Thermodynamic properties and hydration behavior of ye'elimite
Barbara Lothenbach, C.A. Geiger, Edgar Dachs, Frank Winnefeld, A. Pisch
Abstract
Ye'elimite occurs in nature and in calcium sulfoaluminate cements. The thermodynamic properties of synthetic orthorhombic ye'elimite, Ca4Al6O12SO4, were determined experimentally using low-temperature relaxation calorimetry and at higher temperatures using differential scanning calorimetry and drop calorimetry. The heat capacity, Cp, of 480 J/mol/K and standard entropy, S°, of 487 J/mol/K at 298 K were measured. These values compare well with those calculated by density functional theory. Heat of dissolution measurements give an enthalpy of formation, ΔfH°, of −8382.5 kJ/mol for ye'elimite. Heat of reaction measurements for nanocrystalline Al(OH)3 give ΔfH° = −1276 kJ/mol. The resulting calorimetric-based enthalpies were used to calculate the degree of ye'elemite hydration. A solubility product, KS0,ye'elimite, of 10−1.4 at 298 K was calculated from the enthalpy and entropy values. Pore solution composition indicates the presence of a hydroxylated surface layer on ye'elimite, which results in a lower solubility with KS0,surface ≈1019 for the reaction Ca4Al6O12SO4 + 12H2O0 → 4Ca2+ + 6Al(OH)4− + SO42−.