In the present work, we measured the density and the surface tension of CaO–SiO₂–Al₂O₃–R₂O (CaO/SiO₂=0.67, Al₂O₃=20 mass%, R₂O=10.8 mol%, R=Li, Na, K) quaternary melts at elevated temperature using the double-bob Archimedean method and the ring method, respectively.
The density of the CaO–SiO₂–Al₂O₃–R₂O melts decreased with temperature due to the thermal expansion of the melts. In addition, the density of the CaO–SiO₂–Al₂O₃ ternary melt decreased with the addition of the alkali oxides. We converted the density into the molar volume to have a consideration on the microstructure of the melts. The molar volumes of the CaO–SiO₂–Al₂O₃–R₂O melts are proportional to cube of the cationic radius for the alkali ions, which indicates the structural roles of the alkali oxides are mainly charge compensator to AlO₄^5– anions.
The surface tension of the CaO–SiO₂–Al₂O₃–R₂O melts changed only slightly with temperature. The surface tension of the CaO–SiO₂–Al₂O₃ ternary melt increased with the addition of Li₂O. In contrast, the surface tension decreased with the addition of Na₂O or K₂O. As for quaternary system, the surface tension increased with the ionic potential of the additive alkali ions; it is the same tendency with the binary alkali silicate melts. However, the mechanism of the surface tension change with the addition of the alkali oxides has not been clarified in the present study.