Heat of Formation of Calcium Aluminate Monosulfate at 25 °C

The heat of formation of calcium aluminate monosulfate, 3CaO·Al₂O₃·CaSO₄·12H₂O, at 25 °C, and of less completely hydrated samples of the same compound, was determined by the heat-of-solution method, with 2N HCl as the solvent, and 3CaO·Al₂O₃·6H₂O(c) and CaSO₄·2H₂O(c), as the reactants. The results were as follows: ΔH, kj/moleΔH, kcal/mole3CaO·Al₂O₃·CaSO₄·12H₂O(c) Heat of formation  from elements, $$ -2100  from reactants and H₂O(1)-15.0 Heat of solution in 2N HCl- 495.7- 118.5 Change of heat of solution  with H₂O content at 12H₂O, per mole H₂O $$ 1.93 The heat of the reaction (ΔH) is -134.4 kj/mole or -32.1 kcal/mole. The heat of the reaction (ΔH) is -144.9 kj/mole or -34.6 kcal/mole. Values reported earlier for the heat of formation of calcium aluminate trisulfate and of calcium aluminate monocarbonate should be revised by adding -0.9 kcal/mole to each reported ΔH value, with the following resulting values: ΔH from appropriate reactants $$ kcal/mole kcal/mole3CaO·Al₂O₃·3CaSO₄·31H₂O(c) -47.01 -41233CaO·Al₂O₃·3CaSO₄·32H₂O(c) -49.52 -41943CaO·Al₂O₃·CaCO₃·10·68H₂O(c) -19.77 -1957 Conditions for the formation of the monosulfate from solution, and its properties on exposure to moisture, are discussed.