Structural evolution of an Al2O3-MgO•Al2O3 castable in the range 1000-1600°C

Abstract

Thermal and structural characteristics of an alumina-spinel castable (containing 2.5 wt.% CaO) was studied. The specimens were cast (8.0 wt.% water), cured in air at ambient temperature (48 h), then dried at 100°C (24 h) and prefiring at 500°C (3 h). Thermal behavior was analyzed by dilatometries at constant heating rate (5°C/min) up to 1600°C, and at constant temperature (range: 1000-1500°C). Crystallographic phases were determined by XRD and microstructures were analyzed by SEM/EDS. Specimens heat treated at 1000°C and 1200°C showed Al2O3 (A) and MgAl2O4 (MA) as main phases. Between 1200-1300°C, the 2Al2O3∙CaO (CA2) formation is promoted. Calcium hexaluminate 6Al2O3∙CaO (CA6) is detected from 1400°C. The presence of CA6 increases at 1500°C and 1600°C. From dilatometric curves, alternating expansion contraction zones were observed in the range 1000-1600°C. This behavior is associated to (i) the CA2 and CA6 formation, and (ii) the sintering/densification of ceramic grains with probable liquid phase assistance. Thus, considering the temperature profile, generated in service, through this material, both the formation of CA2 and CA6 and the sintering evolution during the first hours, lead to volumetric contractions and expansions in different zones. These volumetric changes generate mechanical stresses that can cause faults in the material.