The role of microsilica in the alkali-aggregate reaction

Mortars containing as reactive aggregates Pyrex, cristobalite and Thames valley sand have been modified by inclusion of microsilica. Their expansivity is correlated with pore fluid analyses, mercury intrusion porosimetry and microstructure, determined by electron microscopy. Afler 180 days, the chemistry of the pore fluid becomes very similar in all mortars made with reactive aggregates. Much transport of silica away from the aggregate occurs and is believed to exert a levelling effect on pore fiuid alkali contents. Microsilica curbs expansion over the first six months, but it is concluded that considerahle potential for further expansion exists. The presence of microsilica as well as a reactive aggregate decreases the effective water/reactive solid (cement + pozzolan) ratio, with the result that the mortars undergo self-dessication and cease to expand. In real situations, where water uptake may occur, dessication is unlikely to occur; differences in the availahility of water constitutes an important distinction between laboratory simulations and ‘real-life’ materials.