The phase equilibria in the CaO–Al₂O₃–SiO₂ ternary system doped with around 5, 10, 15 and 20 wt% B₂O₃ was studied using a high-temperature equilibration followed by rapid quenching technique. Fifteen samples with CaO/SiO₂ (C/S) ratios of 0.3, 0.6 and 1.0 containing 15.6–19.1 wt% Al₂O₃ were equilibrated at a range of temperatures close to predicted liquidus phase boundaries within the B₂O₃-free ternary. Quenched samples were characterised using Scanning Electron Microscopy (SEM) to reveal the equilibrium phase assemblage and Electron Probe Microanalysis (EPMA) to determine the chemistry of individual phases. The liquidus temperatures of the synthetic slags were determined within an uncertainty of ± 10–20 °C. Depending on temperature and composition, anorthite (CaO.Al₂O₃.2SiO₂), pseudowollastonite (CaO.SiO₂), gehlenite (2CaO.Al₂O₃.SiO₂) and tridymite (SiO₂) crystals were observed in equilibrium with the liquid phase at temperatures below the liquidus. Doping with successively higher amounts of B₂O₃ caused the boundaries of the initial primary phase fields to shift position, generally resulting in a reduction of the liquidus temperature. The lowest liquidus temperature was 900 °C for slag with initial composition having C/S = 0.6 and 18.8 wt% B₂O₃ representing a decline of the liquidus by 435 °C compared to the undoped slag. Only one sample (C/S = 1.0, 18.8 wt% B₂O₃) resulted in an increase in the liquidus temperature due to the expansion of the pseudowollastonite phase field at high B₂O₃ contents. Based on the experimental results, B₂O₃ may be a suitable fluxing agent to reduce the smelting temperature in the CaO–Al₂O₃–SiO₂–B₂O₃ quaternary system. A comparison of results with liquidus data for similar experiments using Na₂O flux showed that B₂O₃ was more effective in lowering the liquidus.

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