Impact on aggregate/matrix bonding when a refractory contains zinc aluminate instead of spinel and magnesia-chrome

The high hot strength of MgO-Cr2O3 refractory is often ascribed to its intimate aggregate/matrix bonding. For a fundamental comparison with it, similar to 2 mm aggregates of MgO and Al2O3 were separately embedded in ZnAl2O4 and MgAl2O4 matrices, sintered at 1600 degrees C, and examined. It was found that similarity of thermal expansion coefficient (TEC) between the aggregate and the matrix is critical to achieve good bonding and this is more important than the extent of interdiffusion. The TEC mismatch of >= 5.7 x 10(-6) K-1 caused significant undesirable debonding in MgO aggregate/MgAl2O4 matrix sample and MgO/ZnAl2O4 despite >736 mu m Zn2+ diffusion depth in the latter. Direct bonding, as inferred from a thicker interfacial reaction layer and a greater shift of the aggregate/matrix interface before and after firing, was better in MgAl2O4/ZnAl2O4 combination, followed by tabular Al2O3/ZnAl2O4 and Al2O3/MgAl2O4. Powder X-ray diffraction indicated that the volatilization of ZnAl2O4 at 1600 degrees C in air was negligible compared to MgO-Cr2O3.