Valorization and enhancement mechanism of ferrochrome slag as aggregate for manufacturing ultra-high performance concrete (UHPC)

Ferrochrome slag (FCS) is a by-product discharged in the smelting of ferrochrome alloys. This high-volume waste can be effectively valorized by utilizing it as aggregate in ultra-high performance concrete (UHPC), which is beneficial for resolving the environmental issue induced via its buildup and potential risk of chromium leaching. Additionally, it can also compensate for the shortage of natural aggregate (NA), and reduce the cost of UHPC. Its feasibility as aggregate in UHPC was explored. The enhancement mechanisms of FCS aggregate dosage, pre-wetting water, and steel fiber on the performances of UHPC were systematically investigated. The results indicated that FCS with rough texture, higher water absorption and lower strength can substitute NA in the manufacturing of UHPC. The mechanical and drying shrinkage properties of UHPC mixed with FCS were improved, which was attributed to the synergistic effects including the reduction of effective water-binder ratio and porosity, the enhanced interface structure, and the internal curing to matrix. The water film layer enriched surrounding aggregate absorbed by FCS, which reduced the enrichment and directional growth of portlandite at interface. This also inhibited the water absorption of UHPC, and may hinder the transport of harmful ions inside UHPC. With the incorporation of FCS, its autogenous shrinkage increased, this matter can be ameliorated by the pre-wetted aggregates with water not exceeding 45%–65% of the saturated water absorption rate. Furthermore, the reinforcement of steel fiber on UHPC strength gradually decreased due to the fiber dispersion was affected by the declined workability and aggregate morphology.