Accumulation-hydration effect of steel slag powder and its influence on the performance of sprayed ultra-high performance concrete

This study developed a sprayable ultra-high-performance concrete (SUHPC) based on the pulverized steel slag (SSP), the accumulation-hydration effect of SSP in SUHPC were revealed for the first time, which provides theoretical support for the application of supplementary cementitious materials in SUHPC. Results showed that the sprayed thickness achieved about 30 mm when the relative thixotropy of fresh SUHPC attained 4.27 times that of poured UHPC, and incorporating SSP in partial replacement of Portland cement did not influence SUHPC's pumpability, but slightly reduced the sprayability due to its lower angularity. And the SUHPC hardened matrix still shows a dense microstructure after spraying, with about 1.55 times higher coarse porosity than UHPC, resulting in a slight decrease in both strength and toughness. Due to a low reactivity, SSP mainly acts as a microfiller and resulted in a comparable strength by limiting the dosage below 35 %, but no significant effect on SUHPC toughness. Additionally, both the gradual hydration of SSP and the pozzolanic reaction of silica fume will continue to enhance long-term performance of SUHPC.