potential benefits of flyash in attaining the workability of silica fume concrete

7/30/2019 Potential Benefits of Flyash in Attaining the Workability of Silica Fume Concrete

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Potential Benefits of Flyash in Attaining the Workability of

Silica Fume Concrete

Dr. Jaspal Singh, Professor, Civil Engg Department, Er. Anil Kumar Nanda, Civil Engg

Department, PAU Ludhiana

Introduction

Building industry, is one of the key areas of infrastructure development & for catering to the

requirements of building materials, we are dependent on natural resources. As natural resources

are depleting day by day, we have to think of alternate measuries. Use of industrial wastes for this

purpose is beneficial as by this, not only natural resources are conserved but solution to safe

disposal of industrial waste is obtained. Flyash and silica fume are the promising industrial wastes

which can be easily harnessed in construction. With the increase in the number of coal-based

thermal power plants in India, generation of fly ash has reached enormous proportions. In India,

about 100 million tonnes of flyash is accumulated every year which is generated as waste from

thermal plants. This is causing enough concern as its disposal involves design and installation of

ash ponds covering large areas at each plant site. In spite of concerted efforts on a national scale,

only a very small fraction (around 6%) of the fly ash is put to use in India, compared to its

utilization to a greater extent in other countries.

Silica fume is also a waste by-product from the silicon metal and ferrosilicon alloy industries. The

chief problems in using this material are associated with its extreme fineness and high water

requirement when mixed with Portland cement. However, if used with superplasticizers, we can

attain good workability of concrete.

Workability of concrete plays a vital role in all construction works, affecting the speed of

construction and placing of concrete, which in turn affect the financial aspect of construction

project. The use of silica fume reduces the workability of fresh concrete or mortar due to its very

high specific surface area; however, it improves many of the properties of hardened concrete or

mortar. Earlier workability of concrete was controlled by amount of water added during mixing and

setting characteristics were adjusted with the help of admixtures to modify the properties of

concrete. Nowadays, Superplasticizers are added to concrete to get highly workable concrete. They

are assuming increasing popularity for use in concrete, because of advantages they offer in

handling, placing and compaction of concrete. Though use of superplasticizers is very common in

developed countries, the superplasticizers are not so common in developing countries like India. It

is in this context that effort has been made to study the effect of addition of superplasticizer in

addition to fly ash and silica fume on workability of concrete.

Materials

Portland cement

xOrdinary Portland cement (OPC) of 43 grade (Ultratech) confirming to IS: 8112:1989 was being

used for making concrete. The relevant cement properties experimentally obtained are given in

Table 1.



Table 1: Properties of OPC 43 grade cement

S. No. Characteristics Value obtained

experimentally

Value specified by

IS:8112:1989

1. Specific gravity 2.975 -

2. Standard consistency 32% -

3. Initial setting time 123 min 30 min (minimum)

4. Final setting time 266 min 600 min (maximum)

5.

Compressive strength

3 Days

7 Days

28 Days

29.18 N/mm2

33.78 N/mm2

47.36 N/mm2

23 N/mm2

33 N/mm2

43 N/mm2

The values obtained conform to specifications given in code

Aggregates

i) Coarse Aggregate

The coarse aggregate used were a mixture of two locally available crushed stone of 10 mm and 20

mm size in 50:50 proportion. The aggregates were washed to remove dirt, dust and then dried to

surface dry condition. Specific gravity and other properties of coarse aggregate are given in Table

2. Then sieve analysis of coarse aggregate was done. Proportioning of coarse aggregate was done

and Fineness Modulus was obtained as given in Table 3.

Table 2: Properties of coarse aggregates

Characteristics Value

Colour Grey

Shape Angular

Maximum size 20 mm

Specific gravity 2.63

Table 3 : Fineness modules of proportioned coarse aggregate

IS Sieve

designation

Weight

retained on

sieve in gms(10 mm

aggregates)

Weight

retained on

sieve in gms(20 mm

aggregates)

Average

weight

retained

(gm)

Cumulative

weight

retained

(gm)

Cumulative

@age

weightretained

(gm)

%age

passing

80 mm 0.00 0.00 0.00 0.00 0.00 100.00

40 mm 0.00 0.00 0.00 0.00 0.00 100.00

20 mm 0.00 270 135 135 2.7 97.3

10 mm 2190 4710 3450 3585 71.7 28.3

4.75 mm 2780 20 1400 4985 99.7 0.30

2.36 30 0 15 5000 100 0

PAN - - - - - -



SiO2

AL2O3

Fe2O3

CaO

MgO

SO3

LoI

IR

Lime reactivity

57.41

26.92

5.21

2.05

0.71

0.085

1.18

93.12

59.50

5. Sieve analysis

Sieve no. %age of weight retained

100

140

200

270

Pan

0.00

31.35

26.51

24.72

17.42

Silica Fume

The silica fume used was obtained from Orkla India (Pvt) Ltd (Brand name: Elkem Microsilica 920-

D), Navi Mumbai. Its chemical composition and other properties are given in Table 6.

Table 6 : Physical properties of silica fume

S. No. Characteristics Values

1 Specific gravity 2.26

2. Color Grey

3.

Chemical compositionSiO2

AL2O3

Fe2O3

CaO

MgO

K2O

Na2O

93.800

0.206

0.096

0.426

0.222

0.337

0.107

Super Plasticizer

The super plasticizer used in the study program was Rheobuild SPI obtained by Basf construction

chemicals (India) Pvt. Ltd., Navi Mumbai. It was based on Naphthalene formaldehyde polymer.

The physical and chemical properties of super plasticizer, which was obtained from the company,

conform to IS-9103-1979 and are given in Table 7.

Table 7 : Properties of super plasticizer Rheobuild-SP1

S.

No. Parameter

Specifications (As per IS

9103)

Properties of Rheobuild

SPI

1. Physical state Dark brown free flowing liquidDark brown free flowing

liquid

2.

Chemical name of active

ingredient

Naphthalene formaldehyde

polymers

Naphthalene formaldehyde

polymers



3. Relative density at 250C 1.15 ± 0.02 1.151

4. pH Min 6 7.34

5. Chloride ion content (%) Max 0.2 0.0010

6. Dry material content 32 ± 5 (%) 32.04

7. Ash content 8 ± 5 (%) 8.01

Mix Design by Indian Standard Recommendations

Present investigation includes design of concrete mix (non-air entrained) for medium strength

concrete. The guidelines given in various codes like SP: 23-1982, IS: 10262-1982 and IS: 456-

2000 have been adopted for mix design of concrete.

Table 8 : Quantities per cubic meter for trial mixes with compressive strength

Mix

No.

Water

cement

ratio

Cement

(kg)

Sand

(Kg)

Coarse

aggregate

(Kg)

Average cube

strength at 7

days (N/mm2)

Average cube

strength at 28

days (N/mm2)

1 0.32 579.375 467.58 1108.52 47.21 60.42

2 0.32 540 487.69 1156.27 44.0 58.56

3 0.32 500 508.15 1204.77 48.6 57.31

4 0.32 480 518.00 1229.00 34.85 54.71

5 0.32 450 533.75 1265.40 40.22 44.26

Table 9: Workability with the varying percentage of silica fume & flyash

%

fly ash

4%

silica fume

8%

silica fume

12%

silica fume Reference mix

0 0.870 0.852 0.845

0.9210 0.887 0.860 0.857

15 0.895 0.874 0.869

20 0.902 0.886 0.882

Table 10: Analysis of variance for various percentage of fly ash & silica fume for

compaction factor

Source/Treatment

Mean values of

compaction factor of

reference mix

Mean values of compaction

factor Critical

difference

(C.D.)

Silica

fume

4%

Silica

fume

8%

Silica

fume

12%

Compaction factor with 0

% flyash

0.920.870 0.852 0.845 0.0230

Compaction factor with

10 % flyash0.887 0.860 0.857 0.0224


15 % flyash0.895 0.874 0.869 0.0196


20% flyash0.902 0.886 0.882 0.0221



For the present investigation, it is required to have characteristic compressive strength 40 N

/mm2. the mean target strength is 49.24N/mm2 The compaction factor for the design mix is taken

as 0.9. The maximum size of aggregate is 20 mm (angular). Type of exposure is taken as

moderate and degree of quality control as very good.

Trial Mixes

The quantity of cement obtained after mix design i.e. 579.375 is much more than the maximum

range of cement i.e. 450 kg/m3 as specified in IS 456-2000. So five trial mixes were prepared and

average cube strength were obtained after 7 days & 28 days as given in Table 8.

Workability of Concrete

The compaction factor test was performed to see the effect of addition of silica fume and flyash on

concrete. The workability of reference and all other concrete mixes as detailed in Table 9 was

measured in terms of compaction factor test. It is observed that compaction factor lies between

0.845 to 0.92. Workability of concrete slightly improved with the addition of percentage of flyash

to all the percentage of silica fume. In the case of 4 % silica fume and at 0% level of flyash,

compaction factor was 0.87. With the addition of 10%, 15% and 20% of flyash, compaction factor

improved / increased to 0.887, 0.895 and 0.902 respectively. For 8% of silica fume & at 0% level

of flyash, compaction factor was 0.852. With the addition of 10%, 15% & 20% of flyash,

compaction factor improved to 0.86, 0.874 & 0.886 respectively. Similarly, for 12% of silica fume

and at 0% level of flyash, compaction factor was 0.845. With the addition of 10%, 15%, and 20%

of flyash, compaction factor improved to 0.857, 0.869 and 0.882 respectively. The improvement in

workability with the addition of flyash to the concrete can be explained on the basis of ball bearing

effect of spherical particles of flyash as spherical particles needs less water as compared to other

shapes. Probably, another factor contributing to the improvement in workability is increased

amount of paste in mix which in turn produces a lubricating effect on ingredients of concrete and

helps in achieving a free flowing concrete with closer packing of materials. Conversely, the

workability decreased with the addition of percentages of silica fume to all the percentages of

flyash. At 0% level of flyash, compaction factors were 0.87, 0.852 & 0.845 with the addition of

4%, 8% & 12% of silica fume respectively. At 10% level of flyash, compaction factors were 0.887,

0.860 & 0.857 with addition 4%, 8% & 12% of silica fume. At 15% level of flyash, compaction

factors were 0.895, 0.874 & 0.869 with the addition of 4%, 8%, & 12% of silica fume. Similarly, at

20% level of flyash, compaction factors were 0.902, 0.886 & 0.882 with addition 4%, 8% & 12%

of silica fume. The optimum value of compaction factor was at the replacement level of 24% i.e.

20% of flyash & 4% of silica fume by weight of cement. After the optimum level of replacement of

flyash & silica fume, if we still add silica fume corresponding to 20% of flyash, the compaction

factor starts decreasing. It is due to the fact that surface area is increased due to increased

fineness and greater amount of water is required to get a closer packing which results in decrease

in workability of concrete mixes at higher replacement levels. The variation of workability with

different %ages of flyash and silica fume is as shown in Figure 1.



Figure 1: Workability with the varying percentage of silica fume and flyash Statistical analysis

Effect of various %ages of silica fume and flyash on Workability.

The effect of various %age of silica fume and fly ash on workability was statistically significant at

5% level of significance. The values of critical difference and mean compaction factor are given in

Table 10.

Summary

The workability was determined using compaction factor test. The statistical analysis was applied

on values/results of workability of concrete. All the values/results were found statistically

significant.

From the experimental investigation, the following main conclusions can be drawn:

i. Low water cement ratios like 0.32 can be tried for producing a concrete for commercial

purposes but appropriate superplasticizer compatible with the materials are required to be used.

ii. Optimum level of replacements of cement by flyash obtained from Guru Hargobind Thermal

Plant Lehra Mohabat, Distt. Bhatinda is around 10% for producing medium range of

workability concrete.

iii. Optimum level of replacements of cement by silica fume is around 4% for producing

medium range of workability concrete.

iv. The combination of flyash and silica fume is capable of producing a medium range of

workability of concrete as partial replacement of cement. The optimum replacement levels

of flyash and silica fume are 20% and 4%. This optimum level of combination gave

maximum value of compaction factor i.e 0.902

v. As silica fume & superplasticizer are costly materials and it may not be economical to use

them. But when these materials are used with flyash (a waste), workability is likely to

improve as evident from the investigation carried out by the authors.

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potential benefits of flyash in attaining the workability of silica fume concrete

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