AbstractThe ‘wet’ carbonatation of calcium hydroxide has been compared at 22° and 95° to determine the effect of temperature on the rate of formation, solubility and subsequent rate of ageing of the products, as indicated by changes in lattice structure and in surface area and average crystallite size. Direct precipitation of calcium carbonate from solution by double decomposition has been similarly compared, the conversion of the aragonitic and vateritic products to calcite being studied in solution at 95° and 22°, and in the solid state at temperatures up to 450°. Partly‐carbonatated lime has been hydrated to assess the extent to which the carbonate layer around each lime crystal inhibits further hydration.In ‘wet’ carbonatation at lower pH levels (8–12), especially in the presence of bicarbonate ions, the calcium carbonate layer developed around each lime particle ages more rapidly at the higher temperature, becoming almost non‐porous before carbonatation is complete; at higher pH levels, the carbonate is more porous, and splits off more readily. Similar ageing of calcite inhibits the conversion of aragonite to calcite. In highly alkaline media, at pH 13–14, the conversion of both aragonite and of vaterite to calcite are inhibited at 95°, but occurs readily in the solid state above 400° with considerable loss of surface area, vaterite also sintering extensively at 300°. When partly‐carbonatated lime is hydrated, the presence even of thin carbonate layers reduces the splitting of the lime particles, and the capacity of the carbonate to age afterwards partly