Silica fume (sf) is a by-product of the manufacture of silicon metal and ferro-silicon alloys. The process involves the reduction of high-purity quartz (SiO₂) in electric arc furnaces at temperatures in excess of 2000°c. It is a very finely divided material, many hundreds of times finer than cement. It reacts with lime and water in a similar way to fly ash, also known as pulverised fuel ash (pfa), to produce a binder.

It is added to concrete mixes to increase compressive strength and reduce permeability.Typically it is used in a concentration of 5% to 10% of the weight of cement.

Concrete Society Technical Report 61, Enhancing reinforced concrete durability, gives information on the effects of silica fume on the rates of carbonation and chloride ingress.