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

At low temperature (T<lO°C), CaAl201fio is formed by hydration of high-alumina cements (HACs). HACs are used for rekactory applications; heat treatments then lead to thermal decomposition of CaAl201fio. which first dehydrates to CaAl~04& (X < 10) and finally to the anhydrous phase CaA1204. In CaAlzOl&, aluminium atoms are 6-coordinated, whereas they are 4-coordinated in CaAl20.4. 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 4 3 ~ a . Our innovation is then to study the changes in calcium and aluminium environments during CaAl201dI20 dehydration by X-ray absorption spectroscopy (EXAFS and XANES). Ca K-edge EXAFS spectra show a decrease in Ca-0 distance and a reduction of the number of oxygen neighbours in the fir$ coordination shell. A1 K-edge XANES spectra indicate an important distorsion of the aluminium sites when amorphisation occurs. The relative contents of A14 and A16 are determined by 27Al MAS NMR.