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-O degrees 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 (O2-) with higher Al2O3. Although changes in the structure and viscosity with higher CaO/(SiO2 + Al2O3) were not significant, the symmetric Al-O-0 stretching in the [AlO4](5-)-tetrahedral units decreased. The apparent activation energy for viscous flow varied from 118 to 190 kJ/mol. (C) 2012 Elsevier B.V. All rights reserved.