Smectite clay microstructural behaviour on the Atterberg limits transition

Abstract

Abstract Particle space arrangement is a very important factor that determines the physico-mechanical properties of soil. Formations of three-dimensional (3D) structured networks within gelled or flocculated suspension may prevent clay particles and aggregates from building dense aggregates and by encapsulate water within the ultrathin and closed void network, lead to poor sludge dewatering. To better understand the water retention behaviour of smectite-rich clays, a microstructural investigation was conducted on Amcol Australian bentonite in aqueous suspension in near the liquid limit (LL) and the plastic limit (PL). The investigation was conducted with the aid of synchrotron-powered transmission X-ray microscope tomography (TXM), with subsequent computer reconstruction. Images from the microscopy studies were statistically analysed using the STatistical IMage ANalysing (STIMAN) system. The study found that clay particles form a spanned framework in which mineral particles, aggregates and water-filled voids assemble as hierarchic structural elements. The size of these structural elements was larger in the water suspension and subsequently became smaller as an effect of water loss in the suspension > liquid and > plastic limit conditions. The clay suspension structure was almost isometric, with a low anisotropy coefficient: K α – 9%. This parameter increased to K α – 17% in (LL) and increased further in (PL) conditions to K α – 35%. Voids within structural elements were much smaller than the water filled inter-flock voids, with their median diameter 140 nm (suspension), 120 nm (LL) and 90 nm (PL). Significant differences in Atterberg limits values were observed between powder freshly mixed with water and a seasoned sample. Therefore, careful consideration of the sample mineral composition, clay content and genesis must be given due to preparation for geotechnical examination.