The retarder mechanism of boric acid for setting of low cement castable containing alumina cement was investigated. Al-rich gelatinous materials were precipitated by addition of boric acid to filtrated alumina cement solution with decreasing concentration of ions and pH. However, decreased Al ion and pH by addition of boric acid were increased with time in the suspension. Solubility behavior of the gel was similar to that of Al（OH）3 gel. Selective permeability of the gel covering alumina cement surface to ions or water and drastic change in solubility with pH, are retarder mechanism of boric acid for alumina cement hydration. In castable, increasing of boric acid delayed the setting time, whilst each pH values of capillary water showed almost the same value. Then, Al ion was eluted to the capillary water. These results show that retarder mechanism of boric acid for alumina cement hydration is applicable to castable setting.

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