Mixed Alkali Effect in Viscosity of CaO-SiO₂-Al₂O₃-R₂O melts

The mixed alkali effects of Li₂O, Na₂O, and K₂O on viscosity of CaO-SiO₂-Al₂O₃ melts are investigated by rotating cylinder method. It is found that the single addition of Li₂O, Na₂O, or K₂O to CaO-SiO₂-Al₂O₃ melt will reduce the viscosity of melt, regardless of the Al₂O₃ content, while the degree of reduction follows the sequence of Li₂O > Na₂O > K₂O. For melts containing two alkali oxides, viscosity monotonically decreases when one alkali oxide is replaced by another at all temperature investigated in this paper, but no extrema of viscosity is found like the behavior of electrical conductivity. In addition, the extrema are not found in the slag containing three alkali oxides if changing their relative contents but keeping their total contents constant. Raman spectroscopy is used to illustrate the relationship between viscosity and structure of melt and it is found that degree of polymerization is the major factor affecting the viscosity of melt.