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) and investigated its rheological behavior, mechanical properties and microstructure evolution subjected to the effect of the SSP addition. Results showed that incorporating SSP in partial replacement of Portland cement (PC) did not influence SUHPC’s pumpability (as reflected by plastic viscosity), but slightly reduced the sprayability (as reflected by yield stress and thixotropy) due to its lower angularity. The sprayed thickness achieved about 30 mm when the relative thixotropy of SUHPC mixed with SSP attained 4.27 times that of conventional poured UHPC. The SUHPC was found to entrap air bubbles during spraying, which subsequently transformed into irregular pores with a low sphericity in hardened matrix, 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%. Additionally, both the gradual hydration of SSP and the pozzolanic reaction of silica fume will continuously enhance the matrix and matrix-fiber bond, contributing to the long-term mechanical properties and durability of SUHPC.