Heat of Formation of Calcium Aluminate Monosulfate at 25 °C

The heat of formation of calcium aluminate monosulfate, 3CaO·Al(2)O(3)·CaSO(4)·12H(2)O, 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·Al(2)O(3)·6H(2)O(c) and CaSO(4)·2H(2)O(c), as the reactants. The results were as follows: [Table: see text] The heat of the reaction (ΔH) [Formula: see text] is −134.4 kj/mole or −32.1 kcal/mole. The heat of the reaction (ΔH) [Formula: see text] 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: [Table: see text] Conditions for the formation of the monosulfate from solution, and its properties on exposure to moisture, are discussed.