Kinetics of gypsum dehydration at reduced pressure: an energy dispersive X-ray diffraction study

Gypsum (CaSO(4)center dot 2H(2)O) dehydration kinetics were investigated through energy dispersive X-ray diffraction (EDXD), under reduced pressure (100 Pa), in the temperature range 313-353 K. The process follows the JMAK (Johnson-Mehl-Avrami Kinetic) model. The fitting procedure of the Arrhenius expression provides an activation barrier of 18(2) kcal/mol. Under these experimental conditions, dehydration proceeds via a single-step conversion path gypsum --> gamma-anhydrite. Separate experiments of bassanite (CaSO(4)center dot 0.5H(2)O) dehydration, carried out at similar conditions, indicate, as expected, a faster process gamma-anhydrite being the final product. According to the structural relationships between bassanite and gamma-anhydrite, dehydration should occur via the escape of water molecules along the axis of the channel (c axis) following a one-dimensional behaviour. Therefore, no Avrami model (which implies nucleation and growth of a new phase) can be applied.