EXAFS and ²⁷Al NMR Studies of CaAl₂O₁₄H₂₀ Structural Changes Due to Heat Treatments

Abstract
At low temperature (T<10°C), CaAl₂O₁₄H₂₀ is formed by hydration of high-alumina cements (HACs). HACs are used for refractory applications ; heat treatments then lead to thermal decomposition of CaAl₂O₁₄H₂₀, which first dehydrates to CaAl₂O_4xH_2x (x < 10) and finally to the anhydrous phase CaAl₂O₄. In CaAl₂O₁₄H₂₀, aluminium atoms are 6-coordinated, whereas they are 4-coordinated in CaAl₂O₄. Materials heat-treated between 120°C and 900°C are amorphous and, therefore, only short-range order techniques can be used to study the environment of ions, in particular calcium. Solid-state NMR cannot be used due to low natural occurence of the isotope ⁴³Ca. Our innovation is then to study the changes in calcium and aluminium environments during CaAl₂O₁₄H₂₀ dehydration by X-ray absorption spectroscopy (EXAFS and XANES). Ca K-edge EXAFS spectra show a decrease in Ca-O distance and a reduction of the number of oxygen neighbours in the first coordination shell. Al K-edge XANES spectra indicate an important distorsion of the aluminium sites when amorphisation occurs. The relative contents of A1⁴ and Al⁶ are determined by ²⁷Al MAS NMR.