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

Area of Science:

• Geopolymer chemistry and materials science
• Civil engineering and construction materials
• Sustainable and low-carbon infrastructure development

Background:

• Shield tunnel construction requires specialized grouting materials for stability and durability.
• Traditional grouting materials often have significant environmental impacts.
• There is a need for sustainable, high-performance grouting solutions in underground engineering.

Purpose of the Study:

• To develop and optimize a geopolymer grouting material using red mud (RM) and coal-series metakaolin (CMK) for shield tunnel tail grouting.
• To investigate the influence of key mix design parameters on the fresh, mechanical, microstructural, and electrical properties of the geopolymer grouting.
• To identify both performance-optimal and cost-optimal mix designs for practical engineering applications.

Main Methods:

• Orthogonal experimental design (three-level, four-factor) to systematically study RM/CMK ratio, SiO2/Na2O molar ratio, Na2O content, and water-to-binder ratio.
• Second-order polynomial regression models to analyze the impact of variables on grouting properties.
• Electrical performance evaluation using resistivity and electrochemical impedance spectroscopy (EIS).
• Microstructural and mineralogical characterization using X-ray diffraction (XRD) and scanning electron microscopy (SEM).

Main Results:

• Water-to-binder ratio (w/b) and Na2O content significantly influence grouting fluidity, stability, and setting time.
• Synergistic effects between Na2O content and SiO2/Na2O ratio enhance N-A-S-H gel formation and mechanical strength.
• The optimized geopolymer exhibits a compact microstructure, restricted ion transport, and excellent electrochemical stability, ensuring corrosion resistance.
• A performance-optimal mix achieved 9.12 MPa 28-day strength, while a cost-optimal mix was identified at $20.76/t.

Conclusions:

• The developed geopolymer grouting material effectively meets shield tunnel grouting requirements.
• The material balances performance, economic viability, and environmental benefits, demonstrating strong potential for engineering applications.
• This research contributes to sustainable practices in underground construction through the utilization of industrial byproducts like red mud.