Silica fume pozzolanic activity

Silica fume. Silica fume (SF) is a waste product of manufacturing ferro-sihcon alloys. It consists of an extremely fine powder of amorphous silica. Average particle diameter is about 100 times smaller than that of Portland cement and the specific surface area is enormous 13000-30000 m /kg compared to 300-400 m /kg for common Portland cements. Silica fume shows an elevated pozzolanic activity and is also a very effective filler. For these reasons, addition of silica fume to Portland cement may lead to a very low porosity of the cement paste, increasing the strength and lowering the permeability. It is usually added in the proportion of 5 to 10 % and it is combined with the use of a superplasticizer in order to maintain adequate workability of the fresh concrete. 

In general, results indicate that silica fume, rice husk ash and glass type 11 have an appropriate potential to apply as a part replacement in cement due to their respective pozzolanic activity index values (according to ASTM C618 and C989, and Table 1) and the high pozzolanic characteristics of these materials have an effective impacts on reduction of expansion caused by ASR. 

The pozzolanic activity of silica fume can be demonstrated in the systems Cement-Silica Fume-H20 by estimating the amount of lime in the hydrated product at different times by thermogravimetric analysis. 

Nocun-Wczelik, W., The Hydration of Tricalcirrm Silicate with Active Silica, 4 CANMET/ACl Int. Conf. on Fly Ash, Silica Fume, Slag and Natural Pozzolans in Concr., Istanbul, Turkey, supplemerrtary papers, pp. 485 92 (1992)... 

---