Viscous Behavior of Alumina Rich Calcium-Silicate Based Mold Fluxes and Its Correlation to the Melt Structure

The substitutional effect of SiO2 with Al2O3 content on the viscosity in the calcium-silicate melt-based system containing 12 mass% Na2O and 5 mass% Li2O was studied. Additions of Al2O3 increased the viscosity and higher Al2O3 concentrations modified the dominant silicate network into complex alumino-silicates with the aluminate structure becoming particularly prevalent at an Al2O3 content of 20 mass% and higher. FTIR (Fourier transform infra red) and Raman analysis showed increased amounts of symmetric Al-O-0 and Si-O-Al stretching vibrations with higher Al2O3. Compared to the effect of Al2O3, structural changes and the viscosity with higher CaO/(SiO2+Al2O3) were not as significant. However, the symmetric Al-O-0 stretching and the Si-O-Al in the [AlO4](5-)-tetrahedral units seems to decrease with increased apparent basicity. The apparent activation energy for viscous flow increased with higher Al2O3 due to the increased polymerization of the melt and varied from 129 to 156 kJ/mol.