A Molecular Simulation Study on Sodium-Montmorillonite Clay Soil Stabilization through Calcium-Based Stabilizers

The most prevalent clay mineral found in soil is montmorillonite. Montmorillonite-rich soils also known as expansive soils can pose hazards that bring geotechnical challenges because their swelling or shrinking behaviour arises due to the large water retention capacity of montmorillonite-rich soils. This soil swelling reduces the shear strength of soil and results in differential settlement of foundation, which compromises the structural integrity of the infrastructure. Montmorillonite is made up of multiple-layer structures, and these interlayers contain free cations that enable attachment of water molecules, which cause volumetric expansion of the soil. To prevent swelling, calcium-based stabilizers are often utilized for sodium-montmorillonite (Na-MMT) clay stabilization. These calcium-based stabilizers replace sodium ions with calcium ions with creating a diffuse layer around clay particles that affects the water adsorption capacity of Na-MMT. Therefore, to ensure structural safety and soil stability, it is essential to predict accurate soil properties, which depend on soil-water interactions; a pore-scale study of soil stabilization provides an enhanced understanding of soil-water interactions and water adsorption in the clay, which is responsible for swelling in Na-MMT. This study examines water adsorption and soil-water interactions inside montmorillonite clay pores using the Monte Carlo molecular simulations to quantify the systematic exchange of sodium with calcium cations and their influence on swelling behaviour in montmorillonite. The pore width (multiple of d-spacing) ranges from 10 to 20 Å, and varied pH environment via change in Ca²⁺ cation exchange compositions upto 100% have been simulated under in-situ conditions of temperature range of 288 to 308 K and at a pressure of 1 atm. The ClayFF forcefield was used to modelled Na-MMT clay pore, and the SPCE forcefield was used to modelled the water molecules. The simulated bulk densities of water were validated with literature data at the considered thermodynamic conditions. The Ca²⁺ exchange indicated an influence on the hydration behaviour of Na-MMT and altered the molecular ordering of water inside the pore. The adsorption of water shows dependency on interactions between water and the pore surface, as well as the available pore volume. Furthermore, these simulations analysed the percentage change in cation composition on the surface using local density distribution profiles and pore pressure across the height of the pore. This study aims to provide molecular insights into the performance of calcium-based stabilisers on expansive soils and clay-water interactions, which will help to predict pore pressure, swelling and softening and improved stability of expansive soils.