Effect of CaF₂, B₂O₃ and the CaO/SiO₂ mass ratio on the viscosity and structure of B₂O₃-containing calcium-silicate-based melts

The relationship between the viscosity and structure of B₂O₃-containing calcium-silicate-based mold fluxes and the effects of fluidizers including CaF₂, CaO, and B₂O₃ on the viscosity and their correlation with the structural aspects were studied using a rheometer with Fourier transformation infrared and Raman spectroscopy. The viscosity decreased with increasing CaF₂ addition up to 28 wt% at a fixed CaO/SiO₂ ratio of 0.3, which was related to depolymerization. Furthermore, CaF₂ addition also affected the apparent activation energy for viscous flow, which decreased with increasing CaF₂ content to 105.1 from 151.1 kJ/mol. At higher C/S ratios, the viscosity decreased in the presence of greater Ca²⁺ and O²⁻ supplied from CaO, which subsequently increased the activation energy to 149.7 from 122.0 kJ/mol. With regard to the B₂O₃-melt, polymerization of the network structure was observed by comparing the B₂O₃-free to 4.4 wt% B₂O₃ content. However, the viscosity was relatively constant with increasing B₂O₃ addition. However, the viscosity decreased due to greater simplification of the network structure above 4.4 wt% B₂O₃. The break temperature decreased with greater B₂O₃ addition as the crystallization was inhibited. Furthermore, the apparent activation energy decreased as depolymerization of the network structures occurred above 4.4 wt% B₂O₃.