Some Observations on the Calcium Aluminate Carbonate Hydrates

Two calcium aluminate carbonate hydrates, previously reported in the literature, were prepared and various properties determined. The compound 3CaO·Al₂O₃·CaCO₃·11H₂O, was prepared by precipitation from mixtures of solutions of calcium aluminate, calcium hydroxide and sodium carbonate; and, in less pure form, by several other means. It forms thin hexagonal plates having refractive indices 1.532 and 1.554. From X-ray diffraction, cell parameters, calculated on the assumption of hexagonal symmetry, are a=8.716, c=7.565. On continued exposure to atmospheric carbon dioxide it is decomposed to calcium carbonate and hydrated alumina. The compound 3CaO·Al₂O₃·3CaCO₃·32H₂O, was prepared by precipitation from mixtures of ammonium bicarbonate, calcium aluminate, and calcium hydroxide in aqueous sucrose solution. It crystallized as spherulites of minute needles. Other methods produced individual crystals, some tubular, but only in minute quantities or mixed with other phases. X-ray diffraction indicates isomorphism with 3CaO·Al₂O₃·3CaSO₄·32H₂O. 3CaO·Al₂O₃·3CaCO₃·32H₂O is, in general, less stable than the monocarbonate complex. The needle-form "silicoaluminate" reported by Flint and Wells was re-examined. It was concluded that the phase probably contains CO₂ as well as SiO₂, but the exact composition is in doubt.