Abstract
To identify the hydration products of aggregate-free, low-cement castables (LCC), cement matrices 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. Batches containing approximately 33 wt% HAC, and approximately 67 wt% FA/FS mixtures with ratios of 4.0-9.0 showed optimum particle packing and hydration conditions with the least amount of WOC. This resulted in an increase in cold crushing strength (CCS) of up to 58 MPa after hydration for 3 d. The hydrated batches were composed mainly of unreacted alpha-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, enclosing limited amounts of CAS2 and/or C2AS phases. Such batches are suitable for application as cementing matrices for high-alumina, low-cement castables. The low HAC content with high FA/FS ratio in the presence of more fine alumina in the matrices of such castables leads to significant improvement in hot mechanical properties. 27 refs.