Development and performance evaluation of a novel ultra-high performance concrete based on low-carbon calcium sulfoaluminate cement

Ultra-high performance concrete (UHPC) faces challenges in optimizing workability, strength, and durability while maintaining sustainability. Calcium sulfoaluminate (CSA) cement, known for its high early strength and low carbon footprint, offers a promising alternative binder. However, its application in UHPC requires further optimization to balance mechanical performance and shrinkage resistance. This study developed a novel UHPC using solid waste-based calcium sulfoaluminate cement (SCSA) and investigated the feasibility of substituting silica fume (SF) with steel slag micro powder (SSMP). The physical properties, hydration process, volume stability, microstructure and carbon footprint of the novel UHPC system were systematically examined. The results demonstrated that the SSMP enhances the mechanical strength of UHPC at all ages, with an 80% replacement increasing compressive strength by 25.9% at 3d, 18.8% at 7d, and 13.4% at 28d compared to the control group. XRD, TG, and MIP results indicate that SSMP further promotes hydration by generating C–S–H through its cementitious activity and pozzolanic reactions. Furthermore, when the SSMP replacement ratio does not exceed 40%, it effectively reduces the autogenous shrinkage of UHPC, addressing a key limitation of conventional UHPC. Microstructural observations show that the addition of SSMP reduces harmful pores, and enhances ettringite crystal interlocking. Moreover, carbon footprint analysis indicates that SCSA-UHPC is a greener material with lower carbon emissions, while the incorporation of SSMP as a replacement further reduces carbon footprint by utilizing solid waste. However, the SSMP content should be controlled, as excess content compromises workability and volume stability. These findings highlight the potential of SCSA-UHPC as a sustainable and high-performance material, providing new insights for low-carbon construction applications.