Comparative case study of fresh and spent oil refinery FCC catalysts as cement replacements in UHPM

Ultra-high-performance mortar (UHPM) delivers outstanding mechanical strength and durability but typically depends on clinker-rich binders with a substantial environmental footprint. Spent fluid catalytic cracking (FCC) catalysts from petroleum refineries represent a promising supplementary cementitious material; nevertheless, the comparative effects of catalyst condition (fresh versus spent) on hydration behavior, pore structure, and ductility are still poorly understood. This work investigates UHPM mixtures containing 10–30 wt% fresh or spent FCC catalysts combined with recycled steel fibers at a water-to-binder ratio of 0.20. Compressive strength tests, notched three-point bending, and X-ray micro-computed tomography (µCT) were employed to link microstructural features to mechanical response. The results demonstrate that spent FCC catalyst raised compressive strength by 12% (from 161.5 MPa to 180.8 MPa) and increased residual flexural strength by 30–75% over the full CMOD range, achieving strain-hardening behavior at 20 wt% replacement. Conversely, fresh catalyst decreased compressive strength by 20% and reduced flow spread by 40% at comparable replacement levels. µCT analysis showed lower overall porosity (≈3–4%) and more uniform fiber distribution in the spent catalyst mixtures. Overall, the findings confirm that spent FCC catalyst permits clinker reduction while simultaneously enhancing performance, offering a circular valorization pathway for refinery waste in UHPM.