THE HYDRATION OF THE ALUMINATES OF CALCIUM: V. THE HYDROTHERMAL DECOMPOSITION PRODUCTS OF TRICALCIUM ALUMINATE AT 350 °C.

Abstract

When tricalcium aluminate is treated in saturated steam at 350 °C. hydrolysis occurs with the formation of a crystalline product of the composition 4CaO.3Al₂O₃.3H₂O and calcium hydroxide. The same products result when a 3:1 mixture of calcium hydroxide and hydrated alumina are similarly treated. The calcium hydroxide as well as any hexahydrate of tricalcium aluminate formed at lower temperature may be removed from the hydrothermal product by extraction with a solution of acetic acid buffered with calcium acetate.The 4:3:3 calcium hydroaluminate crystallizes in colourless elongated rectangular plates with parallel extinction, positive elongation, and low birefrigence. The average refractive index is close to 1.627 and the density 2.71 (at 20 °C). The crystals are probably orthorhombic.Appreciable dehydration of the product does not occur on heating below 475° to 500 °C. Dehydration between 650° and 750 °C. in a stream of dry air appears to produce decomposition with the liberation of free alumina and probable formation of the product 12Ca0.7Al₂O₃.The factors affecting the yield of the 4:3:3 calcium hydroaluminate and the mechanism of the hydrothermal reactions of tricalcium aluminate and its hydrates are discussed. Readings of X-ray powder patterns of the 4:3:3 hydroaluminate and its dehydration products are given.