The dehydration kinetics of gypsum at high pressure and high temperature

An in situ dehydration kinetics study of gypsum under water-saturated condition was performed in the temperature and pressure ranges of 383-423K and 343-1085MPa by using a hydrothermal diamond anvil cell and Raman spectroscopy. Kinetic analysis shows that the dehydration rate k increases with pressure, suggesting a negative pressure dependence on dehydration rate. The elevation of temperature can contribute to the dehydration. The n values increase with pressure, indicating that the nucleation process becomes slower relative to the growth process. According to the n values of approximate to 1.0, the dehydration of gypsum is dominated by an instantaneous nucleation and diffusion-controlled growth mechanism. The obtained average activation volume V is equal to 5.69cm(3)/mol and the calculated activation energy E-a and the pre-exponential factor A are 66.9kJ/mol and 4.66x10(5)s(-1). The activation energy may be dependent upon grain size, shape, temperature and pressure, and surrounding water.