Microcalorimetric study of the transformation of CaSO4·2H2O in the 40 and 50 wt% P2O5 solutions at 80 °C

A calorimetric study of the transformation of CaSO4·2H2O (gypsum) in 40 and 50 wt% of P2O5 phosphoric acid solutions is undertaken at 80 °C by dissolving increasing amounts of gypsum in the same volume of liquid. The results show a simple dissolution or its transformation into hemihydrate, CaSO4·1/2H2O, depending on the acid concentration. Kinetic schemes have been developed for these processes. The kinetic and thermochemical parameters were derived iteratively and the enthalpies were compared to experimental values.

► CaSO4·2H2O (gypsum) was dissolved in 40 and 50 wt% P2O5 solutions at 80 °C. ► Dissolution processes were realized in a C80 SETARAM calorimeter. ► The gypsum dissolution in 40 wt% P2O5 solution takes place in one step. ► Gypsum transforms into hemihydrate through a dissolution/precipitation process.