Abstract:
The interaction between dissolved H2O and melt structure on the join CaAl2O4SiO2H2O has been studied with Raman spectroscopy. The total H2O contents ranged from 3 to 10 wt.% with. The spectra are consistent with formation of OH complexes that include all Ca2+ and Al3+ in addition to molecular H2O. 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 SiOH stretching observed in the spectra of SiO2H2O 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 SiOH or (Si,Al)OH bonding exists in the melts. The melts become depolymerized as H2O 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. nonbridging oxygen would be formed per mole H2O dissolved as a Ca..OH complex of Ca(OH)2 type vs. an Al..OH complex of Al(OH)3 type]. With increasing of the melts complexing of OH with Al3+ (Al..OH) probably becomes more important at the expense of complexes with Ca2+ (Ca..OH). Thus, the effect of dissolved H2O on melt polymerization diminishes with . However, the degree of polymerization of the melts (NBO/T) for a given total H2O 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 H2O and as (Si,Al)OH bonds that replace (Si,Al)O(Si,Al) bridging oxygen bonds in the melts.