Dual-Functional Solid Waste-Based Grout for Fracture Sealing and Sustainable Pb(II) Immobilization in Karst

To address the coupled challenges of fracture leakage and heavy-metal contamination at tailings storage sites in karst regions, this study develops an environmentally friendly grouting material dominated by fly ash (FA), steel slag (SS), and desulfurized gypsum (DG) with dual functionality: fracture repair (sealing/reinforcement) and heavy-metal immobilization. An orthogonal design with range analysis identified the optimal formulation as a water-to-binder ratio of 0.8, activator dosage 2%, anti-washout agent 0.4%, and an SS: FA mass ratio of 4:6. The optimized grout exhibits excellent workability and mechanical performance: flowability 215 mm, anti-washout ratio 99.8%, and 28-day compressive strength 8.4 MPa, meeting the engineering requirements for karst fracture remediation. Using XRD, FTIR, TG-DSC, and SEM–EDS, we elucidate that hydration products, primarily C-S–H gel and AFt/ettringite, provide abundant functional sites and porous frameworks responsible for Pb(II) uptake. The adsorption mechanism is dominated by chemisorption(complexation/coordination and ion exchange), with physisorption acting synergistically. The maximum equilibrium adsorption capacity reaches 618.68 mg/g. Combined with its high anti-washout performance and adequate strength, the material maintains structural stability and achieves effective immobilization of heavy metals under static underwater conditions simulating submerged placement. These findings provide a new materials-based pathway and mechanistic basis for the green remediation of karst fractures and the prevention and control of heavy-metal pollution at tailings storage sites.