Action of Water on Calcium Aluminoferrites

The action of water on seven preparations of the calcium aluminoferrite solid solution series, ranging in composition from 6CaO·2Al₂O₃·Fe₂O₃ to 2CaO Fe₂0₃, was studied (a) by leaching the powdered preparations with water, (b) by stirring in a large volume of water at different temperatures from 10 to 100 °C, and (c) by paste hydration at 1, 25, and 45 °C. The initial reaction occurring immediately on contact between the aluminoferrite and water is one of incongruent solution. The Al₂O₃, most of the CaO, and a trace of Fe₂O₃ pass into solution, and a residue of Fe₂O₃, possibly hydrated and amorphous, remains. If the suspension is sufficiently concentrated to give a supersaturated solution, a hydrated precipitate will form. At room temperature the major precipitated phase is 2CaO·Al₂O₃ 8H₂O, or a limited solid solution of the same. At 70 °C a solid solution of 3CaO·Al₂O3 6H₂O, in which about one-tenth of the Al₂O₃ is replaced by Fe₂O₃, is precipitated. The rate of reaction increases with temperature and with the proportion of alumina in the aluminoferrite. Hydration in paste form progresses in a different manner. The hydrogarnet phase produced varies in composition with the parent aluminoferrite, but is always somewhat poorer in Fe_2;O₃ The hexagonal plate phase, 4CaO·(Al₂O₃,Fe₂O₃)·nH₂O is produced from the aluminoferrites high in F(e₂O₃, especially at the lower temperatures. Another platy phase, 2CaO·Al₂O₃·8H₂O (or its limited solid solution) is formed from preparations highest in Al₂O₃, also at lower temperatures. Intermediate members give chiefly the hydrogarnet phase even at 25 °C.