An X-ray absorption study of phases formed in high-alumina cements

X-ray absorption spectroscopy has been used to study the local environment of aluminium and calcium in phases formed in high-alumina cements (HACs). The feasibility of this technique was demonstrated when studying phases with a known structure: CA, the main constituent of HACs, and C₃AH₆, the only thermodynamically stable hydrate. The hydrate CAH₁₀, which is principally responsible for the hydraulic hardening of cements, was then investigated. ²⁷Al magic-angle spinning nuclear magnetic resonance confirmed the octahedral co-ordination of aluminium. Al K-edge X-ray absorption near edge spectroscopy and extended X-ray absorption fine structure spectroscopy spectra showed a strong analogy between the structure of CAHIO and that of gibbsite, AH₃. A structural model for CAH₁₀ based on two rings of six edge-sharing [Al(OH)₆]^3- octahedra is proposed, on the basis of the results obtained at the Al and Ca K-edge.