A green ultra-high performance calcium sulfoaluminate cement concrete containing recycled fine aggregate: Mechanical properties, microstructure and environmental impact

To reduce the carbon footprint and improve the early age properties of ultra-high performance concrete (UHPC), a green UHPC based on calcium sulfoaluminate cement and recycled fine aggregate (CSAC-RUHPC) was developed in this research. The effects of recycled fine aggregate (RFA) substitution rate, curing time and steel fiber (SF) volume fraction on the workability, mechanical properties, microstructure and sustainability of CSACRUHPC were evaluated. Meanwhile, acoustic emission and digital image correlation techniques were employed for analysis. Results showed that the compressive strength, flexural strength, toughness and crack resistance of CSAC-RUHPC declined with the RFA substitution rate increase, while enhanced with the curing time and SF volume fraction increase. The RFA in CSAC-RUHPC promoted the hydration process, but also weakened the interfacial transition zone and increased the porosity. Considering the balance of mechanical strength and environmental benefits, CSAC-RUHPC with 50 % RFA substitution and 2 % SF volume fraction was the optimal choice. In this case, the compressive and flexural strength of CSAC-RUHPC at 4 h reached 68.4 MPa and 11.62 MPa, and at 28 d reached 121.1 MPa and 19.36 MPa, respectively. Noteworthy, compared to ordinary UHPC, the CO2 emissions and embodied energy reduced by about 20-35 % and 18-30 %, respectively.