Modification of Steel Slag Aggregate in Road Engineering: Key Technologies, Performance Enhancements, and Sustainable Prospects

The growing demand for natural aggregates and continued stockpiling of steel slag have increased interest in using steel slag aggregate (SSA) in road engineering. However, delayed hydration of free calcium oxide (f-CaO) and free magnesium oxide (f-MgO), porous and rough surfaces, and the potential release of hazardous elements constrain its long-term application. This review compares aging treatment, surface modification, direct carbonation, microbially induced calcium carbonate precipitation (MICP), and combined treatments from a raw-material heterogeneity and defect-oriented perspective. Their effectiveness is strongly condition-dependent. Aging treatment can control volume expansion, but reaction depth and treatment uniformity remain limited. Surface modification can reduce water absorption and improve interfacial performance but cannot eliminate internal expansive phases. Direct carbonation and MICP can stabilize reactive phases, refine pore structures, and reduce the mobility of some elements, but are limited by mass transfer and equipment requirements, and by mineralization uniformity and ammonium by-product management, respectively. Combined treatments can address multiple defects but increase process complexity, resource consumption, and quality-control requirements. Because material properties and evaluation methods vary among studies, reported performance gains should not be directly used for technology ranking. Instead, technology selection should follow the framework of “raw-material characteristics–dominant defects–preferred technology–engineering boundaries”. From a life-cycle perspective, sustainability depends on balancing resource and environmental benefits against additional treatment burdens. Near-term implementation should integrate raw-material classification, process monitoring, long-term durability and dynamic leaching verification, and the progressive incorporation of key performance and environmental indicators into road-material specifications and engineering acceptance criteria.