Changes of hardness, ultrasound velocity, temperature, concentration and pH-value of low-cement castables were investigated to analyze in detail for three insufficient points on the setting mechanism of the castables, which are (1) the process for coagulation, (2) hydration of alumina cement, and (3) influence of pH-value. The obtained results, which correct an error and reinforce the setting mechanism reported before, are as follows: However we thought that the colloid of the superfine powder becomes unstable if Ca2＋ ion concentration reaches some value and coagulation occurs, we correct that the colloid is already unstable just after the mixing, the instability increase with time and coagulation occurs. The hydration of alumina cement clearly takes place after the setting of the castables and is absolutely different from the setting of the castables. The setting of the castables is hardly under control of the pH-value.

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The changes in the microstructure and slag corrosion resistance of MgO-Al2O3 spinel due to the addition of ZrO2 were investigated. Densification due to ZrO2 addition was significant at the stoichiometric composition of the spinel（MgO＝50mol％）. Furthermore, the densification began at lower sintering temperatures along with the ZrO2 addition, which was confirmed in the sintered bodies containing２vol％ of ZrO2. With increasing the relative density of the sintered bodies, the slag corrosion resistance was improved. In the specimens of２vol％ ZrO2 addition, the slag corrosion resistance was higher than the ZrO2-free specimens with similar relative densities. In the case of 10 vol％ ZrO2 addition, however, degradation in slag resistance was observed. It was finally concluded that the improvement in the slag resistance was resulted from the increase in slag viscosity originating in the elution of ZrO2 component.

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