Interaction between water and melt in the system CaAl 2O 4SiO 2H 2O

Abstract

The interaction between dissolved H 2O and melt structure on the join CaAl 2O 4SiO 2H 2O has been studied with Raman spectroscopy. The total H 2O contents ranged from 3 to 10 wt.% with Al ( Al+Si) =0–0.333 . The spectra are consistent with formation of OH complexes that include all Ca 2+ and Al 3+ in addition to molecular H 2O. No direct evidence for (Si,Al)OH bonds can be discerned in the spectra of hydrous calcium aluminosilicate melt (the 970-cm −1 band from SiOH stretching observed in the spectra of SiO 2H 2O melts is not well resolved in aluminous samples). However, the spectral topology of the fundamental OH stretch bands near 3600 cm −1 can only be rationalized if some SiOH or (Si,Al)OH bonding exists in the melts. The melts become depolymerized as H 2O is dissolved to form Ca..OH and Al..OH complexes. Formation of Ca..OH complexes is a more efficient depolymerization mechanism than that of Al..OH complexes [6 vs. 2 3 nonbridging oxygen would be formed per mole H 2O dissolved as a Ca..OH complex of Ca(OH) 2 type vs. an Al..OH complex of Al(OH) 3 type]. With increasing Al ( Al+Si) of the melts complexing of OH with Al 3+ (Al..OH) probably becomes more important at the expense of complexes with Ca 2+ (Ca..OH). Thus, the effect of dissolved H 2O on melt polymerization diminishes with Al ( Al+Si) . However, the degree of polymerization of the melts (NBO/T) for a given total H 2O concentration is less than that expected by either the Ca..OH or the Al..OH complexing mechanism alone. The excess water is present as molecular H 2O and as (Si,Al)OH bonds that replace (Si,Al)O(Si,Al) bridging oxygen bonds in the melts.