Effect of hybrid steel fibers on the mechanical performances and microstructure of sulphoaluminate cement-based reactive powder concrete

This paper investigated a novel reactive powder concrete (RPC) with high strength and toughness, namely hybrid steel fibers reinforced sulphoaluminate cement-based reactive powder concrete (HSRSRPC). In this study, different contents of MSF (macro steel fiber) and mSF (micro steel fiber) were used for HSRSRPC. The mechanical behaviors of HSRSRPC were studied, including strength, load-deflection curves, strength ratio, toughness index, and cracking behavior. Results indicated that the compressive strength and flexural strength of HSRSRPC were effectively improved with an increase of the content of steel fibers; meanwhile, the flexural toughness highly increased owing to the synergistic effect of hybrid MSF and mSF. The microstructure of specimens was analyzed using XRD, TG-DTG and FE-SEM, and the result shows that the addition of mSF accelerated the hydration process of matrix. Furthermore, the reinforcing mechanism of HSRSRPC was revealed, i.e., AFt, C-S-H, and AH₃, as three hydration products exhibiting fibrillar morphology with different sizes, were formed a lot with mSF supply, and contributed largely to the nanostructure and microstructure of matrix; and MSF and mSF, owing to their own characteristics (including the anchoring, interface and bridging effect), made the major contribution to the microstructure and macrostructure.