In situ study of microstructural evolution of borosilicate glass–sulfate mixture at high temperatures

Microstructural evolution of borosilicate glass–sulfate (BaSO 4 or Na 2 SO 4 ) mixture was observed in situ using a high‐temperature confocal scanning laser microscope. The phase compositions and sulfate retention of the mixture at different temperatures were also examined. The results indicate that for the glass–BaSO 4 sample (abbreviated as BS), the borosilicate glass begins to melt at about 900°C, and the triangular BaSO 4 crystals grow obviously between 900°C and 1100°C. At temperatures above 1100°C, the BaSO 4 crystals gradually melt and decompose. For the glass–Na 2 SO 4 sample (abbreviated as NS), strip‐shaped BaSO 4 crystals begin to form at about 820°C after the borosilicate glass melting, and then gradually melt with further increasing temperature. When the temperature increases to 1000°C, the upper layer of the NS sample possesses Na 2 SO 4 , LiNaSO 4 , and BaSO 4 phases, while the lower layer possesses an amorphous phase. Moreover, the SO 3 retention in the BS sample remains almost unchanged with temperature increases from 900°C to 1100°C, and then decreases with further increasing temperature. The SO 3 retention retained in the NS sample decreases rapidly at temperatures above 900°C. These findings can provide a guidance for vitrifying sulfur‐rich high‐level liquid waste.