Investigating mechanical properties and microstructure of reactive powder concrete blended with sulfoaluminate cement

Super absorbent polymers, expansion agents, and shrinkage reduction agents have been used to improve the self-shrinkage of ultra-high-performance concrete (UHPC), but they may degrade the mechanical properties of concrete. Sulfoaluminate cement (SAC) has been proven to improve the compressive strength and self-shrinkage of UHPC. However, few studies have focused on the role of SAC in the tensile strength and pore structure of reactive powder concrete (RPC), a type of UHPC. In our study, we used compressive strength and splitting tensile strength tests, isothermal calorimetry, X-ray diffraction (XRD), thermogravimetric analysis (TG-DTA), mercury intrusion porosimetry (MIP), and scanning electron microscopy (SEM) to determine the mechanical properties and microstructure of RPC modified by SAC. The results showed that the addition of SAC effectively enhanced the strength of RPC, especially the early strength. Specifically, using an 8 wt% SAC caused an 871.4 % increase in compressive strength and 600% improvement in tensile strength after 8 h of curing. The tensile strength exhibited a strong positive linear correlation with the compressive strength. The rapid hydration of SAC promoted the generation of ettringite in RPC, which significantly increased the cumulative heat release and improved the strength of RPC at an early age; however, its effect was not significant on the later strength. The key reasons for the improvement in the mechanical properties of RPC by SAC modification were pore refinement and aggregate-paste interface improvement. Thus, incorporating SAC into RPC significantly enhance its early-age compressive and tensile strengths, which contributes to accelerate the formwork turnover of RPC components and operational efficiency of the stacking area.