Hydrophobic Modification of Silt: Durability Performance Evolution and Microstructural Stability Under Cyclic Wetting–Drying Conditions

Wet–dry cycling triggers progressive degradation of the physical and mechanical properties of silt soils, severely compromising the long-term serviceability and structural safety of silt subgrade infrastructure. This study proposes a sustainable nano-hydrophobic material (NSHM) modification strategy to enhance the W-D cycle durability of silt. A multi-scale experimental program integrating wettability characterization, mechanical testing and microstructural analysis was conducted to elucidate the modification mechanism, performance attenuation law and microstructural evolution of NSHM-treated silt under cyclic wetting–drying. Results reveal that NSHM effectively imparts robust water repellency to silt, with a distinct dosage threshold effect and a synergistic enhancement from soil relative density. Silt modified with 0.5% NSHM maintains stable hydrophobicity after 5 W-D cycles, with a contact angle reduction of less than 1.3%. Compared with untreated specimens, the 0.5% NSHM-treated silt exhibits only 3%–8% unconfined compressive strength loss, 20%–30% higher cohesion and 8%–20% higher internal friction angle after 5 cycles. The superior durability originates from the synergistic effect of chemical anchoring and physical coating, which firmly immobilizes the hydrophobic network on particle surfaces and overcomes the inherent drawbacks of easy leaching and poor durability in conventional modification methods. This work provides a sustainable soil improvement strategy for W-D-prone regions, with significant engineering value for mitigating performance degradation of geotechnical infrastructure.