Reaction and microstructural insights into UHPC with carbonated steel slag at different substitution ratios

Basic oxygen furnace (BOF) slag, a major type of steel slag generated during steelmaking, offers significant potential as a low-carbon cement replacement. Carbonation pretreatment has been developed to enhance both the filler effect and the pozzolanic reactivity of steel slag powder (SSP), enabling partial replacement of cement with carbonated SSP (CSSP) in ultra-high-performance concrete (UHPC). However, the reaction mechanisms governing CSSP in UHPC remain insufficiently understood. In this study, the phase assemblage, calcium-silicate-hydrate (C-S-H) characteristics, and microstructure of UHPC substituted with CSSP were investigated. The results revealed that the optimal mechanical performance was achieved at a 30 % replacement level, with the compressive strength being 10.0 % higher than that of the untreated SSP mixture. At moderate CSSP contents, carbonation-derived calcium carbonate (CaCO 3 ) acted synergistically as both a micro filler and nucleation site. Consequently, UHPC substituted with CSSP exhibited a higher degree of hydration (DoH), an increased mean chain length (MCL) of C-S-H, and higher pore fractal dimensions than UHPC with untreated SSP. However, at 40-50 % CSSP substitution levels, cement clinker dilution becomes dominant and outweighs the compensatory physical filler effect and chemical contribution of CaCO 3 . Although additional carbonate ions can promote the formation of AFm-type carbonate phases, the reduced availability of reactive clinker limits CH generation and decreases the Ca/Si ratio of the C-S-H gel. Consequently, C-S-H formation becomes insufficient, leading to limited pore refinement, increased pore irregularity, and a less dense UHPC matrix.