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Dr. Kimberly KurtisSchool of Civil Engineering
Georgia Institute of TechnologyAtlanta, Georgia
Structure of theHydrated Cement Paste
Structure of the Hydrated Cement Paste
What do we mean by structure?
Type, amount, size, shape, and
distribution of phases present
macrostructure can be seen
unaided (200 m or larger)
microstructure must been
observed with the aid of a
microscope
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Structure of Concrete
Macroscopically, concrete may be
considered to be composed of 2
phases coarse aggregate and
mortar (paste + fine aggregate) or
aggregate and paste.
heterogeneous distribution
At the microscale, we see that
these 2 phases are not
homogenous themselves!
Aggregate 60-75% of the solid volume of most concretes
The aggregate is principally responsible for the
unit weight, elastic modulus, and dimensional
stability of the concrete because these
properties depend on the physical
characteristics (strength, and bulk density) of the
aggregate.
In addition, porosity, shape and texture of the
aggregate are important for workability,
durability, and strength.
The chemical and mineralogical composition ofthe aggregate is usually less important, with the
exception of some deleterious and some
advantageous reactions.
Aggregate phase is generally stronger, than
the other 2 phases, with some exceptions.
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Hydrated Cement Paste
SolidsC-S-H
CH
Ettringite
Monosulfate hydrate
Residual unhydrated cement
Voids Entrapped air (>1mm)
Entrained air (75-500um)
Capillary pores (macromeso)
Interlayer space(micropores)
Water
Capillary water
Adsorbed water Interlayer water
Chemically combinedwater
Important 3rd Phase in Concrete!In addition to the coarse
aggregate, fine aggregate and
paste (together the mortar
fraction), an important 3rd
phase generally exists the
transition zone (TZ) or
interfacial transition zone
(ITZ)
the interfacial region
between the coarseaggregate and the hcp
10-50 um thick
the weakest link
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Structure of Concrete
Each of the phases may be heterogeneous in its
composition (both solids and voids)
Relative proportions and characteristics of the phases
vary with mixture composition, time, environment, etc.
All of these factors make predictions of
concrete behavior more challenging than
predictions for other materials.
Microstructure
Solids
C-S-H
CH
Ettringite
Monosulfate hydrate
Residual unhydrated cement
Voids
Entrapped air (>1mm)
Entrained air (75-500um
Capillary pores (macro
meso)
Interlayer space(micropores)
Water
Capillary water Adsorbed water
Interlayer water
Chemically combinedwater
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Cement Hydration Reactions
2C3S + 11H C3S2H8 + 3CH
2C2S + 9H C3S2H8 + CH
C3A + 26H + 3CSH2 C6AS3H32
2C3A + 4H + C6AS3H32 3C4ASH12
3C3A + 12H + CH C4AH13
C4AF + 10H + 2CH C6AFH12
Pozzolanic Reaction
Reaction of silica in pozzolan with calcium hydroxide:
xCH + yS + zH CxSyHX+ZHydration
WaterCalcium-silicate
hydrateCalcium
hydroxide
Silica in
pozzolan
Add it ional cemen ti tious C-S-H
In alumino-siliceous pozzolans (e.g. fly ash, slag and metakaolin)
the alumina also participates in reactions with calcium hydroxide
producing various calcium-aluminate hydrates (C-A-H) and calcium-
alumino-silicate hydrates (C-A-S-H).
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Solids: Review
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Microstructure
In solids, microstructural inhomogeneities can lead to serious effects on
strength and other related mechanical properties because these properties
are controlled by the microstructural extremes, not by the average
microstructure.
Thus, the presence of voids, cracks, and other defects play an important
role in determining the performance of the composite material.
Why do these defects exist in concrete?
Why do these defects exist in concrete?
Some voids result from the intrinsic nature of the
cement hydration process
Other voids are introduced intentionally or
unintentionally during mixing and/or placing
Microcracks and cracks can develop due to
mismatch between the components (i.e., different
CTE, E)
Microcracks and cracks can develop due to
loading and environment
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Microstructure
SolidsC-S-H
CH
Ettringite
Monosulfate hydrate
Residual unhydrated cement
Voids
Entrapped air (>1mm)
Entrained air (75-500um
Capillary pores (macromeso)
Interlayer space(micropores)
Water
Capillary water
Adsorbed water Interlayer water
Chemically combinedwater
Microstructure
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Anhydrous cement
Water
Development of MicrostructureDevelopment of Microstructure
C-S-H
CH
Ettringite
Development of MicrostructureDevelopment of Microstructure
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C-S-H
CH
Ettringite
Development of MicrostructureDevelopment of Microstructure
C-S-H
CH
Ettringite
Development of MicrostructureDevelopment of Microstructure
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C-S-H
CH
Ettringite
Development of MicrostructureDevelopment of Microstructure
C-S-H
CH
Monosulfate
Development of MicrostructureDevelopment of Microstructure
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0 5 30 1 2 6 1 2 7 28 90
Minutes Hours Days
Amount
0 5 30 1 2 6 1 2 7 28 90
Minutes Hours Days
Amount
Porosity
CH
Ettringite
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0 5 30 1 2 6 1 2 7 28 90
Minutes Hours Days
Amount
Porosity
CHEttringiteC-S-H
0 5 30 1 2 6 1 2 7 28 90
Minutes Hours Days
Amount Porosity
CH
Ettringite
C-S-H
C-(A,F)-H
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0 5 30 1 2 6 1 2 7 28 90
Minutes Hours Days
Amount
Porosity
CH
Ettringite
C-S-H
C-(A,F)-H
Monosulfate
0 25 50 75 100
Degree of Hydration
0
25
50
75
100
RelativeVolume(%)
Capillary
porosity
C-S-H
Calcium
hydroxide
AFt/AFm
calcium
sulfate
C4AF
C3A
C2S
C3S
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0 25 50 75 100
Degree of Hydration
0
25
50
75
100
RelativeVolume(%)
Capillary
porosityC-S-H
Calcium
hydroxide
AFt/AFm
calcium
sulfate
C4AF
C3A
C2S
C3S
Water-filled porosity
C3S
C4AF
C3A
C2S
CSH2
Hydration ProductsHydration Products
0 25 50 75 100
Degree of Hydration
0
25
50
75
100
RelativeVolume(%)
Capillary
porosity
C-S-H
Calcium
hydroxide
AFt/AFm
calcium
sulfate
C4AF
C3A
C2S
C3S
C-S-H
CH
AFt/AFm
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0 25 50 75 100
Degree of Hydration
0
25
50
75
100
RelativeVolume(%)
Capillary
porosityC-S-H
Calcium
hydroxide
AFt/AFm
calcium
sulfate
C4AF
C3A
C2S
C3S
0 25 50 75 100
Degree of Hydration
0
25
50
75
100
RelativeVolume(%)
Capillary
porosity
C-S-H
Calcium
hydroxide
AFt/AFm
calcium
sulfate
C4AF
C3A
C2S
C3S
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0 25 50 75 100
Degree of Hydration
0
25
50
75
100
RelativeVolume(%)
Capillary
porosityC-S-H
Calcium
hydroxide
AFt/AFm
calcium
sulfate
C4AF
C3A
C2S
C3S
Development of Microstructure
50
40
30
20
10
0
CapillaryPorosity(%)
0.30 0.40 0.50 0.60 0.70 0.80 0.90
W/CM
100% Hydration
Young et al. 1998
Voids: Capillary Porosity Capillary porosity
results from the excess
water used for economy
and workability in the
vast majority of concrete
mixtures.
Al+3
Al+3
Al+3
Ca+2
Ca+2
Ca+2
OH-
OH-
OH-
SiO-
SiO-
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100
50
0
0 20 20 30 40 50 60
Curing time (days)
20
30
40
50
60
CapillaryPorosity(%)
DegreeofHydration(%)
Capillary p orosity
Degree of hydration
Young et al. 1998
Development of Microstructure
Volume ofcapillaryporosity inconcrete is alsorelated todegree ofhydration,which isaffected bycuring (time,Temp, RH)
VoidsThe presence of voidsaffects
Strength
Stress distribution(concentrations)
permeability
freeze/thawresistance
Inverse relationshipbetween strength (fc)
and porosity (p)fc=k(1-p)
3
k=strength of voidlessmortar ~ 34,000 psi
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Transport/Permeability
Capillary
porosity, which
is a function of
w/c, is an
important factor
in determining
the permeability
of the paste.
Permeability = porosity
Solids and Porosity
Powers developed a simple model to estimate the
amount of capillary porosity in a cement paste with
varying degrees of hydration and at different water-
to-cement ratios.
Based on the assumption that 1cm3 of cement
produces 2cm3 hydration product on full hydration.
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Case A: Increasing Degree of Hydration
Consider a paste with w/c of 0.63.
What is the capillary porosity at:
7 days assuming the cement is 50% hydrated?
28 days, 75% hydrated?
365 days, 100% hydrated?
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Case B: Increasing w/c
Assuming 100% hydration, what is the capillary
porosity for w/c=0.70, 0.60, 0.50, and 0.40?
Capillary PorositySome other, simple models describing capillary porosity depend critically
on the volume fractions of water-filled W(t) and total capillary porosity
T(t) and unhydrated cement (t), as a function of time, t. Based on
Power's model for cement hydration, for an ordinary portland cement
paste, these quantities are given by:
T.C. Powers, T.L. Brownyard, Studies of the physical properties of hardened portland cement paste. Bulletin 22,
Research Laboratories of the Portland Cement Association, Chicago, 1948.
K.A. Snyder, D.P. Bentz, Cem Concr Res, 34 (11) (2004) 2045-2056.
where (w/c) is the water-to-cement mass ratio
is the degree of hydration (reacted fraction)
of the cement at time t,
cem is the specific gravity of cement,
exp is the volumetric expansion coefficient for
the "solid" cement hydration products relative
to the cement reacted (often taken to be=1.15),
CS is the chemical shrinkage per gram of
cement (= 0.07 mL/g for sealed conditions and
=0 for saturated conditions)
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Microstructure
1-5 nm
Ult. strength
Adsorption
50 nm, more significant for strength, permeability
Entrained air - spherical voids 70-500um in size; added for
freeze/thaw resistance
Entrapped air - irregular in shape; can be large
Pores > 2.5nm may be filled with air, water (pore solution), or a
mixture
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Classification of Voids in the hcp
Water in the hcp
Ratio of mass of water to mass of cement in a mixture is the
water-to-cement ratio or w/c
When SCMs are used, this is the water-to-cementitious
materials ratio or w/cm
w/c or w/cm may range 0.20-0.80, but 0.40-0.60 is typical
Water is
Introduced to the concrete during mixingNecessary for reaction of cement and SCMs
Permeates the concrete during service
Because the water in concrete contains ions, it is usually
called pore solution and has a high pH
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Pore Solution
0
200
400
600
800
1000
0 20 40 60 80 100
Na+K+Ca++
OH-SO3-
ionconcentration
(x10-3
mol/l)
curing time (d)
Based upon Page and Vennesland, Materiaux etConstructionsV16:19-25, 1981
Some model pore solutions:
High alkali (pH ~ 13.8) 0.55M
KOH + 0.16M NaOH
(Lawrence solution)
Low alkali (pH ~ 13.5) 0.24M
KOH + 0.08M NaOH
Saturated Ca(OH)2 + 0.7M
NaOH
Pore Solution
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Water in hcp
Capillary water - water present in voids larger than 2.5nm- In capillaries >50nm, water exists as free water because its removal
does not cause volume change
- In capillaries 2.5-50nm, removal of water results in shrinkage because
new bonds can form between C-S surfaces
Water in hcpAdsorbed water - water physicallyadsorbed to the solid surfaces in C-S-H- can be removed on drying to RH ~ 30%,resulting in shrinkage
Interlayer water- water associatedwith the C-S-H structure- can be removed only on strong drying toRH ~ 11%, resulting in shrinkage
Chemically combined water - waterthat is an integral part of varioushydration products- lost only on decomposition during heating
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Nature of Composite Materials
In virtually all composite materials, defects are present ingreater density at the interface between the different
constituents
Often, the composite properties are governed by the nature of
the interfaces
www.uf-bio-nano-center.org/ electron.aspImage courtesy of Ben Mohr
Effects of the 3rd Phase
Mehta and Monteiro, 1993
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Interfacial Transition Zone
The ITZ is the region 10-50umwide around coarse
aggregate; characterized by:
Higher local porosity
Greater density of pre-
existing microcracks due to
differential shrinkage and
drying
Larger CH crystals that tend
to be oriented and more prone
to cleavage.
Interfacial Transition Zone: NIST models
Agg Agg
Cement particles (red) around a
model square aggregate at
w/c=0.47, before hydration
After 77% hydration.
Color key: unhydrated cement,
CC--SS--HH, CH, porosity
http://ciks.cbt.nist.gov/garbocz/paper43/node2.html#Figure%201.
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Interfacial Transition Zone
The mechanism by which the transition zone is formed isassociated with the development of water films around theaggregate in fresh concrete that , in effect, create a localregion of with higher water-to-cement ratio.
Wall effect packing effect; aggregate surface acts as awall, making packing of the cement particles inefficient =>high porosity region (more important)
One-sided growth when no aggregate is present,
hydration products grow in all directions. close to anaggregate, growth only occurs on the cement side,contributing to porosity.
Wall EffectPorosity fraction near aggregate surface prior to hydration for two
different cements. The median cement particle diameters of the two
cements are: A1 - 28 m, A7 - 11 m.*
Effects of bleeding ignored.
*Bentz, D.P., Garboczi, E.J., and Stutzman, P.E., in Interfaces in Cementitious Composites, Ed. J.C. Maso (E & FN Spon, London,
1992) pp. 107-116.
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Relative influence of 1-sided growth
http://ciks.cbt.nist.gov/garbocz/paper43/node3.html
Wall effect and 1-sided growth
1-sided growth only (effect apparent
only a few um from aggregate)
Interfacial Transition Zone
Stress-strain behavior for
both the aggregate and
cement paste alone are
nearly linear elastic.
But because of the ITZ,
concrete displays some
nonlinear and inelastic
behavior in compression.
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Interfacial Transition Zone
Interconnectivity of microcracks and pores in TZ also
increases permeability, durability suffers.
http://ciks.cbt.nist.gov/~garbocz/paper72/paper72.html
By tailoring the concrete mixture to
reduce the influence of the ITZ,
strength, E, and impermeability are
increased.
lower w/c
higher cement content
use of SCMs
smaller MSA
reactive dolomitic aggregate lightweight aggregate
extended moist curing
Interfacial Transition ZoneInterfacial Transition Zone
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Interfacial Transition Zone
The microstructural features and mechanical effects of thetransition zone are the subject of some debate.
Some researchers report the presence of a duplex film
consisting of thin layer of CH adhering to the aggregate
surface surrounded by thin layer of rod-like C-S-H [Hewlett,
1998]
Others disagree finding C-S-H to be the solid phase most
often in contact with the aggregate surface [Scrivener and
Gartner, 1988]
In addition, some researchers have come to believe that a
weaker interfacial region does not always exist between thehydrated cement paste and the aggregate [Mindness and
Diamond, 1992].