J. Phys. IV France 7 (1997) C2-1069-C2-1071
EXAFS and 27Al NMR Studies of CaAl2O14H20 Structural Changes Due to Heat TreatmentsN. Richard1, N. Lequeux1, R. Cortès2 and P. Florian3
1 Laboratoire Céramiques et Matériaux Minéraux, ESPCI, 10 rue Vauquelin, 75005 Paris, France
2 Physique des Liquides et Électrochimie, Université Paris VI, 4 place Jussieu, 75005 Paris, France
3 Centre Recherche sur la Physique des Hautes Températures, avenue Recherche Scientifque, 45071 Orléans, France
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 CaAl2O4xH2x (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.
© EDP Sciences 1997