Thermal behaviour and kinetics of dehydration of gypsum in air from in situ real-time laboratory parallel-beam X-ray powder diffraction

Thermal behaviour and kinetics of dehydration of gypsum in air have been investigated using in situ real-time laboratory parallel-beam X-ray powder diffraction data evaluated by the Rietveld method. Thermal expansion has been analysed from 298 to 373 K. The high-temperature limits for the cell edges and for the cell volume, calculated using the Einstein equation, are 4.29 x 10(-6), 4.94 x 10(-5), 2.97 x 10(-5), and 8.21 x 10(-5). Thermal expansion of gypsum is strongly anisotropic being larger along the b axis mainly due to the weakening of H2 center dot center dot center dot O1 hydrogen bond. Dehydration of gypsum has been investigated in isothermal conditions within the 348-403 K range with a temperature increase of 5 K. Dehydration proceeds through the CaSO4 center dot 2H(2)O -> CaSO4 center dot 0.5H(2)O -> gamma-CaSO4 steps. Experimental data have been fitted with the Avrami equation to calculate the empirical activation energy of the process. No change in transformation mechanism has been observed within the analysed temperature range and the corresponding E-a is 109(12) kJ/mol.