Thermal behaviour and kinetics of dehydration in air of bassanite, calcium sulphate hemihydrate (CaSO (sub 4) .0.5H (sub 2) O), from X-ray powder diffraction

Thermal behaviour and kinetics of dehydration in air of bassanite (calcium sulphate hemihydrate, CaSO (sub 4) .0.5H (sub 2) O) 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-->gamma -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 CaO (sub 8-9) 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 alpha 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 E (sub a) of the process of 73(5) kJ/mol. E (sub a) was found to be substantially independent from the kinetic model selected.