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

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–Al2O3–SiO2 (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 (Qn) of the (AlO4) tetrahedra. Ca K-edge XANES spectra also exhibit some chemical dependence of the peaks. Al is distributed in Q2, Q3, and Q4 species at low silica content and progressively enters only Q4 as SiO2 or Al2O3 is introduced. This change in the network polymerization has important implications for interpreting thermodynamical variations with composition in the CAS glasses.