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

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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–O⁰ 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–O⁰ 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.