Effect of Silica Fume on Workability and Compressive Strength of OPC Concrete

Abstract

Silica fume is a by product resulting from the reduction of high – purity quartz with coal or coke and wood chips in an electric arc furnace during the production of silicon metal or silicon alloys. Silica fume is known to improve the mechanical characteristics of concrete. The principle physical effect of silica fume in concrete is that of filler, which because of its fineness can fit into space between cement grains in the same way that sand fills the space between particles of coarse aggregates and cement grains fill the space between sand grains. As for chemical reaction of silica fume, because of high surface area and high content of amorphous silica in silica fume, this highly active pozzolan reacts more quickly than ordinary pozzolans. The use of silica fume in concrete has engineering potential and economic advantage. This paper presents the results of an experimental investigations carried out to find the suitability of silica fume in production of concrete. It is observed that the optimum dose of silica fume is 5% (by weight), when used as part replacement of OPC. The silica fume inclusion increases the workability and strength of concrete considerably.

Full text

J. Environ. Nanotechnol.
Volume 3, No.3 pp. 32-35
ISSN (Print): 2279-0748
ISSN (Online): 2319-5541
doi: 10.13074/jent.2014.09.143086
* Vikas Srivastava Tel.: +919415369170
E-mail: vikas_mes@rediffmail.com
Effect of Silica Fume on Workability and Compressive Strength of
OPC Concrete
Vikas Srivastava
1*
, Rakesh Kumar
2
, V. C. Agarwal
3
and P. K. Mehta
4
1
Civil Engg. Department, SHIATS (Formerly AAI-DU), Allahabad, India
2
Civil Engg. Department, MNNIT, Allahabad, India
3
Civil Engg. Department, SHIATS (Formerly AAI-DU), Allahabad, India
4
Civil Engg. Department, MNNIT, Allahabad, India
Received: 08.05.2014 Accepted: 19.08.2014
Abstract
Silica fume is a by product resulting from the reduction of high – purity quartz with coal or coke
and wood chips in an electric arc furnace during the production of silicon metal or silicon alloys. Silica
fume is known to improve the mechanical characteristics of concrete. The principle physical effect of
silica fume in concrete is that of filler, which because of its fineness can fit into space between cement
grains in the same way that sand fills the space between particles of coarse aggregates and cement grains
fill the space between sand grains. As for chemical reaction of silica fume, because of high surface area
and high content of amorphous silica in silica fume, this highly active pozzolan reacts more quickly than
ordinary pozzolans. The use of silica fume in concrete has engineering potential and economic
advantage. This paper presents the results of an experimental investigations carried out to find the
suitability of silica fume in production of concrete. It is observed that the optimum dose of silica fume is
5% (by weight), when used as part replacement of OPC. The silica fume inclusion increases the
workability and strength of concrete considerably.
Keywords: Silica fume; pozzolan; compressive strength; OPC.
1. INTRODUCTION
By addition of some pozzolanic materials, the
various properties of concrerte viz, workability,
durability, strength, resistance to cracks and
permeability can be improved. Many modern concrete
mixes are modified with addition of admixtures,
which improve the microstructure as well as decrease
the calcium hydroxide concentration by consuming it
through a pozzolanic reaction. The subsequent
modification of the microstructure of cement
composites improves the mechanical properties,
durability and increases the service-life properties.
When fine pozzolana particles are dissipated in the
paste, they generate a large number of nucleation sites
for the precipitation of the hydration products.
Therefore, this mechanism makes paste more
homogeneous. This is due to the reaction between the
amorphous silica of the pozzolanic and calcium
hydroxide, produced during the cement hydration
reactions (Sabir et al., 2001, Antonovich and Goberis,
2003, Rojas and Cabrea, 2002).
In addition, the physical effect of the fine
grains allows dense packing within the cement and
reduces the wall effect in the transition zone between
the paste and aggregate. This weaker zone is
strengthened due to the higher bond development
between these two phases, improving the concrete
microstructure and properties. In general, the
pozzolanic effect depends not only on the pozzolanic
reaction, but also on the physical or filler effect of the
smaller particles in the mixture. Therefore, the
addition of pozzolans to OPC increases its mechanical
strength and durability as compared to the referral
paste, because of the interface reinforcement. The
physical action of the pozzolanas provides a denser,
more homogeneous and uniform paste. Silica fume is
a by product resulting from the reduction of high –
purity quartz with coal or coke and wood chips in an

Vikas Srivastava et al. / J. Environ. Nanotechnol., Vol. 3(3), 32
-35, 2014
elec
tric arc furnace during the production of silicon
metal or silicon alloys. Silica fume is known to
improve both the mechanical characteristics and
durability of concrete. The principle physical effect of
silica fume in concrete is that of filler, which because
of its fineness can fit into space between cement
grains in the same way that sand fills the space
between particles of coarse aggregates and cement
grains fill the space between sand grains. As for
chemical reaction of silica fume, because of high
surface area and high content of amorphous silica in
silica fume, this highly active pozzolan reacts more
quickly than ordinary pozzolans.
The use of silica fume in concrete has
engineering potential and economic advantage. It is
reported by most researchers
(Gafoori and Hamidou,
2007, Yogendran et al., 1987, Khayat et al., 1997,
Ramakrishnan and Srinivasan, 1982, Bayasi, 1993)
that workability is reduced on silica fume inclusion
however Kadri and Dual reported increase in
workability on replacement of cement by silica fume.
It is also reported (Gafoori and Hamidou, 2007,
Yogendran, 1987, Khayat, 1997, Ramakrishnan and
Srinivasan, 1982, Kadri and Dual, 1998) that
compressive strength is increased upto optimum
replacement level of silica fume. Strength of silica
fume concrete is affected by several factors viz. type
of cement, quality and proportion of silica fume and
curing temperature. The main contribution of silica
fume to concrete strength development at normal
curing temperature takes place from about 3 to 28
days. The contribution of silica fume to strength
development after 28 days is minimal (Sakr, 2006).
Bhanja and Sengupta (2003) reported that
inclusion of silica fume in the range of 5 – 25%
increases compressive strength in the range of 6.25 –
29.85% for water cement ratio between 0.26 - 0.42.
Sakr, 2006 reported that at 15% silica fume content
gravel concrete, barite concrete and ilmenite concrete
showed increased compressive strength by 23.33%,
23.07% and 23.52% respectively at 7 days, 21.34%,
20% and 22.58% respectively at 28 days, 16.5%,
18.7% and 22% respectively at 56 days and 18%,
7.14% and 22.80% respectively at 90 days.Dual and
Kadri (1998) reported that at 10% replacement level
compressive strength increased in the range of about
10 – 17 % at different water cement ratio(0.25-0.45).
Khayat et al (1997) reported that at 7.5% replacement
level compressive strength increased in the range of
about 10 – 17 % at different water cement ratio (w/c).
Babu and Prakash (1995)
reported that concrete with
silica fume even upto 40% replacement show strength
higher than that of the control concrete. The
improvements in strength at the different percentages
of replacement of replacement at any water cement
ratio were also varying over a wide range. Khan and
Ayers (1995)
reported 67% increase in compressive
strength at 10% replacement level and 0.38 w/c.
Cong et. al(1992) reported that concrete
containing silica fume as a partial replacement of
cement exhibits an increased compressive strength in
large part because of the improved strength of its
cement paste constituent. Slaniska and Lamacska
(1991) reported that at different replacement level of
cement by silica fume (3.75 – 10.25%) increase in
compressive strength in the range of about 12% - 57%
is observed. Detwiler and Mehta (1989) reported that
silica fume concrete showed improved compressive
strength in the range of 11.56% - 18.89%than the
conventional concrete at different water cement ratio.
In the present study an experimental
programme was conducted to investigate the
suitability of silica fume as partial replacement of
cement and its effect on the compressive strength and
workability of concrete. The referral concrete M
25
was made using 43 grade OPC (Birla) and the other
mixes were prepared by replacing part of OPC with
silica fume. The replacement levels were 5%, 10%,
15%, 20%, 25% and 30% (by weight). This paper
presents the results of this investigation.
2. MATERIAL AND METHODS
For the present investigation, the coarse
aggregate of size 12.5 mm and down from Bharatkup
quarry was used. The sieve analysis of the aggregates
was carried out and the same distribution / FM was
maintained throughout the experiment. The important
properties of the coarse aggregate were: Fineness
Modulus = 6.29; Flakiness index = 20% (< 40% Ok
BS 882-1992); Elongation Index = 7%; Moisture
content = 0.52% (<2% Ok); Crushing value = 18.2%
(<30 Ok); Specific gravity = 2.72 (2.6-2.8).
The fine aggregate used in the investigation
was ‘Jamuna’ sand. The properties of fine aggregate
found as per IS-383 were: Fineness Modulus = 2.5;
Moisture content = 0.52% (<2% Ok); Specific gravity
= 2.54. The gradation of fine aggregate (Zone III) was
maintained throughout the experiment.
Silica fume for the present investigation was
obtained from M/s ELCOM Enterprises, Mumbai. The
silica fume was sieved and the fraction passing 100µ
IS sieve was used in the experiments. The physical
and chemical properties of silica fume viz-a-viz, OPC
are presented in table 1. The binder used in the present
investigation was 43 grade OPC (Birla). The
properties of cement were determined in accordance
with IS – 8112: 1989 were: Fineness = 6.8% (<10%
33

Vikas Srivastava et al. / J. Environ. Nanotechnol., Vol. 3(3), 32-35, 2014
Ok); Consistency = 31%; Initial Setting Time = 60
minutes (>30 minutes Ok) ; Final Setting Time = 480
minute (<600 minutes Ok).
Table 1. Physical and Chemical Properties of Silica
Fume
Properties
OPC
Silica
Fume
Physical
Specific gravity
3.1
2.2
Mean grain size
(μm)
22.5
0.15
Specific area
cm
2
/gm
3250
150000-
300000
Colour
Dark
Grey
Light to
Dark
Grey
Chemical compositions (%)
Silicon dioxide
(SiO
2
)
20.25
85
Aluminium oxide
(Al
2
O
3
)
5.04
1.12
Iron oxide (Fe
2
O
3
)
3.16
1.46
Calcium oxide
(CaO)
63.61
0.2-0.8
Magnesium oxide
(MgO)
4.56
0.2-0.8
Sodium oxide
(Na
2
O)
0.08
0.5-1.2
Potassium oxide
(K
2
O)
0.51
Loss on ignition
3.12
<6.0
For the present investigation, mix design for
M
25
grade of concrete (Target strength = 31.6 MPa)
was carried out using the above coarse aggregate, fine
aggregate, and the binder. The proportion of the
materials by weight was 1:1.89:2.17:0.48 (Cement:
Fine aggregate: Coarse aggregate: Water). To
investigate the effect of silica fume inclusion (as part
replacement of cement), 100 mm cubes were cast for
referral and other mixes having variable silica fume
content. The cement was replaced by silica fume at the
rate of 5, 10, 15, 20, 25, 30 and 35% (by weight). The
workability (Slump value) and the compressive
strength of different mixes were tested at 7 and 28
days as per the procedure laid down in IS: 516 - 1981.
The results obtained from the above investigation are
presented below.
3. RESULTS
The compressive strength of the cubes at
different ages and different silica fume content are
presented in Fig - 1. The slump values and compaction
factor of the different mixes are presented in table - 2.
Table 2. Variation of Slump and Compaction
Factor of different Mixes
Replacement
level (%)
Slump
(mm)
Compaction
Factor
0 (Referral)
25
0.81
5
28
0.82
10
30
0.83
15
30
0.83
20
32
0.84
25
34
0.85
30
37
0.86
35
42
0.87
Fig 1 reveals that optimum replacement level
of cement by silica fume is 5%. At 5% replacement
level the strength of silica fume concrete improved by
12.5% and 18.18% at 7 days and 28 days respectively
as compared to the referral concrete. At all other
replacement levels the strength of silica fume concrete
is lower than the referral concrete, however, the
workability is marginally improved at all replacement
levels. It is reported in the literature that inclusion of
silica fume (5 – 40%) increases the strength in the
range of 6.25 – 67%. In our research work in which
silica fume was included between 5 – 35%, the
increase in strength is observed by 18.18%. The
strength improvement due to silica fume incorporation
in concrete occurs due to chemical and physical
processes, the chemical effect due to the pozzolanic
activity and the physical effect due to the microfiller
action. However, decrease in strength is due to the
reason that silica fume added in excess of that required
for pozzolanic and filler actions results in replacement
of primary binder, that is cement, and hence reduction
in strength.
Fig. 1: Variation of compressive strength with
replacement level of OPC by silica fume.
0
5
10
15
20
25
30
35
40
45
0 5 10 15 20 25 30 35
R e pla c e m e nt L evel (%)
C om pres s ive S treng th (MP a)
7 D ays
28 Days
34

Vikas Srivastava et al. / J. Environ. Nanotechnol., Vol. 3(3), 32
-35, 2014
4
. CONCLUSIONS
The following conclusions are derived on the
use of silica fume in concrete making.
1. The optimum replacement level of
cement by silica fume is found to be 5%
by weight.
2. There is a significant improvement in the
compressive strength of concrete using
silica fume at both 7 and 28 days as
compared to the referral concrete.
3. The workability in case of silica fume
concrete is marginally improved.
4. Beyond optimum silica fume level the
strength decreases but the workability
increases.
ACKNOWLEDGEMENT
Authors of this paper express their sincere
appreciation to the M/s Elkem Enterprises, Mumbai
and Mr. S. Singh of MNNIT, Allahabad for his
support.
REFERENCES
ACI, 234R-96, Guide for the Use of Silica Fume in
Concrete", Reported by ACI, Committee, 234, 1-
51 (1996).
Antonovich V. and Goberis S., The effect of different
admixtures on the properties of refractory
concrete with Portland cement, Mater Sci, 9, 379-
383(2003).
Bhanja, Shantanu, and Sengupta, Bratish "Optimum
Silica Fume content and its Mod of Action on
Concrete, ACI Materials Journal, 100(5), 407-
412(2003).
Bayasi, Z. and Zhou, J., "Properties of Silica Fume
Concrete and Mortar", ACI Materials Journal,
90,(4), 349 – 356 (1993).
Cong, X., Gong, S., Darwin, David and McCabe, S.L.,
"Roll of Silica Fume in Compressive Strength of
Cement Paste, Mortar and Concrete." ACI
Materials Journal, 89(4), 375-387(1992).
Detwiler, R. J. and Mehta, P. K., Chemical and
Physical Effects of Silica Fume on the
Mechanical Behaviour of Concrete, ACI
Materials Journal, 86 (6), 609-614 (1989).
Gafoori, Nader and Diawareand, Hamidou, Strength
and Wear Resistance of Sand -Replaced Silica
Fume Concrete, ACI Materials Journal, 104(2),
206-214 (2007).
Ganeshbabu, K. and Suryaprakash, P. V., Efficiency
of Silica Fume in Concrete Cement and Concrete
Research, 25(6), 1273-1283 (1995).
doi:10.1016/0008-8846(95)00120-2
Kadri, E. H. and Duval, R., Influence of Silica
Fume on the Workability and Compressive
Strength of High Performance Concrete Cement
and Concrete Research, 28(4), 533-547 (1998).
doi:10.1016/S0008-8846(98)00010-6
Khan, S. M. and Ayers, M. E., Minimum Length of
Curing of Silica Fume Concrete, Journal of
Materials in Civil Engineering, 7(3), 134-139
(1995).
doi:10.1061/(ASCE)0899-1561(1995)7:2(134)
Khayat, K. H., Vachon, M. and Lanctot, M. C., Use of
Blended Silica Fume Cement in Commercial
Concrete Mixtures, ACI Materials Journal, 94(3),
183-192 (1997).
Ramakrishnan, V and Srinivasan, V., Silica Fume in
Fibre Reinforced Concrete, Indian Concrete
Journal, 56(12), 326-334 (1982).
Rojas M. F., Cabrea J., The effect of temperature on
the hydration phase of metakaolin – lime-water
system, Cem. Concr. Res, 32, 132-138(2002).
doi:10.1016/S0008-8846(01)00642-1
Sabir B. B., Wild S. and Bai J., Metakaolin and
calcined clays as pozzolona for concrete – A
review, Cem. Concr. Composites, 23, 441 – 454
(2001).
Sakr K., Effects of silica fume and Rice husk Ash on
the Properties of Heavy Weight Concrete"
Journal of Materials in Civil Engineering, 18(3),
367 -376 (2006).
Slanicka, Stefan and Lamacska, B., The Influence of
Condensed Silica Fume on the Concrete Strength
Cement and Concrete Research 21(4), 462-470
(1991).
doi:10.1016/0008-8846(91)90094-X
Yogendran, Y., Langan, B. W., HaqueM. N. and
Ward, M. A., Silioca Fume in High Strength
Concrete, ACI Materials Journal, 84(2), 124-129
(1987).
35