Na2O activity and thermodynamic mixing properties of SiO2–Na2O–CaO melt

Electro-motive force (EMF) measurement using concentration cell with direct contact between sample and reference melts has been carried out to determine the Na2O activity of silicate melt. Thermodynamic mixing properties of silicate melts have been examined based on the temperature and compositional dependences of the Na2O activity. The Na2O activities for soda silicate melt and soda aluminosilicate melt measured by the method of this study are in agreement with previously reported values. The Na2O activities for (100 − x − y)SiO2–xNa2O–yCaO (x = 12–30; y = 5–22, mol%) melts have been measured between 1273 K and 1573 K. The Na2O activity increases with increasing Na2O and CaO content when the melt is a depolymerized composition. The effect of Na2O and CaO contents on the Na2O activity decreases with increasing SiO2 content. They can be interpreted in terms of Gibbs energy of mixing as a concave function of composition. The comparison of partial molar entropy of mixing for soda-lime silicate and sodium silicate melts suggests the existence of local maximum of the entropy of mixing in depolymerized composition of SiO2–Na2O–CaO system. The effect of CaO on redox reaction of iron in silicate melt calculated from the Na2O activity is quantitatively consistent with that measured by voltammetry and wet-chemical analysis.