Effect of the Na/Mg mixing on the structure and properties of aluminosilicate melts

This study documents the mixed modifier effect occurring between Na and Mg in aluminosilicate glasses. Rheological, thermodynamic, and structural data were acquired on compositions with 37.5 mol% Na2O+MgO, 12.5 mol% Al2O3 and 50 mol% SiO2. When Na substitutes Mg, melt viscosity decreases non -linearly, as does glass configurational entropy, showing large deviations from the ideal mixing law. This suggests ordering in the glass atomic structure upon Na/Mg mixing, as supported by structural data. Indeed, as Na2O/(MgO+Na2O) increases, Raman spectra reveal a shift of the 2Q3= Q2+Q4 equilibrium to the left-hand side, while 27Al MAS NMR spectra reveal that the Al coordination tends to 100 % Al[IV]. 23Na MAS NMR data further indicate that the network modifier/charge compensator role of metal cations also changes upon Na/Mg mixing. Those results therefore suggest important changes in melt/glass structure as alkali cations substitute calc-alkaline ones, with important rheological and thermodynamical impacts.