The grain sizes and impurity distribution in Tabular Alumina (TA) and White Fused Alumina (WFA) are significantly influenced by the sintering and fusion processing routes respectively, required to produce these aggregates. Alumina-spinel refractory castables based on TA and WFA, and bonded with calcium aluminate cement, were fired at 1500 °C. In-depth SEM observations revealed that varying possible morphologies of calcium hexa-aluminate (CA 6 ) can exist along the borders of alumina grains, and these morphologies are influenced by the type of alumina aggregate used. While TA grains lead to platelet CA 6 structures, WFA grains promote both platelet and dense, blocky CA 6 structures. These dense, blocky CA 6 structures are located uniquely around certain regions along the WFA borders and appear debonded from the matrix. EBSD results revealed strong orientation relationships between the alumina / CA 6 interfaces for WFA-based castables which were less obvious in the TA-based castables. Grains of several phases within TA-based castables were found to be randomly oriented. On the contrary, the preferred grain orientations of α-Al 2 O 3 , β-alumina and CA 6 phases within WFA-based castables can induce relatively more microcracks due to the coefficient of thermal expansion (CTE) mismatch between the phases during thermal cycling. The generation of well-controlled microcracks makes WFA-based castables potentially suitable for thermal shock applications.
Boateng et al. (Fri,) studied this question.