Near-surface clay sediments: self-assembly and response to ionic concentration gradients

Clay self-assembly, fabric formation and fabric changes are governed by mineral composition, pore fluid pH and ionic strength. This study combines theoretical analyses, extensive data compilation and targeted experiments to investigate underlying processes. Experiments include sedimentation–diffusion tests combined with multi-physics monitoring, including micro computed tomography, optical and acoustic imaging, grazing-incidence X-ray diffraction and nuclear magnetic resonance spectroscopy. Updated fabric maps for 1:1 kaolinite and 2:1 montmorillonite reveal distinct particle associations within the pH–ionic concentration space delineated by well-defined thresholds. The initial void ratio e0 at the onset of sediment formation is primarily controlled by specific surface area, and is higher than the void ratio at the liquid limit; hence, soft sediments may swell towards e0 when flooded – even with brine. Unconstrained fine particles can migrate against ionic concentration gradients (diffusiophoresis), while dense packings hinder ionic diffusion by excluding transport through narrow pore throats. Salt diffusion causes volumetric strain but with minimal fabric changes, underscoring the stability of self-assembled networks. Vertical and horizontal discontinuities may form during ionic diffusion, often nucleating at grain-displacive bubbles formed from excluded gas. These processes help explain phenomena such as soil dispersion, quick clay formation, surface erosion, clogging and clay liner degradation under leachate exposure.