INTERACTION BETWEEN WATER AND MELT IN THE SYSTEM CAAL2O4SIO2H2O

Abstract

The interaction between dissolved H2O and melt structure on the join CaAl2O4SiO2H2O 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 SiOH stretching observed in the spectra of SiO2H2O 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 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.