STUDY OF PORE STRUCTURE OF SILICA FUME CONCRETE FOR ... · influence on most mechanical properties of concrete. Hasselman and Fulrah (8), Wagh et al (9), ... concrete by incorporating,

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International Journal of Civil Engineering and Technology (IJCIET)

Volume 9, Issue 3, March 2018, pp. 920–931, Article ID: IJCIET_09_03_092

Available online at http://www.iaeme.com/ijciet/issues.asp?JType=IJCIET&VType=9&IType=3

ISSN Print: 0976-6308 and ISSN Online: 0976-6316

© IAEME Publication Scopus Indexed

STUDY OF PORE STRUCTURE OF SILICA

FUME CONCRETE FOR OPTIMUM

REPLACEMENT

A. Joe Paulson

Research Scholar, Department of Civil Engineering,

Hindustan Institute of Technology & Science, Chennai, India

A. Melchizedek

P.G. Student, Department of Civil Engineering,

Karunya Institute of Technology & Sciences, Coimbatore, India

R. Angeline Prabhavathy

Professor, Department of Civil Engineering,

Hindustan Institute of Technology & Science, Chennai, India

ABSTRACT

One of the main objectives of the research and development done in concrete is to

improve the performance of concrete. The parameters that were considered are

compressive strength and permeability which are direct indices of durability of

concrete. The compressive strength and permeability could be enhanced by various

methods and means and few of them are increasing the content of binder, decreasing

the water content, proper gradation and minimizing the porous nature of concrete. In

the present work, the pore structure of silica fume concrete is studied considering the

optimum replacement for cement found in previous works, i.e. 13% replacement.

Samples were casted of various grades, viz., M20, M25, M30, M35 and M40 grades

for 0% replacement and 13% replacement and pore structure was studied using

Scanning Electron Microscope (SEM) and chemical analysis was done using Energy

Dispersive X-Ray Spectroscopy (EDAX).

Keywords: Silica fume, replacement, pore structure, interfacial transition zone,

Scanning Electron Microscope, Energy Dispersive X-ray Spectroscopy.

Cite this Article: A. Joe Paulson, A. Melchizedek and R. Angeline Prabhavathy,

Study of Pore Structure of Silica Fume Concrete for Optimum Replacement,

International Journal of Civil Engineering and Technology, 9(3), 2018, pp. 920–931.

http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=9&IType=3

1. INTRODUCTION

With the advent of nano-science in early 2000s, it had a great impact on construction

materials, thus increasing the usage of nano-technology products. These nano-products while

A. Joe Paulson, A. Melchizedek and R. Angeline Prabhavathy


on one side improve the mechanical properties due to accelerated hydration, formation of

small sized crystals, on the other side enhances durability of concrete as very minute pores are

filled by these nano-particles (1,2)

.

Concrete is a composite made of binder, coarse aggregate and fine aggregate. When water

is added to cement and fine aggregate, cement paste is formed. These fill the space between

coarse aggregate and form rock like solid on hardening. Under a microscopic examination,

cement paste in a non-homogeneous and anisotropic matrix composed of irregularly shaped

and unevenly distributed pores attributed to the evaporation of free water and gel pore

formation in the C-S-H hydrates. The pore structure greatly influences the strength

development of concrete(3,4)

.

Pore structure of concrete includes air voids, capillary pores and gel pores. Pore structure

of concrete possesses important role in determining mechanical, durability and transmissive

characteristics of concrete(5,6)

. Parameters such as porosity and pore size distribution of pore

structure of concrete are being employed for evaluation of physical strength of concrete, frost

resistance, permeability, and carbonation resistance of concrete(7)

.

Among all the parameters, it is substantially proved and established that porosity has more

influence on most mechanical properties of concrete. Hasselman and Fulrah (8)

, Wagh et al (9)

,

Liu (10)

and Palchik (11)

approached the problem by estimating the absence of material for

carrying the applied load. An increase in the porosity reduces the strength of concrete. The

strength reduction depends greatly on the size of pore, shape of pore and distribution (12)

.

Porosity in concrete can be due to the presence of entrapped air voids, capillary voids and air

voids, extent varying on different occasions. Entrapped air voids may be as large as 3mm.

Capillary voids or cavities which exist when the spaces originally occupied with water do not

get completely filled with the hydration products of cement. The size of capillary voids ranges

from 10 nm to 1μm. The objective of the present study is to fill these voids with the

ingredients or any other materials compatible to the chemical and physical reaction of

concrete. To meet this objective, evolved supplementary cementitious material (SCM), viz.,

mineral admixtures like condensed silica fume, fly ash, ground granulated blast furnace slag,

metakaolin etc can be used. These SCMs physically and chemically participate in hydration

process.

Silica fume is a by-product obtained during production of silicon metal or ferrosilicon

alloys. One of the most beneficial use of silica fume is mixing it with concrete. Because of its

chemical and physical properties, it is a very reactive pozzolana. Concrete containing silica

fume has very high strength and has more durability. Silica fume is available from suppliers

of concrete admixtures and is added in the production of concrete as a replacement of cement.

The raw materials are quartz, coal and woodchips. The smoke that results from furnace

operation is collected and the ultrafine material less than 1µ is silica fume. Silica fume

consists primarily of amorphous (non-crystalline) silicon dioxide (SiO2). The individual

particles are extremely small, approximately 1/100th

the size of an average cement particle.

Because of its fine particles, large surface area, and the high SiO2 content, silica fume is a

very reactive pozolana when used in concrete. The quality of silica fume is specified by

ASTM C 1240 and AASHTO M307. High-strength concrete is a very economical material for

carrying vertical loads in high-rise structures. Silica-fume concrete with low water content is

highly resistant to penetration by chloride ions. More and more transportation agencies are

using silica fume in their concrete for construction of new bridges or rehabilitation of existing

structures. Silica fume for use in concrete is available in wet or dry forms.

Study of Pore Structure of Silica Fume Concrete for Optimum Replacement


2. LITERATURE REVIEW

Duan et al,(13)

(2013) conducted a study on the pore structure and interfacial transition zone of

concrete by incorporating, slag, silica fume and metakaolin and concluded that the conditions

of SCMs or mineral admixtures improve the micro-structure as well as the compressive

strength due to higher ITZ micro-hardness and denser micro-morphology. SCMs enhance the

micro structure due to micro-aggregate filling and the pozzolanic effect, and the fine particles

bridge the gaps between cement particles and hydration products. Torii et al(14)

(1994)

concluded from an experimental investigation that there was a drastic change in the pore

structure of the samples containing 10% and 15% silica fume, leading to a reduction of coarse

pores larger than 0.1μm and an increase of fine pores smaller than 0.04μm with time. Oltulu

et al(15)

(2014), determined the statistical significance between the pore size as well as pore

size distributions, compressive strengths and capillary absorption coefficients. Further,

reduction in pore volume of samples, pore size distribution becoming finer and ultimately

leading to a higher physical and mechanical properties were established in this work. Bu et

al(16)

(2016) inferred that the properties of concrete are strongly dependent on its pore

structure features, porosity being an important one among them. This study dealt with

developing an understanding of the pore structure-compressive strength relationship in

concrete. Several concrete mixtures with different pore structures are proportioned and

subjected to static compressive tests. The pore structure features such as porosity and pore

size distribution are extracted using mercury intrusion porosimetry technique. A statistical

model was developed to relate the compressive strength to relevant pore structure features.

Kartikeyan et al17

(2014) investigated the effect of using nano-sized mineral (silica fume)

admixtures in concrete as a partial replacement of cement. The silica fume which was used in

this work was ground for 1 hour with varying quantities using planetary ball mill. On

analyzing the results of grinding, it was observed that the grinding was effective in 1 hour and

the size of micro-silica has reduced by 75.45% reaching nano size. Physical tests such as

specific gravity, size identification using particle size analyzer (PSA), micro structure analysis

using Scanning Electron Microscope (SEM), Chemical composition identification by X-Ray

Fluorescent (XRF), Crystalline check for silica using X-Ray Diffraction (XRF) were

performed for samples of both unground and ground micro-silica (Nano silica). Mechanical

properties were obtained by performing strength tests for specimens cast with different

percentages of ground and unground micro-silica in partial replacements such as 5%, 10%

and 15% by weight of cement. The cubes casted with 10% replacement of Nano silica for

cement by weight showed better strength performance. The compressive strength of concrete

where grinded silica fume or nano silica was used showed an improvement of 7.5% over

controlled concrete. Oltulu et al18

(2014) studied the 56-day pore structures of the cement

mortars produced by the addition of silica fume and nano-SiO2 (NS), nano-Al2O3 (NA) and

nano-Fe2O3 (NF) powders in singular, binary or ternary combinations. 3 different proportions

(0.5%, 1.25% and 2.5%) of the binder content were investigated through Mercury Intrusion

Porosimetry (MIP) and Gas Adsorption (BET) analyses. The compressive strengths and

capillary water absorptions of produced mortars were also determined in order to investigate

the effects of changes in pore structure on these properties. Among the 22 mortar groups

produced, NA content of 1.25% yielded the best results on the properties measured by MIP

and BET (total volume of mercury intruded, porosity and specific surface area) as well as the

pore-size distributions. The reduction in pore volume, the pore-size distribution becoming

finer and the improvement in physic-mechanical properties of the mortars after the addition of

Nano-powders could be explained by the filler effect or amount of hydration products of

cement. However, the addition of the powders at proportions in excess of 1.25% resulted in an

increase in the pore volume of some mortars because of agglomeration. Madhanasree et al19

(2016) investigated the influence of partial replacement of cement by silica fume on the



properties of hardened concrete. Properties of hardened concrete viz., 28 day compressive

strength, flexural strength and split tensile strength were determined for different mix

combinations of materials (0%, 12.5%, 13% and 13.5% silica fume on M20, M25 and M30

grades) and compared with the conventional concrete. It was found that 13% replacement of

cement with silica fume yielded maximum 28 day compressive strength, flexural strength and

split tensile strength.

3. RESEARCH GAP

The above literatures show that the pore study is done for various combinations of

replacement and grades of concrete. The optimum replacement was studied and reported. The

pore structure of the concrete with optimum replacement and the same with no replacement

were not studied. Hence, a study is carried out on the pore structure of conventional concrete

and silica fume concrete with a replacement of 13%.

4. MATERIALS USED AND EXPERIMENTAL INVESTIGATION

Ordinary Portland Cement (OPC) 53 grade conforming to IS 12269-1987 was used. River

sand and broken granite jelly of size 20mm and down conforming to IS 383-1970 was used.

Condensed silica fume procured from ELKEM India, Mumbai conforming to IS 15388:2003

was used in this investigation.

Samples of cubes were casted for the following concretes:

Normal cement Concrete – without any replacement of cement and

Silica Fume Concrete – with 13% replacement of cement with silica fume.

Samples are casted for different grades of concrete proportioned as per IS: 10262, viz.,

M20, M25, M30, M35 and M40 grades. Table 1 shows the summary of the specimens with

various replacement levels against grades of concrete and the interested parameters therein.

Table 1 Grade of concrete, percentage replacement and number of samples

S. No. Grade of

Concrete

% replacement of

Cement with silica

fume

Number of

specimens

No of samples for cube

compressive strength

7th

day 28th

day

1 M20 0% and 13% 6 3 3

2 M25 0% and 13% 6 3 3

3 M30 0% and 13% 6 3 3

4 M35 0% and 13% 6 3 3

5 M40 0% and 13% 6 3 3

The pore structure of the concrete samples using (a) SEM (Scanning Electro-

Microscope),(b) EDAX (Energy Dispersive X-ray Spectroscopy is studied. Concrete cubes

150mm size were casted and cured by immersion curing. Compressive strength of these cubes

was found at 7th

day and 28th

days. The samples were collected and were taken for SEM

analysis and EDAX study.

5. RESULTS AND DISCUSSION

The experiments were conducted using Scanning Electron Microscopy (SEM) and Electron

Dispersive X – Ray Spectroscopy (EDAX).



5.1. Scanning Electron Microscopy (SEM) Analysis

In the present study, the change in the morphology due to the incorporation of silica fumes to

concrete was examined with the help of Scanning Electron Microscope (SEM). The main

objective of the addition of silica fume in concrete is to improve the compressive strength

which is due to the micro – structural changes in the cement paste phase as well as in the

Inter-facial Zone around aggregates which was observed using SEM image. The surface

morphology change was observed for all the specimens by varying the curing time,

percentage of silica fumes and also by varying the grade of concrete. On comparing the

conventional concrete and the mixture, from the results, it is observed that there is a

considerable reduction in the size of void space due to the addition of silica fumes.

7th

day of SEM

M20 grade – Fig. 1 and Fig. 2 show the SEM image of M20 grade concrete for 0% and 13%

replacement levels at 7th

day.

Figure 1 M20 grade concrete (0% Replacement) Figure 2 M20 grade concrete (13% Replacement)



day.




day.




M35 grade – Fig. 7 and Fig. 8 shows the SEM image of M30 grade concrete for 0% and 13%


day.


M40 grade – Fig. 9 and Fig. 10 shows the SEM image of M30 grade concrete for 0% and

13% replacement levels at 7th

day.


28th

day of SEM

M20 grade – Fig. 11 and Fig. 12 shows the SEM image of M20 grade concrete for 0% and


day.




M25 grade –Fig. 13 and Fig. 14 shows the SEM image of M25 grade concrete for 0% and


day.




day.




day.




day.




In all the above SEM image analysis we observe that the concrete at 28th

day is hav9ing

less voids when compared with SEM images of concrete at 7th

day. We can hence infer that

28th

day concrete samples are denser than 7th

day concrete samples. The SEM images for the

concrete with silica fume shows that the concrete is intact and percentage of voids are lesser.

From this we can also infer that concrete samples with 13% replacement level are with less

voids or less porous than concrete samples with 0% replacement. Another reason for the

denser and less porous concrete could be increased degree of hydration in case of concretes

where silica fume are incorporated.

The composition of ITZ changes, with incorporation of silica fume, specifically, hydration

product C-S-H gel forms and the content of Ca(OH)2 decreases due to pozzolanic effect. In

addition to this physical action of filling of gap between matrix and aggregate also happens.

The density of ITZ increases as the matrix and aggregate comes together with the addition of

mineral admixture.

5.2. Energy Dispersive X -Ray Spectroscopy (EDAX) Analysis

7th

DAYS EDAX








28th

DAYS EDAX








The comparative study of pore structure for control concrete and silica fume concrete (for

13% replacement levels) is observed to give satisfactory and desired results. Testing being

done on 7th

day, the hydration reaction and formation of compounds would be slightly more

than half of the final structure. The behaviour of micro-structure of concrete changed with

addition of nano materials such as silica fume and also influences the compressive strength of

concrete mixes. The strength of concrete also increases considerably when cement is replaced

with silica fume.



6. CONCLUSION

Evaluation of micro structure of silica fume concrete, for different grades of concrete is the

prime objective of this work. From the observations the following conclusions can be drawn:

Addition of silica fume to concrete can improve micro – structure as micro pores are filled and

the pozzolanic effects gets enhanced. Hence the density of the pore structure becomes higher.

Addition of silica fume, makes ITZ denser with pore size distributed uniformly.

The comparative study of micro-structure shows clearly that addition of silica fume enhances

the hydration reaction in concrete where cement is replaced than in no replacement. With the

enhancement in hydration process, the pore structures are improved significantly. The micro

study reveals that the uniformity and compactness of the matrix is predominantly observed in

the silica fume concrete over reference concrete. The pozzolanic activity of silica fume results

in production of calcium silicate hydrates and calcium alumina-silicate hydrates.

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Documents

STUDY OF PORE STRUCTURE OF SILICA FUME CONCRETE FOR ... · influence on most mechanical properties of concrete. Hasselman and Fulrah (8), Wagh et al (9), ... concrete by incorporating,