Investigation of the macro performance, mechanism, and durability of multiscale steel fiber reinforced low-carbon ecological UHPC

China proposes a carbon peak by 2030 and carbon neutrality by 2060; reducing carbon dioxide emissions and capturing and using carbon dioxide has become a research focus. As a low-carbon ecological cement, sulphoaluminate cement (SAC) has gradually replaced ordinary Portland cement and has become a research hotspot. SAC has the advantages of high early strength, low CO₂ emission, fast setting, and fast hardening. It is widely used in emergency repairs, rapid repairs, and offshore engineering. This paper developed a new low-carbon ecological ultra-high-performance concrete with high strength and excellent durability, namely sulfoaluminate cement-based reactive powder concrete (SACRPC). This work researched the effect of different scale steel fiber on macro performance (including rheological property, workability, and mechanical properties), durability, and micro/ nanostructure of SACRPC. Various characterization methods showed that the addition of mSF (micro steel fiber) improved the pore structure of the SACRPC, it was increased the porosity of the matrix, and provided enough space for the growth of hydration products AFt (ettringite), AH₃ (gibbsite), and C-S-H (Calcium Silicate Hydrate); thus, it accelerated the hydration process of the SACRPC. The hydration products filled the pores of the SACRPC tightly bonded the fiber and the matrix. It was revealed that the multi-level and multiscale reinforced mechanism, namely hydration products, contributed to micro/nanostructure, mSF bridged micro-cracks, and MSF (macro steel fiber) prevented macro-cracks. In the sulfate erosion experiment, SO₄^2- ions entered the matrix and occurred a chemical reaction; AFt regenerated on the surface and inside the SACRPC with a fibrous morphology and smaller crystals. The regenerated AFt filled the pores of the SACRPC and refined the pore structure. The whole process was similar to the self-healing effect of concrete. The unhydrated cement particles reacted with water and formed new hydration products that healed the cracks.