Silica fume (sf) is the byproduct of the manufacture of silicon for the computer industry. 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 pulversided 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.