Molecular Dynamics And Metadynamics Simulation Of Na-,K-,Ca-Montmorillonite

Clay minerals are the most abundant and widely distributed minerals in the Earth's surface sediments.They are important components of clay rocks and soils,including kaolinite,montmorillonite,illite,etc.Montmorillonite is a typical clay mineral.Studying the characteristics of water absorbed swelling and structural strength,and also its physical and mechanical properties are significant to solve the mechanical problems arising in geotechnical engineering applications.In this study,Montmorillonite is the main research object.Firstly,the molecular mechanics and molecular dynamics simulation for different hydration degree with Na-,K-,and Ca-montmorillonite under different temperature and pressure conditions were conducted,the relationship between layer spacing and water content was analyzed to find out the influencing factors and intrinsic mechanism of montmorillonite swelling,the interparticle radial distribution function was used to analyze the structural characteristics of montmorillonite.The diffusion coefficients of interlayer cations and water molecules were calculated under different temperature,pressure s and water content condition for analyzing the dynamical characteristics of montmorillonite interlayer particles.It was found that the increase of montmorillonite interlayer spacing was mainly caused by water absorption and expansion,and it is also related to temperature rising.The interlayer water molecules all gathered around the cation to form two obvious hydration layers.Ca²⁺ has a significant ability to combine water molecules than Na⁺and K⁺.So it is harder to gather water molecules on its surface for Ca-montmorillonite than Na-montmorillonite and K-montmorillonite.The reason for the greater diffusivity of interlayer particles is dependent on higher hydration degree of montmorillonite,higher temperature and lower pressure.Molecular dynamics method was used to simulate Na-montmorillonite,K-montmorillonite and Ca-montmorillonite models in tension and compression.A series of simulations were conducted to investigate the effects of different temperatures and hydration degrees on the mechanical properties of montmorillonite,to obtain the elastic constant tensor by finite deformation of montmorillonite crystals,and then to calculate the average elastic modulus of montmorillonite polycrystals.It was found that the mechanical properties of Na-montmorillonite depend on the temperature and hydration degree,and the elastic properties in x,y and z directions showed obvious anisotropy.The free energy of Na-montmorillonite and Ca-montmorillonite with different number of water molecules from adsorbed states to desorbed states were simulated by metadynamics.The matric potential was derived and calculated by the difference between the adsorption free energy and self-hydration free energy.The matric suction equals to a negative of matric potential.Lastly,the matric suction of Na-montmorillonite with 5%-30%water content was found between 4.2 and 0.19GPa,and the matric suction of Ca-montmorillonite was between 4.4and 0.0014GPa.