Silica fume-Definition,its production process,Its influence on various concrete such as fresh and hardened concrete , its mixing and curing process and also its available forms and real time examples of silica fume in projects in India and its benefits regarding productivity of economical and faster construction.

1. SILICA FUME BY,
M.MITHRA

2. CONTENTS
PRODUCTION COMPOSITION
INFLUENCE
ON VARIOUS
CONCRETES
BENEFITS
SILICA
FUME

3. WHAT IS SILICA FUME?
Byproduct of producing silicon metal or
ferrosilicon alloys
Very reactive artificial pozzolanic admixture
It is basically spherical in shape of size 1 micron
and diameter of about 0.1 micron.

4. SILICA FUME PRODUCTION PROCESS
Silicon metal and alloys are produced
in electric furnaces.
 The raw materials are quartz, coal,
and woodchips.
The smoke that results from furnace
operation is collected and sold as
silica fume, rather than being
landfilled.

5. SILICA FUME PRODUCTION
PROCESS
Raw
materials
Incineration
vapourizationcondensation
Silca fume
collected

6. SILICA FUME COMPOSITION
Silica
fume
Consists
of SIO2
1/100th of
cement
particle
Very reactive
because of
its fineness
Quality by –
ASTM C
1240 &
AASHTO M
307

7. SILICA FUME USED WITH WHAT &
STRENGTH ACHIEVED BY?
Silica fume + superplasticizer backbone of modern High
performance concrete.
Benefits of silica fume technology – “Concrete International”,1998 by
Michael Shydlowski, President, Master Builder, Inc states.
It should be realised that silica fume by itself, do not contribute to the
strength dramatically, although it does contribute to the strength property
by being very fine pozzolanic material and also creating dense packing
and pore filler of cement paste.
 The high strengths of high performance concrete containing silica fume
are attributable, to a large degree, to the reduction in water content
which becomes possible in the presence of high dose of superplasticizer
and dense packing of cement paste.

8. SILICA FUME EFFECTS AND IT’S
INDIAN SCENARIO
There is no virtue in avoiding silica fume if it is available and
economical, as its use simplifies the production of high performance
concrete and makes it easier to achieve compressive strengths in the
range of 60 to about 90 MPa.
 Gives higher strength.
Necessary ingredients for making high strength and high
performance concrete.
 In India, silica fume has been used very rarely.
Nuclear Power Corporation was one of the first to use silica fume
concrete in their Kaiga and Kota nuclear power projects.

9. REAL TIME EXAMPLES
Silica fume was used for one of the flyovers at Mumbai where,
for the first time in India 75 MPa concrete was used (1999).
At present, India is not producing silica fume of right quality.
Recently, Steel Authority of India has provided necessary
facilities to produce annually about 3000 tons of silica fume at
their Bhadravathi Complex. It appears that the quality of silica
fume produced by them needs upgradation.

10. Bandra-Worli
sea link project,
Mumbai

11. SILICA FUME PROPERTIES
Microsilica is initially produced as an ultrafine undensified
powder
At least 85% SiO2 content
Mean particle size between 0.1 and 0.2 micron
 Minimum specific surface area is 15,000 m²/kg
Spherical particle shape

12. AVAILABLE FORMS OF
SILICA FUME
Forms Bulk density
(Kg/m³)
Undensified forms 200–300
Densified forms 500-600
Micro pelletised forms 600-800
Slurry forms 1400

13. POZZOLANIC ACTION
More reactive
The reactivity of a pozzolana can be quantified by measuring
the amount of Ca(OH)2 in the cement paste at different times.
 In one case, 15% of microsilica reduced the Ca(OH)2 of two
samples of cement from 24% to 12% at 90 days and from
25% to 11% in 180 days.
Most research workers agree that the C – S – H formed by the
reaction between microsilica and Ca(OH)2 appears dense
and amorphous.

14. INFLUENCE ON FRESH CONCRETE
 Water demand increases in proportion to the amount of microsilica added.
(about 1% for every 1% of cement substituted)
 Therefore, 20 mm maximum size aggregate concrete, containing 10%
microsilica, will have an increased water content of about 20 litres/m³.
 Measures can be taken to avoid this increase by adjusting the aggregate
grading and using superplasticizers. The addition of microsilica will lead to
lower slump but more cohesive mix.
 The microsilica make the fresh concrete sticky in nature and hard to handle.
 It was also found that there was large reduction in bleeding and concrete
with microsilica could be handled and transported without segregation.

15. INFLUENCE ON FRESH CONCRETE
It is reported that concrete
containing microsilica is
vulnerable to plastic shrinkage
cracking and, therefore, sheet or
mat curing should be considered.
Microsilica concrete produces
more heat of hydration at the
initial stage of hydration. However,
the total generation of heat will
be less than that of reference
concrete.

16. INFLUENCE ON HARDENED
CONCRETE
 Concrete containing microsilica showed outstanding characteristics in the
development of strength.
 Modulus of elasticity of microsilica concrete is less than that of concrete
without microsilica at the same level of compressive strength.
 As regards, the improvement in durability aspects many published
reports, of this investigation carried out, indicate improvement in
durability of concrete with microsilica. There are some investigations
indicating contradiction, particularly with reference to resistance against
frost damage.
 Effective for alkali-aggregate reaction, but addition of silica fume in
small quantities actually increases the expansion.

17. MIXING OF SILICA FUME
 By far the most popular application of microsilica is in the 50 : 50 slurry
form; as it is easy to store and dispense.
 The work by Hooton among others showed that, for equivalent
microsilica additions, slurry produced significantly higher compressive
and tensile strengths.
 The slurry needs to be kept agitated for a few hours in a day to avoid
gelling and sedimentation.
 Presently in India, Mc-Bauchemie (Ind) Pvt. Ltd., supply the silica fume
slurry under the trade name “Centrilit Fumes”.

18. CURING OF SILICA FUME
Curing is probably the most important aspect of microsilica
concrete as the material undergoes virtually zero bleeding.
If the rate of evaporation from the surface is faster than the
rate of migration of water from interior to the surface, plastic
shrinkage takes place.
In the absence of bleeding and slow movement of water from
interior to the surface, early curing by way of membrane
curing is essential.

20. USES OF SILICA FUME
High-strength concrete, economical material for carrying
vertical loads in high-rise structures.
Until a few years ago, 6,000 psi concrete was considered
to be high strength.
 Today, using silica fume, concrete with compressive
strength in excess of 15,000 psi can be readily produced.
The structure shown at the above right used silica-fume
concrete with a specified compressive strength of 12,000
psi in columns reaching from the ground through the 57th
story.

21. USES OF SILICA FUME
The greatest cause of concrete deterioration in
the US today is corrosion induced by deicing or
marine salts.
Silica-fume concrete with a 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.

22. CONCLUSIONS
Cement replacement upto 10% with silica fume leads to
increase in compressive strength.
The maximum replacement level of silica fume is 10 % for
M30 grade of concrete.
Due to the use of the micro silica in a OPC concrete the life of
that concrete is increased 4 to 5 times than the OPC
concrete.
The use of micro silica in high strength concrete leads to
economical and faster construction.

23. REFERENCES
Shetty M.S., Concrete Technology, S.Chand and Company
Ltd. Delhi, 2003.
Silica Fume Association(SFA)