Recycling of shield muck with industrial solid wastes for low-carbon synchronous grouting materials: A full case study from laboratory investigation to field implementation

This study employs ground-granulated blast-furnace slag, desulfurized gypsum, and coal fly ash to solidify shield-tunneling muck for the on-site preparation of synchronous grouting materials. The influences of curing-agent content (CAC) and muck suspension density (SD) on the grout properties were investigated in detail. The response surface methodology was adopted to establish nonlinear response models that accurately reflect the performance responses under the coupled effects of these two factors. Additionally, the microscopic hydration mechanism was analyzed via SEM, XRD and Zeta potential measurements. The NSGA-Ⅲ algorithm was utilized to optimize the volume fractions of grout components (curing agent, soil particles, and water). The optimized mix proportion was subsequently validated through full-scale engineering applications. The results indicate that SD and CAC exert a significant nonlinear coupling effect on the grout performance. This geopolymer system delivers excellent solidification performance for mucky clay-rich shield muck suspensions. Its 28-day unconfined compressive strength (UCS) reaches a maximum of 9.1 MPa with an average value of 2.89 MPa, which is markedly higher than the 1.17 MPa achieved by conventional cement–fly ash grout. With mechanical performance as the primary consideration, the optimal volume fractions are determined as 71.08% water, 6.19% curing agent, and 22.72% soil particles. The water content of the grout directly governs the activation degree of cementitious precursors and the types of hydration products formed; a higher SD promotes more thorough hydration but may induce larger pore structures. Field test results demonstrate that, compared with conventional cement-fly ash-based synchronous grouting materials, the novel grout reduces segment uplift by 30–40% and lowers injection pressure by 10–15%, thus weakening the disturbance to the surrounding soil. Owing to its excellent flowability and high filling efficiency, the grouting volume is increased by 15–20%.