The mechanism of the step I and step II of thermal decomposition of 3CaO · Al2O3 · 6D2O was studied. The presence of Ca(OD)2 was proved in the products of the first step of decomposition. In the calorimeter cell of the Dupont 990 thermoanalyser the enthalpy changesΔHr,I=59.2kJ/mole D2O for step I (210–410°) andDHrII,1=69.0 kJ/mole D2O for the first fast part of the step II (“stage 1 of the step I”, encompassing the temperature interval 410–560°) were measured. This indicates that the dissociation of Ca(OD)2 is not the only transition taking place in the first fast part (stage 1) of the step II.
A static method is described for measurement of the equilibrium values of temperature, pressure and composition of the solid phase, and determination of the number of degrees of freedom in heterogeneous systems containing a gaseous phase, including water vapour. With this method, it has been found that in the system formed in the thermal dehydration of CaC2O4·H2O a solid solution of monohydrate and anhydride coexists in equilibrium with water vapour. The composition of the solid solution changes with the temperature and the pressure of water vapour. The dehydration enthalpy of the solid solution referred to 1 mol H2O does not depend on its composition. It was found to be ΔHdeh=(69+-3) kJ·mol−1.