The isothermal heat of hydration of MgSO4 hydrates was studied by humidity controlled calorimetry. Two hydrates, starkeyite (MgSO4·4H2O) and a mixture of MgSO4 hydrates with summary 1.3 mol H2O were investigated. The solid-gas reactions were initiated at 30°C and 85% relative humidity. The heat of hydration was determined
in a circulation cell in the calorimeter C80 (Setaram). The crystal phases formed after the hydration process were analyzed
by thermogravimetry (TG) and X-ray powder diffraction (XRD).
Starkeyite reacted with the water vapour to the thermodynamic stable epsomite and the MgSO4 hydrate mixture with 1.3 mol water to hexahydrite. The hydration heats of starkeyite and the mixture were determined to be
−169±3 and −257±5 kJmol−1, respectively.