Effect of Al ₂O ₃ on the viscosity and structure of calcium silicate-based melts containing Na ₂O and CaF ₂

The effect of Al ₂O ₃ on viscosity in the calcium silicate melt-based system containing Na ₂O and CaF ₂ was investigated and correlated with the melt structure using FTIR (Fourier transform infrared) spectroscopy, XPS (X-ray photoelectron spectroscopy), and Raman spectroscopy. Substituting SiO ₂ with Al ₂O ₃ modified the dominant silicate network into a highly structured alumino-silicate structure with the aluminate structure being particularly prevalent at 20 mass% of Al ₂O ₃ and higher. As the melts become increasingly polymerized with higher Al ₂O ₃ content, the fraction of symmetric Al-O ⁰ stretching vibrations significantly increased and the viscosity increased. XPS showed a decrease in the amount of non-bridged oxygen (O ^-) but an increase in bridged oxygen (O ⁰) and free oxygen (O ^2-) with higher Al ₂O ₃. Although changes in the structure and viscosity with higher CaO/(SiO ₂ + Al ₂O ₃) were not significant, the symmetric Al-O ⁰ stretching in the [AlO ₄] ^5--tetrahedral units decreased. The apparent activation energy for viscous flow varied from 118 to 190 kJ/mol.