Influence of preferred orientation of clay particles on the diffusion of water in kaolinite porous media at constant porosity

Abstract

Solute transport in natural or artificial compacted clay porous media is receiving particular attention in the contexts of waste storage and the design of materials with tuneable physical properties. In these contexts, the porosity is commonly considered as a primary parameter controlling the diffusional properties of water and solutes in these systems. However, little attention has been given to the role played by anisotropy in the particle orientation. In this study, the influence of the preferred orientation of clay particles on the water diffusion anisotropy in two kaolinite porous media obtained by compaction and centrifugation methods (for a constant porosity value of ~0.5) was investigated by coupling experiments and simulations. An increase in the preferred orientation of kaolinite particles, as quantified by X-ray scattering analysis, was found to be logically associated with an enhanced anisotropy in water diffusion obtained from pulsed gradient spin echo attenuation measurements by nuclear magnetic resonance of protons. Brownian dynamics simulations performed on three-dimensional virtual porous media, mimicking the shape and orientation of the particles in the samples, led to calculated water diffusion coefficients in agreement with experimental data. Once validated, this computational work was extended to a wide range of degrees in the preferred orientation of particles. The results showed that this parameter leads to an increase and a decrease in pore water diffusion coefficients along and across the mean orientation plane, respectively, up to a factor ~2. The directional diffusion anisotropy was also found to range between 1 and ~5 for the most isotropic and anisotropic organisations, respectively. This study hence provides quantitative insights into the impact of the preferred orientation for the prediction of water diffusion in compacted clay media.