Development and Properties of Rapid-Hardening and High-Fluidity UHPC-Based Grout with Sulfoaluminate Cement and Wollastonite Fibers

This study develops a rapid-hardening and high-fluidity ultra-high-performance cement (UHPC)-based grout by incorporating calcium sulfoaluminate (CSA) cement as an early strength component, along with steel and wollastonite fibers as hybrid reinforcements. The UHPC grout proportions containing different contents of CSA cement and wollastonite fibers were designed to investigate fluidity, mechanical properties, hydration kinetics, microstructure and chloride resistance. The results showed that both CSA cement and wollastonite fibers significantly enhanced the compressive and flexural strength of UHPC grout. The incorporation of CSA cement led to rapid compressive strengths of 23 MPa at 6 h and 75.9 MPa at 1 day, marking a significant enhancement compared to the reference group and indicating excellent early-age performance. CSA cement accelerated the hydration process of the UHPC grout and promoted formation of more ettringite. Wollastonite fibers and U-type expansive agent (UEA) further improved the mechanical performance through bridging and physical filling effects. Moreover, CSA cement and wollastonite fibers effectively optimized expansion behavior and refined pore structure of the UHPC grout, and improved its chloride penetration resistance. Although both components influenced the fluidity of the grout, the UHPC grout still maintained high fluidity, offering a promising outlook for its potential use in demanding engineering applications.