Abstract Products characterised by X-ray diffraction peaks at 15.6, 13.1 and 10.8 Å, analogous to those formed in the alkali-aggregate reaction, have been prepared by reacting calcium hydroxide with solutions of potassium water glasses at temperatures between 50 and 90°C. The 13.1 Å phase is a dehydration product of the 15.6 Å phase. When prepared through solution, the crystals were in the form of laths with irregular ends. The 10.8 Å phase crystallised as plates which, like the alkali-aggregate reaction product, often occurred in the form of rosette-shaped clusters. Chemical compositions, X-ray powder data, scanning electron micrographs, plots of energy dispersive X-ray analysis, infra-red spectra and curves of simultaneous thermogravimetric and differential thermal analyses of the products are presented. The effects of various treatments on crystal stability are also given.