2O32-
to SO42-
) 3842.4
GGBFS in Water Calculations
Max Concentration of Thiosulfate (After 28 days) (mg/L, ppm) 211.22
Mass of Thiosulfate (After 28 days) (mg) 21.12
Mass of Sulfur in Thiosulfate (After 28 days) (mg) 12.08
Mass % of Sulfur (from CaS) Oxidized to Form Thiosulfate 10.60
Max Concentration of Sulfate (After 28 days) (mg/L, ppm) 671.11
Mass of Sulfate (After 28 days) (mg) 67.11
Mass of Sulfur in Sulfate (After 28 days) (mg) 22.40
Mass % of Sulfur (from CaS) Oxidized to Form Sulfate 19.65
Total Mass % of Sulfur Oxidized 30.25
GGBFS in Basic Solutions Calculations
GGBFS in Ca(OH)2 + NaOH Calculation
Max Concentration of Thiosulfate (After 35 days) (mg/L, ppm) 316.44
Mass of Thiosulfate (After 35 days) (mg) 31.64
Mass of Sulfur in Thiosulfate (After 35 days) (mg) 18.10
Mass % of Sulfur (from CaS) Oxidized to Form Thiosulfate 15.87
Max Concentration of Sulfate (After 35 days) (mg/L, ppm) 138.77
Mass of Sulfate (After 35 days) (mg) 13.88
Mass of Sulfur in Sulfate (After 35 days) (mg) 4.63
Mass % of Sulfur (from CaS) Oxidized to Form Sulfate 4.06
Total Mass % of Sulfur Oxidized 19.93
GGBFS in NaOH Calculation
Max Concentration of Thiosulfate (After 28 days) (mg/L, ppm) 424.71
Mass of Thiosulfate (After 28 days) (mg) 42.47
Mass of Sulfur in Thiosulfate (After 28 days) (mg) 24.29
Mass % of Sulfur (from CaS) Oxidized to Form Thiosulfate 21.31
Max Concentration of Sulfate (After 28 days) (mg/L, ppm) 1016.62
Mass of Sulfate (After 28 days) (mg) 101.66
Mass of Sulfur in Sulfate (After 28 days) (mg) 33.93
Mass % of Sulfur (from CaS) Oxidized to Form Sulfate 29.76
99
Total Mass % of Sulfur Oxidized 51.07
Aggregate and GGBFS in Basic Solutions Calculations
300 ppm Thiosulfate and Hematite in Ca(OH)2 + NaOH Calculation
Maximum Concentration of Thiosulfate (mg/L, ppm) 300
Mass of Thiosulfate (mg) 30
Mass of Sulfur in Thiosulfate (mg) 17.16
Max Concentration of Sulfate (After 35 days) (mg/L, ppm) 15.58
Mass of Sulfate (After 35 days) (mg) 1.56
Mass of Sulfur in Sulfate (After 35 days) (mg) 1.34
Mass % of Sulfur Oxidized to Form Sulfate 7.81
GGBFS and Hematite in Ca(OH)2 + NaOH Calculation
Max Concentration of Thiosulfate (After 35 days) (mg/L, ppm) 352.6
Mass of Thiosulfate (After 35 days) (mg) 35.26
Mass of Sulfur in Thiosulfate (After 35 days) (mg) 20.17
Mass % of Sulfur (from CaS) Oxidized to Form Thiosulfate 17.69
Max Concentration of Sulfate (After 35 days) (mg/L, ppm) 154.93
Mass of Sulfate (After 35 days) (mg) 15.49
Mass of Sulfur in Sulfate (After 35 days) (mg) 13.27
Mass % of Sulfur (from CaS) Oxidized to Form Sulfate 11.64
Total Mass % of Sulfur Oxidized 29.33
300 ppm Thiosulfate and Magnetite in Ca(OH)2 + NaOH Calculation
Maximum Concentration of Thiosulfate (mg/L, ppm) 300
Mass of Thiosulfate (mg) 30
Mass of Sulfur in Thiosulfate (mg) 17.16
Max Concentration of Sulfate (After 42 days) (mg/L, ppm) 142.11
Mass of Sulfate (After 42 days) (mg) 14.21
Mass of Sulfur in Sulfate (After 42 days) (mg) 4.74
Mass % of Sulfur Oxidized to Form Sulfate 27.62 Table A10 – Sulfur mass balance calculations for ion chromatography analysis
