AbstractSamples of calcium oxide and calcium hydroxide have been carbonatated under ‘dry’ conditions using carbon dioxide gas, and under ‘wet’ conditions using carbon dioxide dissolved in water or in alkaline media, viz., ammonium or sodium carbonates and bicarbonates. The progress of these carbonatations has been followed by determining changes in surface area and lattice structure, and the surface properties of the final products have been compared with those of calcium carbonate precipitated from solution at similar pH levels.In either ‘dry’ or ‘wet’ conditions, the carbonatation proceeds inwards from the outside of each particle by an advancing interface mechanism, being inhibited by the low porosity of the calcium carbonate layer normally developed around each particle. In ‘wet’ conditions, the growth and porosity of the calcium carbonate layer depends largely on the pH of the surrounding solution, since this determines the rate of formation and the solubility of the calcium carbonate. The tendency of the layer to age is related to the growth and ageing of calcium carbonate particles precipitated from solution by d