Microstructural Evolution and Mechanical Response of UHPS Incorporating Steel Slag Fine Aggregate and Different Morphology Steel Fibers

To mitigate ecological stress caused by natural aggregate extraction and steel slag stockpiling, replacing manufactured sand (MS) with steel slag fine aggregate (SSFA) in ultra-high-performance shotcrete (UHPS) is a promising sustainable strategy. By adjusting the sand–cement ratio (0.7–0.9), steel slag content (0–75%), and the composite ratio of two types of steel fibers. The present study explores the effect of SSFA replacing MS on the mechanical properties of UHPS and conducted a microstructural analysis. While ensuring workability, as the sand–cement ratio increases from 0.7 to 0.9, the mechanical properties of UHPS continue to improve; as the steel slag content increases from 0 to 75%, the compressive strength and flexural strength of UHPS show a gradual upward trend, increasing by 1.15 times and 1.23 times, respectively. The optimal split tensile strength was determined to be at a steel slag dosage of 50%. In comparison with straight steel fibers, hooked-end steel fibers have been shown to enhance compressive strength by an average of 9%, flexural strength by an average of 11%, and splitting tensile strength by an average of 19%. Furthermore, the synergistic effect of steel slag as fine aggregate and steel fibers demonstrated a positive impact on the energy absorption of UHPS. Therefore, an appropriate proportion of steel slag can effectively improve the mechanical properties of UHPS and promote the recycling of industrial solid waste steel slag.