The substitutional effect of SiO
2 with Al
2O
3 content on the viscosity in the calcium-silicate melt-based system containing 12 mass% Na
2O and 5 mass% Li
2O was studied. Additions of Al
2O
3 increased the viscosity and higher Al
2O
3 concentrations modified the dominant silicate network into complex alumino-silicates with the aluminate structure becoming particularly prevalent at an Al
2O
3 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 Al
2O
3. Compared to the effect of Al
2O
3, structural changes and the viscosity with higher CaO/(SiO
2+Al
2O
3) were not as significant. However, the symmetric Al–O
0 stretching and the Si–O–Al in the [AlO
4]
5–-tetrahedral units seems to decrease with increased apparent basicity. The apparent activation energy for viscous flow increased with higher Al
2O
3 due to the increased polymerization of the melt and varied from 129 to 156 kJ/mol.
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