Selection of binders for in situ spinel refractory castables

The expansive behavior of alumina–magnesia refractory castables is usually associated with in situ spinel formation. Nevertheless, when bonded with calcium aluminate cement (CAC), this class of materials can present additional expansion reactions due to CA2 and CA6 formation. Considering that these reactions impart a further contribution to the material's overall volumetric change, the objective of this work has been to analyze the effect of partial or complete replacement of CAC by hydratable alumina (HA). Taking into account that this substitution would affect various castable processing steps, properties such as the mechanical strength (during curing, intermediate or high temperatures), linear change behavior during heating, creep and thermal shock resistance were evaluated. In general, CAC-containing castables led to better mechanical strength and thermal shock resistance, whereas HA-containing castables presented higher creep resistance, lower apparent porosity and better volumetric stability. Due to the substantial reduction of the overall expansion of alumina–magnesia castables, the addition of hydratable alumina was pointed out as an interesting alternative to attain designed expansion levels.