Development and performance of red mud-based geopolymer grout enabling utilization of solid wastes

In response to the low-carbon strategy, this study develops a geopolymer grouting for shield tunnel tail grouting using red mud (RM) and coal-series metakaolin (CMK) as primary raw materials. A three-level four-factor orthogonal experimental design was used to investigate the effects of RM/CMK mass ratio (RM/CMK), molar ratio of SiO₂ to Na₂O (SiO₂/Na₂O), Na₂O content (Na₂O%) and water-to-binder ratio (w/b) on the multi-performance characteristics. Second-order polynomial regression models revealed the dominant factors affecting fresh properties, mechanical strength and microstructural compactness. Electrical performance was evaluated using resistivity and electrochemical impedance spectroscopy (EIS), while X-ray diffraction (XRD) and scanning electron microscopy (SEM) characterized mineral composition and microstructure. Multi-objective optimization was performed to determine both performance-optimal and cost-optimal mix designs. Results indicate that w/b and Na₂O% are the primary factors influencing grouting fluidity, consistency, stability, and setting time, while the synergistic effect between Na₂O% and SiO₂/Na₂O promotes N-A-S-H gel formation and strength development. Electrical and microstructural analyses show a compact microstructure with restricted ion transport, indicating excellent electrochemical stability and corrosion resistance. The performance-optimal mix (RM/CMK = 1.32, SiO₂/Na₂O = 1.56, Na₂O% = 3.97%, w/b = 0.52) achieved 28-day strength of 9.12 MPa and the cost-optimal mix (RM/CMK = 1.25, SiO₂/Na₂O = 1.27, Na₂O% = 2.11%, w/b = 0.45) cost 20.76 USD/t. The material meets shield tunnel grouting requirements, balancing performance, economy and environmental benefits, offering strong engineering application potential.