Bauxite based low and ultralow cement castables

Abstract

Five batches consisting of 90 wt-% graded bauxite aggregate with 10 wt-% of different cement matrices were investigated for the processing of low (LCC) and ultralow (ULCC) cement castables. The influence of variation in high alumina cement (HAC) content and fine alumina/fumed silica ratio on the physical and thermomechanical properties of the hydrated castables before and after firing at 1400°C was studied. The results were interpreted in the light of solid phase composition and microstructure, on the basis of XRD and SEM characterisation. As high alumina cement content decreases from 5·0 to 3·3 wt-% and fine alumina/fumed silica ratio increases from 1·5-2·3 to 2·3-9·1, in the matrices of LCC and ULCC respectively, a gradual decrease in green bulk density and cold crushing strength occurs. The inverse of this behaviour occurs after firing the hydrated samples for 2 h at 1400°C. All processed LCC and ULCC samples show high thermal shock resistance on successive quenching from 1000°C to ambient temperature, high volume stability on refiring for 2 h at 1500°C, and high loadbearing capacity on firing under load up to 1600°C. The temperatures corresponding to maximum expansion T0 and the beginning of subsidence T0 0·5 gradually increase from 1400 to 1500°C and from 1560 to > 1600°C respectively on increasing the fine alumina/fumed silica ratio from 1·5-2·3 in LCC to 2·3-9·1 in ULCC samples, owing to the formation of acicular mullite.