Study of gypsum dehydration by controlled transformation rate thermal analysis (CRTA)

The dehydration of gypsum (CaSO4 . 2H(2)O) into calcium sulfate hemihydrate (CaSO4 . 0.5H(2)O) and anhydrites (gamma-CaSO4 and beta-CaSO4) was carried out by heating under a constant pressure of water vapor using Controlled transformation Rate Thermal Analysis. No intermediate formation of calcium sulfate hemihydrate occurred under a pressure of water vapor of 1 and 500 Pa (at the slow reaction rates used), whereas the transformation of gypsum into calcium sulfate hemihydrate was observed at 900 Pa. This transformation occurred without any lattice transformation when micron-sized needle-shaped crystals of gypsum were used, although a partial lattice transformation was observed when the starting sample was a centimeter-sized single crystal of gypsum. (C) 1998 Academic Press.