Hydration sequence controls polymer-induced water repellency and hydraulic response in illite-rich clay

Although geomaterial hydrophobisation is well established, the role of hydration sequence in polymer-induced water repellency of illite-rich clay remains insufficiently constrained. A butyl-acetate-assisted polydimethylsiloxane (PDMS) formulation was applied to a multi-mineral illite-rich clay through two pathways: hydrophobisation before wetting (HW) and wetting before hydrophobisation (WH). Wettability, soil water repellency, hydraulic behaviour, structural stability and microstructure were examined using contact-angle measurements, water drop penetration time (WDPT), molarity of an ethanol droplet (MED), hydraulic tests, soil-water characteristic curves (SWCCs), slaking tests, microscopy, mercury intrusion porosimetry (MIP) and comparative molecular-scale simulations. The HW pathway generated an atypical, non-monotonic repellency curve. Extreme hydrophobicity remained resilient at oven dryness and peaked (WDPT ∼18,000 s) near 8–10% moisture. HW specimens showed a lower degree of saturation, up to 73% lower apparent hydraulic conductivity, and substantially greater resistance to infiltration-induced slaking. Conversely, pre-wetting (WH) produced a less persistent water-repellent state with a rapid, monotonic decline in repellency after minor hydration, while apparent conductivities remained comparatively higher. Microstructural observations indicate that the dominant capillary fabric was not substantially reorganised within the accessible pore-size range. This finding points to a hydraulic response governed primarily by altered interfacial wetting and incomplete saturation, rather than by dominant large-scale pore blockage. Under their assumed starting configurations, comparative molecular-scale models were consistent with stronger electrostatic screening in PDMS-rich dry-surface configurations and residual water–mineral affinity in the mixed pre-hydrated configuration. Consequently, path-dependent surface hydration governs both the persistence of engineered water repellency and its ensuing hydraulic consequences in multi-mineral illite-rich clay.