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

Dipartimento di Scienze della Terra, Sapienza Università di Roma, P.le Aldo Moro 5, I-00185 Roma, Italy

^* Corresponding author, e-mail: paolo.ballirano{at}uniroma1.it

Thermal behaviour and kinetics of dehydration in air of bassanite (calcium sulphate hemihydrate, CaSO₄·0.5H₂O) 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 -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_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 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_a of the process of 73(5) kJ/mol. E_a was found to be substantially independent from the kinetic model selected.

Key-words: bassanite, CaSO₄·0.5H₂O, thermal behaviour, kinetics of dehydration, X-ray powder diffraction, Rietveld method.