Controlled-rate thermal analysis - Kinetic study in thermal dehydration of calcium sulfate dihydrate

Kinetics of consecutive reactions with two-stage dehydration of calcium sulfate dihydrate has been examined by means of controlled-rate thermal analysis (CRTA). Under these conditions the intermediate formation of the hemihydrate was shown clearly. Several different decomposition rates were used in the CRTA experiments, in order to estimate the apparent activation energy without preliminary knowledge of the actual mechanism. This technique was useful in situations where kinetic models can not be discriminated by a single CRTA experiment and using the master shape of the CRTA curves. The kinetic equations governing the dehydration of CaSO4 . 2H(2)O were determined, and the corresponding mechanisms for the two steps were (a) nucleation and growth of nuclei, A4, and (b) a phase boundary controlled reaction, R3. It is concluded that CRTA offers significant advantages in this field of study when dealing with dehydration in consecutive reactions.