Setting mechanism of low-cemeilt castables was investigated from several viewpoints. From the results, the setting mechanism was concluded as follows. (1) The setting of the castables is coagulation of hydrophobic colloid of super fine ceramic powder stabilized by deflocculant. In mixing water of the castables Ca2+ ions dissolve from alumina cement. When Ca2+ ion concentration increases above a certain value, the colloid becomes unstable by Ca2+ ions and the coagulation occurs. (2) The role of the alumina cement in the mechanism is a Ca2+ ion supplier. (3)Ca2+ ion dissolution rate is considered to be determined by Ca element diffusion in the surface layer on alumina cement grams. (4) In the castables containing sodium tripolyphosphate (STTP), chelate is formed initially and secondary precipitate is formed by the reaction of Ca2+ ions and tripolyphosphate ions. After STTP consumes by the reaction, Ca2+ ion concentr;ltion mere;lses and the coagulation occurs.

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