ijetae_0812_80.pdf

International Journal of Emerging Technology and Advanced Engineering

Website: www.ijetae.com (ISSN 2250-2459, Volume 2, Issue 8, August 2012)

472

Effect of Partial Replacement of Cement by Silica Fume on

Hardened Concrete Dilip Kumar Singha Roy

1, Amitava Sil

2

1Professor, Department of Civil Engineering, N. I. T., Durgapur,M. G. Avenue, Durgapur, W.B., Pin-713209 2Scientist, IPIRTI, Field Station Kolkata, 2/2, Biren Roy Road (West) Sarsuna, Kolkata-700061

Abstract - The use of Silica Fume (SF) in short period of time had one of the most dramatic impacts on the industrys ability to routinely and commercially produce SF modified

concrete of flowable in nature but yet remain cohesive, which

in turn would develop both high early and high later-age

strengths including resistant to aggressive environments. This

paper features an experimental study on the nature of SF and

its influences on the properties of fresh and hardened

concrete. In the present study, an attempt has been made to

investigate the strength parameters of concrete made with

partial replacement of cement by SF. Very little or no work

has been carried out using silica fume as a replacement of

cement. Moreover, no such attempt has been made in

substituting silica fume with cement for low/medium grade

concretes (viz. M20, M25). Properties of hardened concrete

viz Ultimate Compressive strength, Flexural strength,

Splitting Tensile strength has been determined for different

mix combinations of materials and these values are compared

with the corresponding values of conventional concrete. The

present investigation has been aimed at to bring awareness

amongst the practicing civil engineers regarding advantages

of these new concrete mixes.

Key words Compressive strength, flexural strength, normal concrete, silica fume(SF) concrete, split tensile

strength.

I. INTRODUCTION

During the last three decades, great strides have been

taken in improving the performance of concrete as a

construction material. Particularly Silica Fume (SF) and fly

ash individually or in combination are indispensable in

production of high strength concrete for practical

application. The use of silica fume as a pozzolana has

increased worldwide attention over the recent years because when properly used it as certain percent, it can

enhance various properties of concrete both in the fresh as

well as in hardened states like cohesiveness, strength,

permeability and durability. Silica fume concrete may be

appropriate in places where high abrasion resistance and

low permeability are of utmost importance or where very

high cohesive mixes are required to avoid segregation and

bleeding.

Silica fume is a by- product in the production of silicon

alloys such as ferro-chromium, ferro-manganese, calcium silicon etc. which also creates environmental pollution and

health hazard. From the study carried out by Ray [1] , it is

found that compressive strength increased by about 21%,

flexural strength by 35% and split tensile strength by 10%

when silica fume was added (5-12.5) % with a increment of

2.5% on a high slump concrete. Joshi [2} observed that

reduction in cement content at fixed water cement ratio was

not detrimental to fresh and hardened concrete properties

and may actually improve performance when silica fume

was added as 10% by weight of cement content.

During the viaduct construction between J.J hospital and

Crawford market in Mumbai, Saini [3] has undergone a research work based on high performance concrete (HPC)

of grade M75 where SF was added @10% by weight of

cement to ensure durability of the structure. They found

28days compressive strength of HPC varied between 79.6

to 81.3 MPa indicating good control of quality of concrete.

Basu (4] developed HPC with SF that can reduce micro-

cracks which tends to develop around the interface between

bulk hydrated cement paste and anhydrous cement particles

or un reacted pozzolanas. A study has been carried out by

Thomas [5] on controlling alkali-silica reaction (ASR) in

concrete with particular emphasis on development of a new standard practices in Canada. He concluded that use of

blended cement containing low alkali Portland cement

mixed with SF would be a viable means of controlling

expansion in concrete.

From the research work done by Lewis [6], it has been

observed that there is a considerable reduction in rebound

from (35-15)% by addition of SF which also increased the

pumpability of high wokability mix having slump value

above 250mm. Incorporation of SF and other admixtures

could be important in the production of HPC in order to

obtain superior mechanical and durability properties as stated by Roncero [7].



473

He found that water demand of cement SF system in a paste of normal consistency increases with temperature and

decreases with incorporation of superplasticizers until a

certain dosage.

During the extensive research work carried out by

Vishnoi [8], it was concluded that SF concrete has a

capability to withstand abrasion erosion with better construction feasibility, workability and surface finish.

Kanstad [9] made a investigation work to assess the crack

sensivity of HPC with different SF contents and found that

effect of variation of SF content was of minor importance

compared to other factors viz degree of insulation,

environmental condition.

Keeping in view of the above aspects, an attempt has

been made to replace cement by SF to develop a cost

effective modified concrete, i.e, SF concrete. Considering

this aspect, the present paper reports a study on the effect

partial replacement of cement by SF (from 5% to 10% with a step of 2.5%) on M20 grade concrete designed with 53

grade Ordinary Portland Cement, sand and coarse

aggregate. As SF is costlier than cement, addition to

cement will further enhance the cost, which may not be

economically viable. The present study has, therefore,

made an attempt to use SF as cement replacement materials

for low/medium grade concrete (M20) used for general

construction purposes with a view to achieve the desired

strength parameters of the concrete higher grade.

II. EXPERIMENTAL METHOD

A. Materials

53 grade Ordinary Portland Cement, Zone II sand, 12.5

mm and 20 mm down graded aggregate, commercial Silica

Fume Grade 920-D (specific surface = 21.4, bulk density =

620 Kg/m3) have been used for various composites.

Designed Mix Proportion has been used as (normal

concrete) 1 : 1.485 : 3.143 for M20 grade concrete with the

following ingredients :

a) Cement = 533.05 Kg/m3 b) Sand = 791.58 Kg/ m3 c) Well graded aggregate(20mm size) = 1675 Kg/m3 d) Coarse aggregate / Fine aggregate ratio was 2.11

and water cement ratio was 0.45 for all mixes.

B. Sample Preparation and Properties Studied

Aggregates, Cement and SF have been charged into the

mixer machine in succession with appropriate proportions

for dry mix followed by addition of water and then rotated

sufficiently to achieve uniform and high workable mix.

The concrete has been placed in 150 mm cube, 150mm

diameter 300mm high cylinder and 100mm 100mm 500mm prism moulds and vibrated with standard vibrator.

Curing regime has been taken as 24 hours in mould with

hessian clothes at (20 24)0 C followed by under water curing until the day of testing.

In the fresh state, compaction factor of each mix have been

measured. In hardened state, 7 days and 28 days compressive strength of cubes and cylinder, split tensile

strength and flexural strength have been measured.

III. TEST RESULTS AND DISCUSSIONS

A. Fresh State

1) Mix Character: Due to superfine nature of SF particles, SF concrete has shown more cohesiveness than

standard ordinary Portland cement concrete. All the mixes

have exhibited satisfactory character in relation to

segregation and bleeding. But with the increase in

percentage of SF, the stickiness in concrete was observed.

2) Workability: In all the mix, the compacting factor, i.e,

workability increases as percentage of SF is increased from 5% till 10%. SF concrete is just as susceptible to poor

workmanship as ordinary concrete and all normal site

operations should be performed to the optimum

requirements.

B. Hardened State

1) Compressive Strength: Table I and figure 1 depicts that when cement is replaced by SF, the maximum 7 days

cube compressive strength observed as 17.85 N/mm2

(4.32% higher over normal concrete) and 28 days strength

obtained as 33.93 N/mm2 (19.6% higher). The maximum 7

days and 28 days cylindrical compressive strength are

found to be 15.94 N/mm2 (4.32% higher) and 25.56 N/mm

2

(16.82% higher) respectively when cement is replaced by

SF as shown in figure 2. From the properties exhibited by

concrete using silica fume replacing cement, it is observed that, there may be some marginal loss of strength initially

but the same improves effectively both with the age and

incorporation of SF in place of cement. The increase in

strength development is due to the fact that silica fume

dissolves in saturated solution of Ca(OH)2 within few

minutes. As soon as enough Portland cement has hydrated

to result in saturation of the pore water with Ca(OH)2,

Calcium Silicate Hydrate (C-S-H) gel is formed on the

surface of silica fume particles. This C-S-H gel produced

by SF concrete has a lower C : S ratio than that resulting

from the hydration of Portland cement concrete without silica fume.



474

TABLE I

COMPRESSIVE STRENGTH OF CUBE AND CYLINDER AT 7 AND

28 DAYS OF CURING

33.93

25.56

28.37 28.3726.44

21.88 20.4621.38

0

5

10

15

20

25

30

35

40

0 2.5 5 7.5 10 12.5

% Cement replaced by SF

Cube

& C

ylinder

Com

pre

sive

Str

ength

(N

/mm

2)

Cube Compressive strength

Cylinder Compressive strength

Figure 1. Variation of 28 days Cube and Cylinder compressive

strength for different percentage of SF replacing Cement

2) Split Tensile Strength: It is clear from the Fig. 2 that

the maximum cylindrical split tensile is found to be 3.61

N/mm2 at 10% cement replaced by SF (38.58% more than

that of normal concrete). With an average silica fume

dosage of about 40 kg, there will be approximately 1 km2 of surface area capable of reacting with the calcium

hydroxide that is formed as the cement hydrates. For low

silica fume content, the strength improvement is marginal

because the volume of silica fume is inadequate to cover

the surface of all coarse aggregate particles. As the

percentage of silica fume is increased, there is an

improvement in packing, i.e., action of it as a filler material

resulting in improvement of the interfacial bond between

the aggregate cement matrix. As a result, a sharp increase in tensile strength is observed.

3.61

4.93

2.6

2.14

2.78

3.33

3.934.07

0

1

2

3

4

5

6

0 2.5 5 7.5 10 12.5

% Cement replaced by SF

Fle

xura

l &

Split

Ten

sile

Str

ength

(N/m

m2)

Split Tensile

Strength

Flexural Strength

Figure 2. Variation of 28 days Flexural and Split Tensile strength for

different percentage of SF replacing Cement

3) Flexural Strength: The maximum 28 days flexural

strength of SF concrete is found to be 4.93 N/mm2 which is

21.13% higher with respect to that of the normal concrete

for 10% cement replaced by SF as depicted in figure 2.

This value is far more than the value calculated from the

expression 0.7 fck (where fck is the characteristic strength of concrete) as specified by IS: 456-2002 (Indian Standard Specifications). Silica fume besides reacting with free lime

of cement and contributing to the development of the

strength, bind themselves tightly with cement hydrates in

the form of flocks and makes more space for the hydration

products (C-S-H gel) of cement grains. As SF content is

increased, the bond of the hydrated cement paste with the

aggregate, chiefly for the larger particles, is greatly

improved allowing the aggregate to participate in stress

transfer in a better way and hence, the increase in the

strength is observed.

It is clear from the above discussion that till 10%

replacement of cement by SF, there are marked

improvement in compressive strengths, split tensile

strength and flexural strength. This type of SF concrete may be recommended in places where high early strength

development is essential. This high strength SF concrete

which is being produced with a designed mix of lower

grade, i.e, M20 and the same can be used as a suitable

alternative of lower grade normal concrete. With all high

performance materials for making this type of concrete,

appropriate care should be taken in its use and proper

guidelines may be sought from the concerned authorities to

ensure the maximum benefits.

IV. ECONOMIC ANALYSIS

Now according to local rate (West Bengal, India),

Cost of Cement = Rs. 7.20 per Kg

Cost of Sand = Rs. 0.89 per Kg

Cost of Stone chips = Rs. 1.60 per Kg

Cost of Silica Fume (Condensed) as supplied by M/s

ElKem Materials (Mumbai, India) Rs. 17 per Kg.

The cost of SF concrete of M20 grade (Only 10% SF by weight of cement) = Rs. 7,745.56

Sl

No.

Percent

SF

replacing

cement

Compressive Strength (N/mm2)

Cube Cylinder

7

days

28

days 7 days 28 days

1 0 17.11 28.37 13.58 21.88

2 5 14.60 26.44 10.85 20.46

3 7.5 16.74 28.37 12.17 21.38

4 10 17.85 33.93 15.94 25.56



475

Cost of Conventional Concrete of M25

= Rs. 8,467.50

The economic analysis reveals that the target mean

strength of next higher grade concrete namely M25 is

achieved in 28 days after replacing 10% of cement by silica

fume from the designed mix proportion of M20 grade.

Thus, an obvious recommendation can be put forwarded to use the M25 grade of silica fume concrete as a supplement

of M20 grade normal concrete. This is seen from the above

cost analysis and which certainly confirms the reduction in

the cost of construction of M25 grade SF concrete by about

4.0% in comparison to that of the M20 grade normal

concrete.

V. CONCLUSION

It may be concluded that use of silica fume is a

necessity in production of not only for high strength

concrete but also for low/medium strength concrete as this

material facilitate the adoption of lower water - cement

material ratio and better hydration of cement particles

including strong bonding amongst the particles. From the

study it has been observed that maximum compressive strength (both cube and cylinder) is noted for 10%

replacement of cement with silica fume and the values are

higher (by 19.6% and 16.82% respectively) than those of

the normal concrete (for cube and cylinder) where as split

tensile strength and flexural strength of the SF concrete

(3.61N/mm2 and 4.93N/mm2 respectively) are increased by

about 38.58% and 21.13% respectively over those (2.6

N/mm2 and 4.07 N/mm2 respectively) of the normal

concrete when 10% of cement is replaced by SF.

As the SF concrete is more compact and thereby more

durable in nature and hence with some degree of quality

control, it may be used in places of construction where there is a chance of chemical attack, frost action etc.

Moreover with 10% of cement replaced by silica fume, the

characteristic strength of higher grade of cement concrete

namely M25 is achieved only by using the M20 grade

designed mix proportion and consequently this SF concrete

can certainly be used as a supplement to M20 grade normal

concrete with at least 4% of cost reduction.

Lastly with good quality control, high early strength can

be achieved in SF concrete which may be useful in various

structural constructions such as high-rise buildings,

bridges, chimneys, machine foundations, run ways etc., wherein, the timeframe of completion vis--vis the

economy is an important driven factor for the targeted

purpose as well as for the contractors and owners alike as

this concrete will provides quick stage by stage or floor to

floor construction.

REFERENCE

[1] Ray, I, De, A and Chakraborty, Fifth Conference on Concrete Technology for High Slump Concrete, Vol 1, p.p 86-93

[2] Joshi, N. G. Bandra Worli Sea Link: Evolution of HPC mixes

containing Silica Fume, Indian Concrete Journal, (Oct. 2001), pp. 627-633

[3] Saini, S, Dhuri, S. S, Kanhere, D. K, Momin, S. S. High

Performances concrete for an urban viaduct in Mumbai, Indian

Concrete Journal, (Oct. 2001) , pp. 656-664.

[4] Basu, P. C. : NPP containment structures Indian experience in Silica

Fume based HPC, Indian Concrete Journal, (Oct. 2001), pp. 656-664.

[5] Thomas, M D. A. Using Silica Fume to Combat ASR in Concrete,

Indian Concrete Journal, (Oct. 2001), pp 656-664

[6] Lewis, R. C. , Hasbi, S. A. : Use of Silica Fume concrete :Selective case studies, Indian Concrete Journal, (Oct. 2001), pp. 645-652.

[7] Roncero, J., Gettu, R., Agullo, L. , Vazquez, E. : Flow behaviour of

superplasticised cement pastes : Influence of Silica Fume, Indian

Concrete Journal, (Jan. 2002), pp. 31-35.

[8] Vishnoi, R. K. , Gopalakrishnan, M. : Tehri Dam Project : Silica

Fume in High Performance Concrete for Ensuring Abrasion Erosion

Resistance, Proceedings organised by Indian Society for

Construction Materials and Structures, (February 2003), pp. 28-40.

[9] Kanstad, T, Biontegaard, O, Sellevold, E. J, Hammer, T. A. and

Fidjestol, P. Effect of Silica Fume on Crack Sensivity, Concrete International, (Dec. 2001), pp 53-59

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