Microsilica increases the strength and durability of concrete by increasing its uniform distribution, increasing the volume of hydration products, as well as decreasing the average size of pores in the cement paste.
Silica fume, also known as microsilica or condensed silica fume, is a mineral admixture of very fine glassy spheres of silicon dioxide. This material is produced by high-temperature electric arc furnaces during the manufacturing process of ferrosilicon and metallic silicon.
In addition, Microsilica reduces carbonation and chloride permeability. Thus, microsilica concrete protects reinforcements and embedments strongly from aggressive agents.
Condensed silica fume consists primarily of pure silica in non-crystalline form and contains more than 90% silicon dioxide (Detwiler et al., 1996). Small amounts of magnesium, iron, and alkali oxides are also present. Most particles are smaller than 1 mm with an average diameter of 0.1 mm. ASTM C 1240 limits the percent retained on the 45-mm sieve to 10%, the moisture content to 3%, and the loss on ignition (LOI) to 6%.
A substitute for portland cement is condensed silica fume. Portland cement can be substituted by silica fume at a rate of 9-15%.
Using silica fume concrete creates the following advantages:
Because of its high surface area, condensed silica fume multiplies the amount of water needed in the mix, so it is best to use it in conjunction with a water-reducing admixture. During the first 7 days, it must be protected from drying shrinkage from silica fumes/portland cement. Using condensed silica fume with cement requires it to be thoroughly dispersed.It is costly to use silica fume in cement. Comparatively to Portland cement ($90/ton), condensed silica fume costs $400 to $1000/ton.This alternative cementing material, or any alternative cementing material, should be used with portland cement only after creating test mixtures to determine proper proportions for the expected properties.
It is a dry, purified, micronised, high performance pozzolanic mineral admixture formulated to produce concrete with special performance qualities. Microsilica or fumed silica is used world wide in concrete to ensure better resistance to chloride, sulphate, chemical/ acid attacks as well as to produce high grade concrete