Silica fume, a by-product in the manufacture of ferrosilicon and also of silicon metal, is a very efficient pozzolanic material, though there are some problems associated with its use in concrete. The use of silica fume in concrete should be limited to specialized applications that can take full advantage of its unique properties. The chief problems in using this material are associated with its extreme fineness and high water requirement when mixed with Portland cement.


As an admixture, small quantities of silica fume, 5 to 10 percent by weight of cement, can be added to concrete. The resulting loss in slump is compensated for either by the addition of more water or the use of superplasticizers. In either case, there is a marked increase in compressive strength as compared with the control mix. This is particularly so with the use of superplasticizer. Also, silica fume can be used as a partial replacement for cement. The percentage replacement may vary from 0 to 30 percent. Though this does not change the weight of the cementitious materials, there is an increase in the water demand because of the extreme fineness of silica fume. In order to maintain the same water- (cement plus silica fume) ratios, superplasticizers are used to maintain the required slump. This approach also results in an increase in compressive strength at the age of 3 days and thereafter.

Silica fume is a highly efficient pozzolanic material and has a considerable potential for use in concrete. Because of the relatively limited supply of the material, it is recommended that it be used judiciously to solve specific problems and to make specialized products. Its indiscriminate use in concrete is not recommended.