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 CrO42-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 CrO42-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.
