Comparative study of effect of raw and densified silica fume in the paste, mortar and concrete

The differences of effect of silica fume in paste, mortar and concrete were studied by determining the non-evaporable water content of pastes, measuring the compressive strengths of pastes, mortars and concretes containing 5% and 10% raw silica fume or densified silica fume with water-to-binder ratios (W/B) of 0.29 and 0.24 and investigating the properties of interfacial transition zone between hardened paste and aggregate. The results show that silica fume can significantly increase the hydration degree of paste. The addition of silica fume trends to increasing the compressive strengths of hardened pastes, mortars and concretes, and the strength activity index of densified silica fume in concrete is the highest while that in paste is the lowest. The agglomeration of silica fume has been found in blended paste which is hardly seen in concrete. The silica fume can improve the interface bond strength between hardened cement paste and aggregate. The crystalline orientation degree, the crystalline size and the content of calcium hydroxide at the interface are obviously decreased by adding silica fume. The different dispersion and the improvement of the interfacial transition zone are the main factors causing the different role of silica fume in paste, mortar and concrete.