Water and the glass transition temperature of silicate melts

Literature data on the effect of water on the glass transition in silicate melts are gathered for a broad range of total water content c_w from 3 × 10⁻⁴ to 27 wt%. In terms of a reduced glass transition temperature T_g^*=T_g/T_g^GN, where T_g^GN is T_g of the melt containing c_w≈0.02 wt% total water, a uniform dependence of T_g^* on total water content (c_w) is evident for silicate melts. T_g^* decreases steadily with increasing water content, most strongly at the lowest water content where H₂O is dominantly dissolved as OH. For water-rich melts, the variation of T_g^* is less pronounced, but it does not vanish even at the largest water contents reported (≈27 wt%). T_g^* vs. c_w is fitted by a three-component model. This approach accounts for different transition temperatures of the dry glass, hydroxyl and molecular water predicting T_g^* as a weighted linear combination of these temperatures. The required but mostly unknown water speciation in the glasses was estimated using IR-spectroscopy data for hydrous sodium trisilicate and rhyolite.