Evolution of solidification structure in steel slag: mechanisms, microstructural control, and implications for sustainable construction materials

Mineral evolution and morphological transformations during steel slag solidification critically influence densification and its performance as a construction material. Although steel slag is commonly used as cement admixture and aggregate, its application in high-grade materials remains limited because disordered crystallization during cooling produces a loose and porous structure. This study investigates the crystal evolution of steel slag under various cooling conditions and explores a pathway to form a dense structure. Focusing on non-equilibrium solidification, microstructural evolution and porosity formation under rapid quenching, the residual liquid fraction was regulated and directional filling was introduced. Using XRD, SEM-EDS, hardness testing and empirical density calculations, the phase transformation process and densification behavior were systematically evaluated. Results show that rapid quenching preserves MgO rich and Fe rich non-equilibrium phases while retaining part of the liquid phase. Porosity is highly sensitive to temperature: above 1200℃ the remaining liquid fills interstitial spaces and stabilizes the structure, whereas below 1200℃ the liquid phase rapidly disappears, final phases precipitate and significant solidification shrinkage occurs. Retaining the liquid phase also modifies the composition and distribution of high-strength minerals, greatly enhancing both micro hardness and macro hardness. Once the liquid phase disappears at or below 1127℃ the hardness of individual phases and the overall matrix decreases markedly. These findings provide a theoretical basis for optimizing the solidification and cooling processes, with practical significance for enhancing the densification, mechanical performance, and environmental value of steel slag.