Abstract
Montmorillonite is the principal constituent of many expansive soils. The swelling of expansive clays upon hydration can cause significant distress to infrastructure. On the other hand, the swelling characteristics lead to many beneficial applications of these clays, such as barrier materials and in nanocomposites. The montmorillonite hierarchical structure plays a vital role in the swelling of expansive clays. The hierarchical structure of montmorillonite consists of four levels, clay mineral layers, tactoids, aggregates, and assembly of aggregates. The clay tactoid is considered as the fundamental clay particle, which consists of several clay mineral layers stacked in the Z direction. We report here for the first time a molecular mechanics study of the clay tactoid. The molecular model of sodium-montmorillonite (Na-Mt) tactoid was built, and its mechanical properties were evaluated using Molecular Dynamics (MD) and Steered Molecular Dynamics (SMD) simulations. The mechanical response of Na-Mt tactoid was evaluated under compression, tension, and shear. Compression tests revealed a short linear characteristic with a compression modulus of 125 GPa followed by a nonlinear portion with increasing modulus approaching 640 GPa, which is the modulus of a single clay mineral layer. The tensile pulling of the tactoid also indicated an influence of the tactoid nanostructure with the influence of multiple neighboring layers of tactoid. The shearing simulations indicated first a mechanical response with a modulus of 10 GPa ending with locking of interlayer cations into tetrahedral cavities followed by small displacements exhibiting a modulus of 70 GPa. All of the mechanical behaviors were related to the evaluated interaction energies between clays and cations during loading. These studies elucidated key mechanisms of swelling clay tactoid response to loading.
Published Version
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