Structure characteristics and mechanical properties of kaolinite soils. I. Surface charges and structural characterizations

In this study on the surface charges and structural characterization of kaolinite clay, it is found that the highly pH dependent edge charges are important to the interparticle forces and associated fabric formations. When the pH is below the isoelectrical point of the edge surfaces, IEPedge, the Coulombian attraction between faces and edges dominates, which favors the formation of an opened edge-to-face (EF) association and a higher final sediment volume. At pH > IEPedge, the double-layer repulsion prevails at all surfaces to result in a deflocculated and dispersed fabric, which tends to form denser soil packing in the sediment. Increasing the ionic strength (with NaCl as the electrolyte) leads to the opposite behavior in clay suspensions below and above the IEPedge: the degree of fabric flocculation and the final sediment volume decrease with an increased ionic strength at pH < IEPedge but increase at pH > IEPedge. The measured pore-size distribution reveals a distinct dual-porosity characteristic in the pH 4 with salts (0.15 mol/L NaCl) specimen: it consists of inter-aggregate spaces (~1.10 µm) and intra-aggregate pores (~0.30 µm). Soils with a higher degree of EF flocculation due to the prevailing Coulombian or van der Waals attraction can render a higher liquid limit.