Microsilica

Microsilica, commonly known as silica fume, is a highly reactive, ultrafine form of silicon dioxide (SiO2) produced as a byproduct of silicon metal or ferrosilicon alloy manufacturing. It consists of spherical particles typically 0.1 to 0.3 micrometers in diameter and exhibits a very high specific surface area, which causes a tendency to agglomerate if not properly dispersed in a cementitious mix.

Used as a pozzolanic material in cement and concrete, microsilica reacts with calcium hydroxide released during cement hydration to form additional calcium silicate hydrate (C-S-H). This reaction refines the pore structure, reduces capillary porosity, and improves strength and durability, including resistance to chloride ingress, sulfate attack, alkali-silica reaction, and freeze–thaw cycles.

In concrete, microsilica is typically added as a partial replacement for cement or as a highly active supplementary cementitious material. Typical dosage ranges are about 5–15% by mass of cement, with higher dosages possible in high-performance concretes when using appropriate superplasticizers to maintain workability. Excessive dosages without proper dispersion can lead to reduced workability or increased shrinkage if curing is inadequate.

Handling considerations include ensuring thorough dispersion to avoid dry clumps, using admixtures such as superplasticizers, and following safety guidelines for dust control. Microsilica is widely used in precast, shotcrete, repair mortars, and durable concrete applications, and, as a byproduct of industrial silicon production, can contribute to resource efficiency when replacing cement.