The influence of magnesia powders on the setting of the alumina-magnesia low-cement castables was investigated and discussed the setting mechanism of the low-cement castables. When the grain size of the magnesia powder in the castable was made smaller, Mg2＋ ion concentration in the capillary water increased and the setting was accelerated. Accordingly, Mg2＋ ions were influenced to the setting. Not only Mg2＋ ions but also Ca2＋ ions, which are originated from impurity, were dissolved to the water from the magnesia powder. In the alumina cement free alumina-magnesia castables, the setting was controlled by ions dissolved from the magnesia powder. Changes in characteristics with setting of the alumina cement free castables were similar to that alumina cement included except the time dependence. The role of alumina cement in the setting of the low-cement castables is a Ca2＋ ion supplier.

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