Effect of calcium on dissolution and precipitation reactions of amorphous silica at high alkalinity

A better understanding of silica dissolution-precipitation reactions at high pH aqueous solutions allows for promotion of favorable (e.g.; pozzolanic) reactions and mitigation of deleterious (e.g.; alkali-silica) reactions in concrete. In this paper, the kinetics and products of silica glass dissolution are studied as a function of solution pH, temperature, and availability of calcium. It was observed that dissolution rate versus time increases linearly with pH and reaches a maximum at pH = 14, with slower dissolution at higher alkalinities. In solutions with similarly high pH, but saturated with portlandite, glass dissolution is significantly slower. This is due to formation of a dense, low porosity, and strongly bonded C-S-H layer on the surface of glass, which serves as a barrier against diffusion of OH^- and alkali ions towards the substrate glass. This protective layer forms only when Ca is abundant and portlandite saturation can be maintained on a local scale.