Effect of Al2O3 on the viscosity and structure of calcium silicate-based melts containing Na2O and CaF2

The effect of Al2O3 on viscosity in the calcium silicate melt-based system containing Na2O and CaF2 was investigated and correlated with the melt structure using FTIR (Fourier transform infrared) spectroscopy, XPS (X-ray photoelectron spectroscopy), and Raman spectroscopy. Substituting SiO2 with Al2O3 modified the dominant silicate network into a highly structured alumino-silicate structure with the aluminate structure being particularly prevalent at 20 mass% of Al2O3 and higher. As the melts become increasingly polymerized with higher Al2O3 content, the fraction of symmetric Al–O0 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 (O0) and free oxygen (O2−) with higher Al2O3. Although changes in the structure and viscosity with higher CaO/(SiO2 + Al2O3) were not significant, the symmetric Al–O0 stretching in the [AlO4]5−-tetrahedral units decreased. The apparent activation energy for viscous flow varied from 118 to 190 kJ/mol.

► Substitution of Al2O3 for SiO2 increased the viscosity.
► Highly structured alumino-silicates prevalent at Al2O3 20 mass% and higher.
► FTIR and Raman showed Al-O0 stretching vibrations increased with Al2O3 content.
► XPS analysis showed a decrease in O− but an increase in O0 and O2−.