Effect of zircon additions on low and ultra-low cement alumina and bauxite castables

Two series of low- and ultra-low alumina castables were prepared from calcined Chinese bauxite and pure alumina aggregates. Both series consisted of 90 wt.% aggregate and 10 wt.% high alumina cement (HAC), with HAC replaced by 2, 4, 6 and 8 wt.% zircon to obtain castables containing an in situ mullite–zirconia composite in the matrix to improve their microstructure, physico-mechanical and refractory properties. The samples were characterized in terms of densification parameters (bulk density and apparent porosity), cold crushing strength (CCS), XRD, SEM and thermal shock resistance (TSR) to understand the effect of zircon additions on the castable composition and properties. Generally, the results revealed that castables containing bauxite aggregate showed higher densification and mechanical properties due to the presence of some impurities in the bauxite aggregate which help liquid phase sintering, and consequently, improved the mechanical properties. Castables containing pure alumina exhibited higher refractory properties due to their higher purity, i.e. the absence of a liquid phase. The emergence of refractory phases such as alumina, mullite and zirconia was observed to have beneficial effects on physico-mechanical and refractory properties due to the development of morphologies which interlocked other phases, and to pore filling from expansible reactions.