Microstructure And Mechanical Properties Of Kaolinite Based On Molecular Dynamics

With the development of“The Belt and Road”,the construction of infrastructure systems,such as transportation,energy resource engineering and hydraulic engineering,have been booming.Many engineering accidents and natural disasters related to rock and soil destruction often occur in the process of engineering construction,which threaten the safety of engineering projects.Therefore,understanding the deformation and strength characteristics of rock and soil is very important for maintaining the safety and stability of engineering buildings.Among the rock and soil mass in various engineering environments,clayey soil is common.Most of studies on clayey soil focus on macroscale and microscale experiments.However,the multi-order nature of clay structure determines that there will be a lack of understanding of clayey soil if only macroscale and microscale characteristics are studied.It is urgent to study the structure and mechanical properties of clay on the microscale.Molecular dynamics simulation can provide detailed information about the structure and dynamic properties of clay minerals at the atomic level.Therefore,this method was used in this paper to deeply explore the structure and mechanical properties of clay on the macroscale.In this thesis,the kaolinite that abound in tropical and subtropical soils of South China is selected as the research object to study the deposition behavior under the influence of kaolinite combination forms and flexibility,and the influence of salt solution environment on the structure and physical properties of the kaolinite-salt solution system.At the same time,through the friction simulation,the microscale friction properties of kaolinite and hydrated kaolinite are obtained.The main results are as follows:（1）Through the initial combination variation and bending simulation of kaolinite plates,the equilibrium configuration after deposition and bending modulus,elastic modulus of kaolinite were obtained respectively.The strong interaction between alumina surfaces and silica surfaces of kaolinite dominates the deposition behavior.After the internal interaction,various kaolinite combinations tend to form the surface-to-surface overlapping equilibrium configuration with the highest density,and a few combinations will form the equilibrium configuration with lower density.The bending modulus of kaolinite plates in x and y directions is 0.307×10^-17N·m and 1.010×10^-17N·m,respectively,and the elastic modulus is 88.3 GPa and 290.7 GPa,respectively,indicating that kaolinite plates are more likely to bend in the x direction.（2）Combined with the conclusions obtained in（1）,a conceptual model of clay deposition behavior considering the influence of kaolinite combination forms and flexibility is proposed.Different initial combinations of kaolinite and their flexibility will affect the microstructure of kaolinite sediments.The combination forms can basically determine the final stacking mode in the sediment.On this basis,considering the influence of the flexibility of kaolinite,the average pore size of sediment model is smaller,the pore size distribution tends to be uniform,the pore volume is smaller,and the overall density of sendiment is larger compared with the kaolinite sendiment model of rigid clay plates.（3）The relative movement between the flow layers in the solution can be achieved by making the kaolinite plates move relative to each other in the kaolinite-water-salt system,and the effects of solution concentration and cation valence on the dynamic behaviors and physical properties of salt solution in the system were comprehensively studied.Through these characteristics,the essence of the influence of salt solution on the electrical double layer was clarified.The ions in solution have two movement modes,including the"limited movement mode"and the"free movement mode",and the water molecules in the system can be divided into four types:strong bonding water in the gibbsite surface adjacent zone,strong bonding water in the siloxane surface adjacent zone,weak bonding water in the sub-intermediate zone and bulk water in the intermediate zone.The mobility of four types of water molecules increases in turn.In the condition of high salt solution concentration and high cation valence,the compression essence of the electric double layer in kaolinite-water-salt system is that the increase of solution concentration and cation valence makes the water molecules in the weak bound water layer move to the bulk water layer,resulting in the decrease of the viscosity of pore salt solution in kaolinite-water-salt system.（4）Through the friction simulation results under different hydrostatic stress,the friction mechanical properties of kaolinite and hydrated kaolinite on microscale are obtained.The microscale friction coefficientμof kaolinite-7（?）,halloysite-8.6（?）and halloysite-10（?）systems are 0.035,0.048 and 0.033,respectively.The friction angleφof three systems are 2.270,2.900 and 2.263,respectively.The cohesion c of three systems obtained by two calculation methods are 0.210,0.229,0.067 GPa and 0.203,0.289,0.102 GPa,respectively.The cohesion calculation results of two methods are different,but basically consistent.The simulation results fill a comprehensive understanding of the mechanical characteristics of kaolinite and hydrated kaolinite,and provide the more effective and accurate theoretical basis for macroscale mechanical experiments and prevention of engineering accidents and geological disasters.