| Expansive clay minerals are the key objects of disaster prevention and control in geomechanics,and also one of the most important geomaterials in civil engineering.They have strong hydrophilicity,and the interlayer is easily softened after hydration.A small amount in the soil can cause significant changes in macroscopic physical and mechanical properties,thus playing a critical role in problems involving swelling,deformation,and failure in geomaterials.To ensure the engineering construction safety of these"special soils"with nanoscale size and molecular-scale water-sensitive characteristics,it is necessary to understand the hydration characteristics and swelling mechanisms of clay minerals themselves.Traditional theories and methods are subject to many limitations when they encounter small-scale geotechnical problems.However,molecular dynamics（MD）method can solve these problems.This subject is based on the urgent hydration properties and micro-mechanisms in expansive clay minerals.And the basic physical and mechanical properties of the main expansive clay in geotechnical engineering including montmorillonite（MMT）under adverse conditions such as hydration are studied based on MD method from the nanoscale.The quantitative analyses of their thermodynamic swelling properties,tensile,compressive and shear mechanical properties,and their interaction mechanisms with interlayer water and polymer are carried out by writing MD calculation Perl script.The main research work and conclusions are as follows:（1）The crystalline swelling and energetic evolution of different clay minerals after initial hydration are studied.The internal matter structure under different hydration amounts is analyzed based on the mass density script.The detailed distribution information of interlayer water is obtained based on the 3D concentration script.The study indicates that even with a slight increase in interlayer hydration,the clay volume increases significantly,the total energy decreases,and the system stability deteriorates;Due to the asymmetric illite-montmorillonite（I-M）interlayer and mixed counterions,the hydration thermodynamic properties,energy evolution and matter distribution of I-M mixed layer clay（MLC）are between the stable illite and the hydratable MMT;With the increase of external temperature,the volume of hydrated clay mineral increases,the density decreases,the total energy and potential energy decrease;the thermal expansion and energy decrease of MMT and MLC at room temperature（278K～298 K）are relatively larger;As the external pressure increases,the volume decreases,the density increases,and the energy does not change.This work makes up for the lack of key mechanism about crystalline swelling within small spacing in traditional theories,and screened out MMT and I-M MLC with strong expansiveness,which lays the foundation for subsequent research on hydration interactions and hydration mechanical properties.（2）The deep causes of interlayer swelling of expansive clay minerals and their interactions with water are investigated.The difference in swelling behavior between I-M MLC and MMT is first compared,and the effects of asymmetric interlayer and mixed counterions on crystalline swelling and clay-water interactions in hydrated MLC are analyzed.Moreover,the molecular dynamics simulation of continuous hydration of clay,which is difficult to achieve by traditional methods,is performed based on the water-insertion script.It captures the evolution of quantitative properties such as basal spacing d,interaction energy,and many hydrogen bonds in the clay interlayer,increasing hydration for the first time through the interaction energy and the H-bond number MD calculation scripts.The study shows that I-M mixed layer clays（MLCs）have smaller swelling compared to pure MMT;With increasing hydration,the clay-clay interaction energy and the clay-ion interaction energy drop,while the clay-water interaction energy increases;MLCs have stronger clay-ion interactions;The hydrogen atoms in H2O molecules preferentially form H-bonding coordination with the oxygen atoms on mineral surface,followed by H-bonding between the water molecules in the clay interlayer;unlike the H bonds in Na-MMT,which are evenly distributed among the interlayers,the water in MLCs preferentially forms many H-bonds in the M-M interlayer,followed by the I-M interlayer,and the number is relatively small.This work provides the perception of the molecular mechanism for initial swelling and clay-water interaction in the widespread MLCs;From this study,it will help in disclosing underlying changes in dynamics,energy,and intermolecular interaction for expansive clay.（3）The basic mechanical properties and internal structural mechanism of the most important expansive clay MMT are studied under interlayer hydration and different stress states.Based on the stress-strain script and CLAYFF force field,the MD simulation and stress-strain analysis of MMT under tensile,compressive and shear stress are conducted with different hydration amounts,which were difficult to achieve in the past.And their mechanical properties,failure mechanism and microstructure evolution are determined at different stress stages.The study demonstrates that the weakening effect of interlayer hydration on mechanical properties is obvious,including ultimate stress and elastic modulus,and this weakening effect is greater in the early stage of hydration;The mechanical properties of MMT are obviously anisotropic;the Z direction tensile modulus is much smaller than the in-plane,and is also greatly affected by the hydration amount,and its strain variation is approximately polynomial,while the plane X and Y directions are close to linear;Stress has the greatest influence on the mechanical behavior of Z direction,and the generation of tensile and compressive strains decreases and increases the elastic modulus,respectively;when the ultimate tensile stress is reached,large deformation and tensile failure occur until the layer separation;The higher the layer charge density,the denser the bound-water film,the more hydrogen bonds formed,the smaller the volume and lattice length c,and the stronger the tensile mechanical properties;The interlayer is the main cause of deformation and dominates the mechanical properties of MMT.The compression curve under low stress is close to linear elasticity,while under high compressive stress,it has nonlinear compression-hardening characteristics;its mechanical modulus and strength under Z-direction compressive stress are much larger than the small and unreliable tensile stress direction,showing strength differential effect;The compression modulus and related conclusions obtained by nanoindentation test are close to the compressive MD simulation;the indentation load increases with the indentation depth,and shows an increasing trend of compression-hardening;the compression phenomenon of hydrated MMT is more obvious,and the nonlinear compression-hardening characteristics are also more prominent.The stress-strain relationship under low shear stress is close to linear elasticity,and then enters the strain softening stage,while under high shear stress,MMT with different hydration amounts produces large strain deformation and slip dislocation after the ultimate stress;compared to the dry state that is close to linear and difficult to deform,the shearing phenomenon of hydrated clay mineral is more obvious;The main failure mechanism under shear stress is the decohesion and failure of interlayer,resulting in shear sliding between clay sheets along the interface direction,or shear dislocation along the lateral direction;the shear stress reduces hydrogen bonding interactions in the system;The slip dislocation is more likely to occur in the interface direction,and its shear modulus and shear strength are an order of magnitude lower than in the lateral direction;Under the action of hydration and tensile and compressive stress,it is mainly the lattice length c and lattice angleβthat change;the interfacial shear stressτzxmainly causes the changes of lattice length a and lattice angleβ;the lateral shear stressτxy mainly causes the changes of lattice length c,lattice angleαand lattice angleβ,and the significant change of angle mainly occurs before and after the ultimate stress.The nanoscale results of this work are the basis for the study and modeling of the mechanical characteristics of expansive clay.（4）The rheology and clay-binding ability of PHPA polymer on the surface of expansive clay mineral are first studied.Based on the MD method,the shear rheological behavior and interaction mechanism of hydrated MMT-polymer composite system are analyzed at different shear rates,temperatures,and polymer concentrations.In addition,the shear rheological experiments and environmental scanning electron microscope（ESEM）observation of mixed fluids are executed.The MD study indicates that PHPA interacts with the MMT surface,adsorbs on it to form a viscous film,and inhibits its swelling;As the shear rate increases,the shear stressτincreases,and the viscosityηdecreases,with evident shear thinning;The shear stressτand viscosityηdecrease with temperature and increase with PHPA concentration.The experimental study shows that as the shear rate increases,the shear stress increases nonlinearly,while the viscosity decreases nonlinearly;The higher the polymer concentration,the greater the shear stress,the greater the overall viscosity;At the low concentrations（0kg/m3 to 0.05 kg/m3）,the yield stress increases approximately linearly with concentration,but as the concentration continues to increase,the slope decreases;The thixotropy of composite system increases non-linearly with concentration.The ESEM images demonstrate that the mixed sample has higher integrity and degree of cohesion,and its surface is smoother and more regular than the individual MMT particles;the polymer is adsorbed to the clay mineral surface in the form of fluffy network structure,forming a coating of viscous film,which slows the penetration of interlayer water molecules.This work reveals the interaction mechanism between MMT and polymer,and identifies the influencing factors and action modes of composite system engineering behavior.Based on the key issues in geomechanics,this paper proposes to study the basic physical and mechanical properties of expansive clay and its interaction with interlayer water and polymer,using the advantages of MD for small-scale geotechnical physical and mechanical behavior prediction.It conducts basic exploration in the following aspects:macro（micro）control and mechanism interpretation of geotechnical disasters,evaluation and prediction of soil properties and its adaptability for engineering,study and modeling of mechanical characteristics of swelling clay,and full use of MMT-polymer supporting fluids.This research method performs the MD calculation simulations that were difficult to achieve in the past by writing Perl script,and extracts the required parameters to achieve the quantitative analysis of basic properties and intrinsic mechanisms,providing technical support for explaining the swelling,deformation and failure phenomena and mechanisms in geotechnical materials.The popularization and use of MMT-polymer composite fluids also contribute to the recycling of discarded expansive clay. |
