Action of water on calcium aluminoferrites

Abstract

The action of water on seven preparations of the calcium aluminoferrite solid solution series, ranging in composition from 6CaO ? 2A12O3 ? Fe2O3 to 2CaO ? Fe2O3, 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 A12O3, 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 ? AI2O3 ? 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 Fe2O3. The hexagonal plate phase, 4CaO ? (AI2O3,Fe2O3) ? nH2O is produced from the aluminoferrites high in Fe2O3, especially at the lower temperatures. Another platy phase, 2CaO ? AI2O3 ? 8H2O (or its limited solid solution) is formed from preparations highest in AI2O3, also at lower temperatures. Intermediate members give chiefly the hydrogarnet phase even at 25 ?C.