Phase Transformations and Properties Evolution of Alumina-Based Refractory Castables Containing ZnO and SiO2

Although most of the studies presented in the literature are focused on MgAl2O4 formation and its role on alumina-based refractories performance, ZnO has been reported as a promising spinel inducer. Aiming to investigate the influence of ZnAl2O4 (ZA) and MgAl2O4 (MA) generation on the properties of alumina-based castables, three vibratable compositions containing calcium aluminate cement or hydratable alumina as binders and 1 wt% of silica fume, were evaluated in this work. Flexural strength, apparent porosity, hot elastic modulus, corrosion cup-tests, thermodynamic simulations, were carried out to analyze the performance of such ceramics. The results indicated that ZnAl2O4 was mainly formed above 800 degrees C, favoring an earlier sintering of the samples. Besides that, the softening of the castables was observed above 1200 degrees C, which resulted in the elastic modulus decay of the samples during their first heating cycle due to the formation of SiO2-rich liquid phase in the resulting microstructure. Cement-free samples obtained after calcination (600 degrees C for 5h) presented enhanced corrosion resistance when placed in contact with molten slag at 1500 degrees C. Although, silica fume addition to the castables negatively affected their corrosion performance, it helped to counterbalance the expansion associated with the spinel and calcium aluminates formation.