Kinetics and mechanism of the carbonation of ettringite

Synthetic, phase pure ettringite has been exposed to a moist CO2 atmosphere at 68 and 88"/o relative humidity (RH) both at 21-25°C and at 4(degree)C. Physically, the ettringite was carbonated in the form of powders and pellets, pressed to ~80% theoretical density. Nucleation of decomposition products is a slow process, requiring a definite induction period, the duration of which decreases with both increasing temperature and RH. The surfaces of compacted ettringite pellets, like loose powders, decompose to mixtures of calcium carbonate, calcium sulphate hydrate (both gypsum and hemihydrate), alumina gel and water but the interior of pellets retain water with the result that recrystallization occurs and AFm forms. Pellets more reliably predict the behaviour of industrial ettringite-based cements than loose powders, which are not observed to form AFm.