The chemistry of ‘alkali-aggregate’ reaction

Abstract To clarify ideas about the alkali aggregate reaction, the reaction between sodium hydroxide and various forms of silica has been studied. Variations with time in the sodium, hydroxyl and silica concentrations in solution have been followed and the volume change of solid silica immersed in solution has been measured. If the silica is an absorbant and reactive form such as silica gel, there is an immediate drop in both sodium and hydroxyl concentrations. Subsequently, as the silica dissolves, sodium concentration rises again. Both the final concentration of dissolved silica and the volume expansion of solid silicate pass through a maximum at an intermediate total Si0 2 /Na 2 0 mole ratio.