MOLECULAR DYNAMICS SIMULATION OF CRYSTALLINE SWELLING AND FAILURE PROPERTIES OF MONTMORILLONITE CLAY

BABAK ABBASI

Clay minerals play considerable importance in geotechnical, agricultural, and pharmaceutical applications. They are made of plate-like particles with micrometer dimensions and play a critical role in problems involving swelling, deformation, and failure in geomaterials. Understanding of these phenomena and the parameters that influence them requires studies at the nanoscale. Molecular dynamics is utilized here to study effects of different interlayer cations including Li+, Na+, K+, and Cs+ on the crystalline swelling, structure, and the dynamic properties of interlayer water in Wyoming-montmorillonite. The hydration thermodynamics of swelling is quantified using the hydration and immersion energies. Interlayer structure and mobility of water molecules and cations are investigated at different hydration levels from 0 to 500 mgwater/gclay. The effects of temperature on the mobility of interlayer water and cations are studies using the self-diffusion activation energies. The variation of the density states of spectral frequencies are studied for vibrational modes of interlayer water. It is found that the structure and dynamic properties of interlayer not only depend on the size of the exchangeable cations, but also on the hydration state and molecular structure of clay minerals. Failure mechanisms of dry clay sheets of Na-montmorillonite are investigated under uniaxial tensile and compression loadings using MD simulation and in the canonical ensemble. Stress-strain response of hydrated clay in uniaxial compression show nonlinear behavior which is accompanied with formation of extra layers of water in the interlayer. The nanoscale mechanism of the sliding of clay platelets at different states of hydration and hydrostatic stress is studied in the isobaric-isothermal ensemble. The molecular structure of interlayer water, number of hydrogen bonds, and their configuration are examined during shear loading. The maximum average shear stresses coincide with the formation of the first, second, and third water layers. At constant hydrostatic stresses, the average shear stresses did not show a consistent behavior when the water content was varied. However, the average shear stress was higher at larger hydrostatic pressures. The average values of nanoscale cohesion in the range of water contents of 100 and 400 mgwater/gclay were C ̅_ave=0.11 GPa and φ ̅_ave=3.3°.

MOLECULAR DYNAMICS SIMULATION OF CRYSTALLINE SWELLING AND FAILURE PROPERTIES OF MONTMORILLONITE CLAY

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