How Micro Silica works | Silica Fumes | Micro Silica hydration CSH Gel formation | Elkem metallurgy

Silica fume is very fine pozzolanic material composed of ultra fine, amorphous glassy sphere (average diameter,
0.10-0. 15~m) of silicon dioxide (Si02) produced during the manufacture
of silicon or ferro-silicon
by electric
arc furnaces at temperature
of over 2 000”C. The micro silica is formed when SiO gas produced in the furnace
mixes with oxygen, oxidizes to Si02, condensing into the pure spherical particles of micro silica that form the
major part of the smoke or fume from the furnace. These fumes are collected and bagged called silica fume. It is
also known as condensed
silica fume and micro silica.
Condensed silica fume
is essentially silicon dioxide (more than 90%) in noncrystalline form. Since it is an airborne
material like fly ash, it has spherical shape. about 100 times smaller than
average cement particles. Silica fume has specific surface area of about 20,000 m2/kg, as
against 230 to 300 m2/kg. Silicafumes simplifies the production of high performance concrete and makes it easier to achieve
compressive strengths in the range of 60 to about 90 MPa. For higher strengths, the use of
silica fume is essential.”
Indian Scenario: 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.
Silica fume was also used for one of the flyovers at Mumbai where, for the first time in
India 75 MPa concrete was used (1999). Silica fume is also now specified for the construction
of proposed Bandra-Worli sea link project at Mumbai.
At present, India is not producing silica fume of right quality. In India, however, the silica fume of international quality is marketed by Elkem Metallugy
(P) Ltd., 66/67, Mahavir Centre, Sector 17, Vashi, Navi Mumbai-400 703.
Since silica fume or microsilica is an important new material, let us see this material in
some detail.
"
microsilica is initially produced as an ultrafine undensified powder
"
at least 85% SiO2 content
"
mean particle size between 0.1 and 0.2 micron
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minimum specific surface area is 15,000 m2/kg
"
spherical particle shape.
Available forms
Microsilica is available in the following forms:
"
Undensified forms with bulk density of 200-300 kg/m3
"
Densified forms with bulk density of 500-600 kg/m3
"
Micro-pelletised forms with bulk density of 600-800 kg/m3
"
Slurry forms with density 1400 kg/m3.
Standard grade slurry pH value 4.7, specific gravity 1.3 to 1.4,
Pozzolanic Action
Microsilica is much more reactive than fly ash or any other natural pozzolana. 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.
nfluence on Fresh Concrete
Water demand increases in proportion to the amount of microsilica added. The increase
in water demand of concrete containing microsilica will be 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.
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.
Presently in India, Mc-Bauchemie (Ind) Pvt. Ltd., supply the silica fume slurry
under the trade name “Centrilit Fumes”.

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How to make micro silica