Al speciation and Ca environment in calcium aluminosilicate glasses and crystals by Al and Ca K-edge X-ray absorption spectroscopy

Abstract

X-ray absorption near-edge structure (XANES) spectroscopy at both Ca and Al K-edges has been used to investigate the environment of Al and Ca in peralkaline glasses obtained, by classic quenching methods, in the CaO–Al 2O 3–SiO 2 (CAS) system and in calcium aluminate glasses obtained by ultra-quench from laser heating. Al K-edge XANES spectra exhibit remarkable variations of the structural features with composition. The spectra of the glasses are compared to those of calcium aluminate crystals. The analysis reveals a correlation between some peaks and the degree of polymerization (Q n ) of the (AlO 4) tetrahedra. Ca K-edge XANES spectra also exhibit some chemical dependence of the peaks. Al is distributed in Q 2, Q 3, and Q 4 species at low silica content and progressively enters only Q 4 as SiO 2 or Al 2O 3 is introduced. This change in the network polymerization has important implications for interpreting thermodynamical variations with composition in the CAS glasses.