Crystallization behavior of B₂O₃-Containing blast furnace slag under varying basicity and cooling rates: A high-temperature confocal laser scanning microscopy analysis

The crystallization behavior of blast furnace slag during cooling dictates its mineral composition and subsequent reuse as a secondary resource. This study examines B₂O₃-containing blast furnace slag under different basicity and cooling conditions to optimize vitrification and waste heat recovery. High-temperature confocal laser scanning microscopy (CLSM), differential scanning calorimetry (DSC), viscosity-temperature analysis, and FactSage thermodynamic modeling were used to analyze phase transformations. Results indicate that B₂O₃ reduces the slag's melting temperature, enhances fluidity, and delays crystallization across all cooling rates. The slag with 4 wt% B₂O₃ and a CaO/SiO₂ ratio of 1.20 experiences insufficient undercooling and high viscosity, suppressing nucleation and crystal growth, favoring the formation of highly reactive glassy slag. High cooling rates induce dendritic growth, while lower rates promote bulk structures. Melilite is the dominant crystalline phase. B₂O₃ inhibits spinel formation while facilitating Mg₃B₂O₆ precipitation, stabilizing the liquid phase and reducing crystallization temperature. At low basicity, calcium aluminosilicates dominate, whereas at high basicity, calcium-rich phases and Mg₃B₂O₆ alter melting behavior. JMA-Ozawa kinetics analysis shows that B₂O₃ increases the Avrami index from 1.01 to 1.56, shifting crystallization from one-dimensional to multi-dimensional growth. This study provides theoretical guidance for optimizing slag granulation, waste heat recovery, and resource utilization, offering innovative strategies for industrial applications.