Thermal decomposition of materials with layered structures: Dehydration of calcium sulphate dihydrate single crystals

The kinetics of the thermal dehydration of CaSO 4 · 2H 2O single crystals, which have a layered structure, has been studied in vacuo by means of a quartz crystal microbalance. It has been found that the rate of the reaction interface advance in the crystallographic [010] direction is controlled by diffusion of water molecules through the layer of the solid product, whereas in the [001] direction it is controlled by the chemical step. To explain the experimental results, a topochemical model has been suggested. The kinetic parameters for the [001] direction are: activation energy ( E) = (19.8 ± 0.4) kcal mol and frequency factor ( v) = (1.5 ± 0.9) · 10 14 s −1.