Phase evolution and hydraulic properties of low cement castables matrixes

Abstract

To identify the hydration products of aggregate-free, low cement castables (LCC), cement matrixes were examined. Two series of cementing batches based on 33·3 and 50·0 wt-% high alumina cement (HAC) were processed by adding ultra-fine calcined alumina/fumed silica mixtures (FA/FS) with weight ratios of 1·0, 1·5, 2·3, 4·0, and 9·0 to each series. The hydrated batches were investigated for their hydraulic properties. Batches showing the highest cold crushing strength with minimum water of consistency (WOC) and reasonable setting time (ST) were selected and characterised with respect to phase composition, microstructure, and microchemistry, before and after firing up to 1400°C, by XRD, DTA, and SEM techniques. Cementing batches containing ≈33 wt-% HAC, and ≈67 wt-% FA/FS mixtures with ratios of 4·0–9·0 show optimum particle packing and hydration conditions with the least amount of WOC. This results in increases in cold crushing strength (CCS) of cementing batches up to 58 MPa after hydration for 3 d. The hydrated batches are composed mainly of unreacted α-Al₂O₃ particles bonded by CAH₁₀, AH₃, and C₂ASH₈ phases. On firing up to 1400°C, the hydrated phases are transformed into anhydrous CA₂ and CA₆ patches enclosing limited amounts of CAS₂ and/or C₂AS phases. Such batches are suitable for application as cementing matrixes for high alumina low cement castables. The low HAC content with high FA/FS ratio in the presence of more fine alumina in the matrixes of such castables leads to significant improvement in their hot mechanical properties.