Crystallization behavior of calcium silicate phases in molten steelmaking slag

Complex mineral phase structure, variable physicochemical properties, and uncertainty in the crystallization behavior of steel slag pose significant challenges to its treatments and industrial applications. This study aims to elucidate the evolution of steel slag in the solidification process the melt. Details of calcium silicate phase precipitation were investigated through thermodynamic calculation, in-situ, and ex-situ analysis, respectively. The results of the theoretical calculations were significantly different from the results of the ex-situ analyses. According to the crystalline thermodynamic analysis of the basic oxygen furnace (BOF) slag, the phases in the slag contain CaO, MgO, Ca ₃SiO ₅ (C ₃S), and liquid phases in the temperature range of 1400–1600 °C. However, according to the analyses of ex-situ measurement data, in this temperature range, the BOF slag exhibits the generation and evolution of Ca ₂SiO ₄ (C ₂S), Ca ₂Fe ₂O ₅ (C ₂F) phases in addition to the mentioned crystalline phases. Combined with in-situ analysis, it was found that as the temperature decreases and the holding time increases, the size of calcium silicate crystals increases due to the synergistic effects of crystal aggregation and growth. The crystallization growth process of C ₃S follows a certain growth sequence, with its morphology evolving from aggregated dendritic structures towards irregular shapes with sharp angles. The complete decomposition of C ₃S decreases the compositional fluctuations in the reaction system, leading to a more stable interface between C ₂S and CaO.