EXAFS AND AL-27 NMR-STUDIES OF CAAL2O14H20 STRUCTURAL-CHANGES DUE TO HEAT-TREATMENTS

At low temperature (T<10 degrees C), CaAl2O14H20 is formed by hydratio n of high-alumina cements (HACs). HACs are used for refractory applica tions; heat treatments then lead to thermal decomposition of CaAl2O14H 20, which first dehydrates to CaAl2O4xH2x (x < 10) and finally to the anhydrous phase CaAl2O4. In CaAl2O14H20, aluminium atoms are 6-coordin ated, whereas they are 4-coordinated in CaAl2O4. Materials heat-treate d between 120 degrees C and 900 degrees C are amorphous and, therefore , only short-range order techniques can be used to study the environme nt of ions, in particular calcium. Solid-state NMR cannot be used due to low natural occurence of the isotope Ca-43. Our innovation is then to study the changes in calcium and aluminium environments during CaAl 2O14H20 dehydration by X-ray absorption spectroscopy (EXAFS and XANES) . Ca K-edge EXAFS spectra show a decrease in Ca-O distance and a reduc tion of the number of oxygen neighbours in the first coordination shel l. Al K-edge XANES spectra indicate an important distorsion of the alu minium sites when amorphisation occurs. The relative contents of Al-4 and Al-6 are determined by Al-27 MAS NMR.