# Microstructure, Transport, and Mechanics of Compacted Clay Simulated at the 0.1 μm Scale (1400 Smectite Clay Particles) Using a Coarse-Grained Model with Explicit Counterions

## TL;DR

This paper uses a coarse-grained model to simulate compacted clay at a 0.1 μm scale, bridging the gap between atomistic and continuum-scale models to study its microstructure and transport properties.

## Contribution

The study introduces a coarse-grained framework that simulates compacted smectite clay at a mesoscale, capturing features like tactoid formation and anisotropic pore networks.

## Key findings

- The model captures mesoscale heterogeneity, including tactoid formation and hierarchical porosity in compacted clay.
- Compaction and counterion composition significantly influence pore size, transport, and electrochemical behavior.
- Swelling pressure predictions may require simulations at larger scales (up to 1 μm).

## Abstract

Clay-rich geomaterials play a critical role in many subsurface
systems. The macroscale properties of these materials (low permeability,
high ionic conductivity, high swelling pressure, etc.) are sensitive
to molecular-level adsorption and hydration interactions at clay–water
interfaces. Efforts to develop multiscale simulation approaches to
predict these properties reveal a scale gap between atomistic simulations
(typically limited to systems smaller than 10 nm) and continuum-scale
models (which use computational grid elements with dimensions ≳
10 μm). In this study, we present a coarse-grained (CG) framework
that partly bridges this gap by simulating compacted smectite clay
assemblages with dimensions of 0.1 μm containing 1,400 clay
particles across a range of dry densities (1,050 to 1,850 kg·m–3) and Na/Ca counterion compositions (Na fraction ranging
from 0.2 to 1). The simulated systems, along with their reconstructed
binary three-dimensional pore networks, are used to evaluate the microstructure,
pore size distribution, tortuosity, ion diffusivity, and swelling
pressure of compacted smectite clay. Results show that our approach
captures important features of the mesoscale heterogeneity of compacted
clays, including tactoid formation, hierarchical porosity, and anisotropic
pore networks. Results also reveal how compaction and counterion composition
govern emergent behaviors, including dominant pore sizes, directional
transport, and electrochemical response. This work highlights the
potential of CG simulations to bridge molecular and continuum scales
and to advance geotechnical and environmental applications involving
clay-rich materials as well as related nanoporous media such as geopolymers
and calcium-silicate-hydrate. However, the results also suggest that
accurate prediction of certain microstructural and mechanical properties
(e.g., swelling pressure) may require even larger-scale systems on
the order of 1 μm.