Thermal behaviour and kinetics of dehydration in air of bassanite, calcium sulphate hemihydrate (CaSO4 • 0.5H2O), from X-ray powder diffraction

Thermal behaviour and kinetics of dehydration in air of bassanite (calcium sulphate hemihydrate, CaSO40.5H2O) have been investigated in situ real-time using laboratory parallel-beam X-ray powder diffraction data. Thermal expansion has been analyzed between 303 and 383 K at increments of 5 K. The bassanite ! g-anhydrite conversion starts at 388 K and is completed at 408 K. Thermal expansion of hemihydrate is isotropic and not related to the expansion of CaO8–9 polyhedra. Lattice parameters and volume dependence from T is linear within the studied temperature range. Kinetics of dehydration has been investigated from isothermal diffraction data collected at 10 temperatures between 378 and 423 K with steps of 5 K using a fresh sample at each temperature. Transformed fraction a vs. t curves were fitted with seven different kinetic models. The best fit was found for the Avrami-Erofe’ev equation that provided an empirical activation energy Ea of the process of 73(5) kJ/mol. Ea was found to be substantially independent from the kinetic model selected.