Mesoscale Modeling and Properties of Clay Aggregates

The clay phase of many natural soils comprises a micro-structure of clay aggregates. These can be formed during sedimentation, due to van der Waals attraction between negatively charged particle surfaces in saltwater environments, or can occur in partially saturated soils where colloidal iron acts as a cementing agent. In order to understand the formation of clay aggregates and their role in affecting properties at the macroscale/continuum level, we have carried out multiscale analyses, initially considering the formation and properties of the aggregates. Nanoscale numerical simulations consider interactions between two clay platelets. The analyses focus on Wyoming montmorillonite (Na-smectite) and use the CLAYFF force field to describe pairwise interactions between ions within the clay and surrounding bulk water (i.e., Coulombic and van der Waals forces). The analyses establish the potential of mean force at different spacings between the layers for edge-to-edge and face-to-face interactions. The results are then used to calibrate the Gay-Berne (GB) potential that represents each platelet as a single-site ellipsoidal body. It is then possible to simulate the process of aggregation for an assembly of clay platelets in mesoscale simulations. These simulations find that aggregates of Na-smectite typically form in face-to-face stacks with 3–8 platelets. The particle assemblies become more ordered and exhibit more pronounced elastic anisotropy at higher confining pressures. The computed elastic stiffness properties are in good agreement with previously measured nanoindentation moduli over a wide range of clay-packing densities.