Utilisation and Activation Strategies of Steel Slag for Ultra-High-Performance Concrete: A Comprehensive Review

The use of ultra-high-performance concrete (UHPC) in practical applications has increased rapidly; however, its high cement consumption raises significant concerns regarding carbon emissions and sustainability. Steel slag (SS) as a cement replacement in UHPC offers a cost-effective mitigation strategy. SS shows partial mineralogical similarity to cement but exhibits intrinsically low hydration reactivity. Appropriate activation strategies are essential to enhance the performance of steel slag-UHPC (SS-UHPC). Existing reviews have primarily focused on steel slag in normal concrete or on isolated activation methods, leaving a critical gap in understanding its application in UHPC. The novelty of this review article lies in the integrated analysis of the interrelationships among the intrinsic properties of SS, its activation techniques, and the multi-scale performance of SS-UHPC. Review shows that untreated steel slag powder (SSP) exhibits intrinsically low hydration reactivity and may adversely affect early-age strength and microstructural development. Among various activation methods, carbonation treatment of SSP shows particular promise for enabling high-volume cement replacement (up to 40%) in UHPC. Upon accelerated carbonation, calcium-bearing phases in steel slag are converted into stable calcium carbonates (CaCO 3 ) and reactive silica gel, thereby improving volume stability and enhancing both pozzolanic and hydraulic reactivity. In addition, the substitution and activation of SS in UHPC and normal strength concrete (NSC) are comparatively discussed. This review identifies key research gaps, particularly the insufficient understanding of the relationships between the carbonation efficiency of SS and the resulting mechanical performance of SS-UHPC, as well as durability and engineering-scale considerations. • Systematic review of steel slag utilisation in UHPC systems. • Integrated analysis of activation methods and multi-scale performance. • Carbonation strategy enables high-volume cement replacement in UHPC. • Critical evaluation of hydration, ITZ, durability, and LCA impacts. • Identifies key gaps linking carbonation degree to structural performance.