As a result of examination on the hexavalent chromium sorption behavior of calcium aluminate, the following results have been obtained. Each type of calcium aluminate sorbs hexavalent chromium in liquid phase according to hydration. The hexavalent chromium decreasing rate of calcium aluminate is larger than that of OPC or BB, and its sorption capacity is also large. According to estimation of Cr sorption capacity per gram of various of calcium aluminates, the following results were obtained : ca. 65 mg for CA, ca. 98 mg for C₁₂A₇, and ca. 152 mg for C₃A. When calcium aluminate sorbs Cr according to hydration, it takes Cr into the hydrated product in an intact form of hexavalent chromium. Formation of CrO₄^2-type U-phase was confirmed in a case of high Cr concentration. The cause of this was considered due to that in a system with high Cr concentration, by main component of Cr standard solution is K₂Cr₂O₇, also the addition of sodium hydroxide for controlling pH to neutral state, both of alkali metal ion and CrO₄^2-concentration became high. Compared with calcium silicate, OPC and BB, calcium aluminate is resistant to the leaching of hexavalent chromium that was sorbed according to hydration, which leads to its excellency in holding ability for hexavalent chromium.

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Tobermorite (Ca₅ Si₆ H₂ O₁₈·4H₂O) compacts with and without addition of 5 mass% gypsum were prepared by the hydrothermal method at 180°C for 24 hours under vapor curing conditions. The results of TG DTA and XRD of these samples show that the decomposition of tobermorite to wollastonite and silica occurs at about 820°C. At the decomposition, the compacts shrank due to crystalline phase transformation and sintering. The linear shrinkage measurements indicate that the shrinkage of the compact due to the decomposition is decreased from 14 to 10% by adding 5 mass% gypsum. The addition of gypsum increased the size of tobermorite flake like crystals and formed large pores in the compacts. These large pores in the compacts inhibited the sintering after the decomposition of tobermorite, which resulted in the decrease of the thermal shrinkage of the compacts at high temperatures.

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