Interaction of silica fume with calcium hydroxide solutions and hydrated cement pastes

Abstract

The interactions of silica fume (SF) with calcium hydroxide and with Portland cement have been studied using microscopy-based techniques and x-ray diffraction. Digestion of SF in calcium hydroxide solution for up to 120 days showed it to persist in the form of agglomerates coated with amorphous calcium silicate hydrates. Similar SF microstructures were found in cement pastes hydrated for up to 180 days, including those produced by very high shear mixing. These results show that SF persists in cements long after the principal hydration reactions have ceased. They also demonstrate the difficulty in obtaining satisfactory dispersion of SF in cement, due to the high agglomerate strength of the SF. It is the behavior of these agglomerates, rather than that of the individual SF particles, which controls the pozzolanic activity.