Silica fume, or microsilica, is a condensed gas, the by-product of metallic silicon or ferrosilicon alloys produced by electric arc furnaces. (While both terms are correct, microsilica (MS) is a less confusing name.) The Canadian standard CAN/ CSA-A23.5-M86, Supplementary Cementing Materials, limits amorphous SiO2 to a maximum of 85% and oversize to 10%. Many MS contain more than 90% SiO2.
MS has an average diameter of 0.1 to 0.2 m, a particle size of about 1% that of portland cement. Because of this small size, it is not possible to utilize MS in its raw form. Manufacturers supply it either densified, in a slurry (with or without water-reducing admixtures), or pelletized. Either the densified or slurried MS can be utilized in concrete. The pelletized materials is densified to the point that it will not break down during mixing.
Because of its extremely small size, MS imparts several useful properties to concrete. It greatly increases long-term strength. It very efficiently reacts with the Ca(OH)2 and creates a beneficial material in place of a waste product. MS is generally used in concrete with a design strength in excess of 12,000 psi. It provides increased sulfate resistance to concrete, and it significantly reduces the permeability of concrete. Also, its small size allows MS to physically plug microcracks and tiny openings.
