Evaluation method for hydration resistance of magnesia fine powder was examined for application of castable with the following finding: (1) By way of evaluation method for hydration resistance, submerged magnesia fine powder in water was treated in an autoclave under moist pressure of 0.49MPa. It turned out that this method was desirable for evaluation method for the hydration resistance of magnesia fine powder for application of monolithic refractories, especially castable. (2) It turned out, that B_2O_3 content in magnesia fine powder was higher, the hydration resistance was better. (3) Therefore alumina-magnesia castable with magnesia containing B_2O_3 component can be expected to show higher density when it is applied in the actual use, because it was difficult to expand.
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