CRITIC–Entropy-Weighted TOPSIS-Based Mix Ratio Optimization of Carbide Slag–Polypropylene Fiber-Modified Expansive Soil

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.