This study systematically investigates the engineering characteristics of lead-contaminated red clay stabilized by calcium lignosulfonate and ordinary Portland cement composite binders. A series of experiments were conducted to evaluate the effects of lignosulfonate contents (0%, 0.25%, 0.5%, 1%, 2%), cement content (4%, 6%, 8%, 10%), and lead ion concentration (0%, 0.1%, 1%) on the mechanical properties, permeability characteristics, and leaching behavior. Key findings include the following. (1) Based on the highest mean UCS values observed in this study, the best-performing formulations were 1% lignosulfonate + 4% cement for uncontaminated soil, 0.5% lignosulfonate + 4% cement for 0.1% lead, and 0.25% lignosulfonate + 10% cement for 1% lead. (2) The permeability coefficient initially decreases and then increases with lignosulfonate addition, with maximum reductions of 65.9% and 44.4% for 0.1% and 1% lead contamination under their respective best-performing formulations under these specific test conditions. (3) The leaching concentration of 0.1% lead-contaminated soil met the national standard (<5 mg/L). Critically, however, the 1% lead-contaminated soil failed the TCLP test, with a leaching concentration of 37.3 mg/L, vastly exceeding the regulatory limit. This constitutes a treatment failure for environmental safety purposes, rendering the concurrent mechanical strength improvement irrelevant. (4) Microstructural and X-Ray Diffraction analyses (SEM and XRD) suggest that lignosulfonate improves soil structure by promoting the formation of C-S-H gel and ettringite (3CaO·Al₂O₃·3CaSO₄·32H₂O), whereas high lead concentrations inhibit ettringite formation. This research provides a theoretical foundation for the multi-criteria evaluation and application of lignosulfonate–cement composites in lead-contaminated soil remediation.

To address expansive soil swelling–shrinkage distress and the limitations of single carbide slag (brittle failure), pure fiber (limited strength), and cement (high carbon emissions), this study develops a multi-indicator optimization method for carbide slag–polypropylene fiber composite improvement. Twenty-five full-factorial tests were conducted under three schemes: single slag, single fiber, and composite improvement. Six indicators (USR, LSR, VSR, cohesion, internal friction angle, UCS) were used to construct a CRITIC–entropy–TOPSIS model. The new insight brought by this research lies in coupling combination weighting with TOPSIS to eliminate the deviation of one-sided optimal formulas biased toward swelling inhibition or strength growth obtained by conventional single weighting and conventional range analysis, filling the gap of a multi-objective evaluation system balancing indicator conflict and dispersion, and realizing multi-index balanced optimization. The optimal C8P0.3 (8% slag + 0.3% fiber) has a relative closeness of 0.93. Compared to raw soil, USR drops by 77.07%, and cohesion and UCS increase by 164.30% and 71.91%; compared to C8P0, cohesion and UCS increase by 8.56% and 10.16%, relieving brittleness. XRD/SEM reveal that slag hydration forms rigid C-S-H networks while fibers create flexible 3D networks, jointly achieving rigid–flexible synergy that fills pores and bridges cracks. This work provides an optimization approach for low-carbon expansive subgrade stabilization.