Influence of red mud substitution on strength, hydration process, and microstructural in GGBS-based UHPC

The utilization of granulated blast-furnace slag (GGBS) as a cement substitute in the production of GGBS-ultra-high performance concrete (UHPC) is attracting increasing attention. However, the economic and carbon emission benefits of using GGBS as a cement replacement are becoming less pronounced. On the other hand, the extremely low water-to-binder ratio in UHPC hinders the rapid hydration of GGBS particles. Therefore, it is necessary to develop a method to optimize the incorporation of GGBS into UHPC. In this study, an alkaline solid waste, red mud (RM), was introduced to construct an RG-UHPC system to investigate its performance and feasibility. The results indicate that, compared to conventional UHPC, the RG-UHPC system exhibits certain performance losses, yet it improves fluidity, reduces drying shrinkage, and decreases porosity. In comparison with GGBS-UHPC, the RG-UHPC system demonstrates enhanced early-age strength. Hydration kinetics analysis reveals that the hydration of RG-UHPC involves multiple simultaneous reaction processes, with RM accelerating the apparent reaction rate during the nucleation and growth (NG) stage. Multifractal analysis further indicates that RM refines large pores and enhances matrix stability. The hydration mechanism and decoupling analysis of RG-UHPC was also discussed. Through environmental and economic assessments, RG-UHPC is shown to be both cost-effective and environmentally friendly, demonstrating potential value for applications in engineering and construction.