High chloride content calcium silicate glasses

Chloride is known to volatilize from silicate glass melts and until now, only a limited number of studies on oxychloride silicate glasses have been reported. In this paper we have synthesized silicate glasses that retain large amounts of CaCl₂. The CaCl₂ has been added to the calcium metasilicate composition (CaO·SiO₂). Glasses were produced via a melt quench route and an average of 70% of the chloride was retained after melting. Up to 31.6 mol% CaCl₂ has been successfully incorporated into these silicate glasses without the occurrence of crystallization. ²⁹Si MAS-NMR spectra showed the silicon being present mainly as a Q² silicate species. This suggests that chloride formed Cl-Ca(n) species, rather than Si-Cl bonds. Upon increasing the CaCl₂ content, the T_g reduced markedly from 782 °C to 370 °C. Glass density and glass crystallization temperature decreased linearly with an increase in the CaCl₂ content. However, both linear regressions revealed a breakpoint at a CaCl₂ content just below 20 mol%. This might be attributed to a significant change in the structure and is also correlated with the nature of the crystallizing phases formed upon heat treatment. The glasses with less than 19.2 mol% CaCl₂ crystallized to wollastonite, whilst the compositions with CaCl₂ content equal to or greater than 19.2 mol% are thought to crystallize to CaCl₂. In practice, the crystallization of CaCl₂ could not occur until the crystallization temperature fell below the melting point of CaCl₂. The implications of the results along with the high chloride retention are discussed.