Synchrotron Small Angle X-Ray Scattering of Nanoscale Porosity Evolution in Kaolinite and Montmorillonite

A comprehensive understanding of nanoscale microstructural evolution (2–100 nm) is essential for interpreting the hydro-mechanical behavior of clayey soils, yet its quantitative characterization remains challenging. This study employs synchrotron small-angle X-ray scattering (SAXS) to investigate the wetting-induced microstructural evolution of two end-member clays viz Texas montmorillonite and Georgia kaolinite under high-suction conditions. The suction is deliberately controlled from 304 MPa to 4 MPa for Texas montmorillonite and from 153 MPa to 4 MPa for Georgia kaolinite during wetting. The features of the SAXS scattering curves reveal distinct hierarchical structures: Texas montmorillonite exhibits a three-zone pattern with two characteristic peaks, whereas Georgia kaolinite displays a two-zone pattern defined by a single peak. These results indicate a three-level hierarchical nanoscale structure in Texas montmorillonite, and a two-level structure in Georgia kaolinite. Fractal dimension analysis further shows that interparticle pore heterogeneity in Texas montmorillonite initially increases and then decreases upon wetting, while the inter sub-stack surface roughness increases. In contrast, Georgia kaolinite exhibits nearly unchanged interparticle pore roughness throughout wetting.