Silica Fume | Its influence on fresh and hardened concrete

What is Silica Fume:

• Silica fume, also referred to as microsilica or condensed silica fume, is another material that is used as an artificial pozzolanic admixture.
• It is a product resulting from reduction of high purity quartz with coal in an electric arc furnace in the manufacture of silicon or ferrosilicon alloy.
• Silica fume rises as an oxidised vapour. It cools, condenses and is collected in cloth bags.
• It is further processed to remove impurities and to control particle size. 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.

Properties of Silica Fume:

• It is extremely fine with particle size less than 1 micron and with an average diameter of about 0.1 micron, about 100 times smaller than average cement particles. Silica fume has specific surface area of about 20,000 m2/kg against 230 to 300 m2/kg.
• Silica fume has become one of the 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.
• 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.

Silica fume | Influence 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.

Silica fume | Influence on Hardened Concrete

• Concrete containing microsilica showed outstanding characteristics in the development of strength. It has been also found out that modulus of elasticity of microsilica concrete is less than that of concrete without microsilica at the same level of compressive strength.