【作者】 杨宇；

【导师】 徐国元；

【作者基本信息】 华南理工大学 ， 道路与铁道工程， 2022， 博士

【摘要】 膨胀性黏土矿物是岩土力学领域灾害防治的重点对象,也是土木工程中最为重要的地质材料之一。它们的亲水性较强,并且层间水化后容易软化,土壤中即使含量不多,也会对其宏观物理力学性能产生明显影响,从而在岩土材料的膨胀、变形和失效等问题中发挥着关键作用。要保障这些具有纳米级尺寸和分子尺度水敏特性的“特殊土”的建设安全,则需了解黏土矿物本身的水化特性和膨胀机理,但传统的理论和方法,在遇到小尺度岩土问题的求解时会受到诸多限制,而分子动力学方法则可以解决这些问题。本课题立足于膨胀性黏土矿物研究中亟待获得的水化性质和微观机理,基于分子动力学（MD）方法,从纳米尺度研究了蒙脱石等岩土工程中较为主要的膨胀性黏土矿物,在水化等不利条件下的基本物理力学性质,并通过编写MD计算Perl语言脚本的方式,定量分析了其热动力学溶胀特性、拉压与剪切力学特性、以及其与层间水、聚合物间的相互作用机制。主要研究工作及结论如下:（1）研究了不同黏土矿物初始水化后的层间膨胀及能量学演化规律,基于质量密度分布脚本剖析了其在不同水化量下的内部物质结构,基于3D浓度脚本获取了层间水分布的细节信息。研究表明,即使层间水化量轻微增加,黏土矿物体积也会明显增大,总能量减少,系统稳定性变差;由于不对称的夹层与混合的Na⁺、K⁺,伊-蒙混合层黏土矿物的水化热动力学性质、能量演化和内部物质分布介于稳定的伊利石和易于水化的蒙脱石之间;随着外部温度升高,水化黏土矿物体积增大,密度减小,总能量与势能减少;蒙脱石和混合层黏土矿物在常温环境下（278 K～298 K）的热膨胀和能量减少幅度相对更大;而随着外部压力升高,体积减小,密度增大,能量无变化。相关工作弥补了传统理论关于小间距晶层膨胀关键机理的缺失,筛选出了膨胀性较强的蒙脱石和伊-蒙混合层黏土矿物,为后续的水化相互作用、水化力学特性等研究奠定了基础。（2）研究了膨胀性黏土矿物层间膨胀的深层次原因以及其与水之间的相互作用,比较了伊-蒙（I-M）混合层黏土矿物（MLCs）与纯蒙脱石（MMT）之间溶胀行为的差异,分析了MLCs中不对称的夹层与混合的阳离子对于晶层膨胀的影响,通过水分子插入脚本,开展了传统手段难以进行的,黏土矿物不断水化的MD计算模拟,通过编写相互作用能和氢键数目分布MD计算脚本,捕捉了基础层间距、相互作用能和氢键数目等量化的参数随水化量的变化情况。研究表明,伊-蒙混合层黏土矿物相比于蒙脱石有着更小的溶胀;随着水化增加,黏土与黏土之间、黏土与阳离子之间的相互作用能会不断下降,而黏土与水之间的相互作用能则会增加;MLCs有着更强的黏土-离子相互作用;水中的氢原子会优先与矿物表面氧形成氢键配位,其次才是夹层水分子间;与MMT中的氢键均匀分布不同,MLCs中的水会优先在M-M夹层中形成较多的氢键,接下来才是I-M夹层,且数目相对较少。相关工作提供了伊-蒙混合层黏土矿物初始溶胀的分子机制及矿物-水相互作用,揭示了膨胀性黏土矿物在动力学、能量和分子间相互作用的潜在变化规律。（3）研究了最为主要的膨胀性黏土矿物蒙脱石在层间水化和不同应力状态下的基本力学性质及内在结构机理,通过编写施加应力-计算应变脚本和嵌入CLAYFF力场,开展了过去方法难以实现的,不同水化量蒙脱石拉压和剪切应力下的MD计算模拟与应力应变分析,确定了其不同应力阶段的力学特性、失效破坏机制和微观结构演化。研究表明,层间水化对力学性能的弱化效应明显,包括极限应力和弹性模量,且在水化初期弱化幅度会更大;蒙脱石的力学性质各向异性明显,Z方向的拉伸模量远小于平面内,受水化量的影响也较大,并且其应变变化近似多项式的形式,而平面X和Y方向则接近线性;应力对表面Z方向的力学行为影响最大,且拉、压应变的产生会减小和增加弹性模量,当达到极限拉应力后,会产生大变形和拉伸破坏,直至整层的分离;层电荷密度越高,结合水膜越密实,形成氢键数目越多,体积和晶格长度c越小,抗拉力学性能越强;夹层是大部分形变的主要原因,并且支配着蒙脱石的力学性能。低应力下的压缩曲线接近线弹性,高应力下则具有非线性的压硬特征;其在Z方向压缩应力下的力学模量及强度,要远大于很小且不可靠的拉伸应力方向,表现出拉、压强度不对称的力学特性;纳米压痕试验获得的压缩模量及相关结论,与压缩MD模拟较为接近;压痕荷载会随着压入深度的增加而增大,呈现出压硬性的增长特征,并且水化后的压缩现象更明显,非线性压硬特征也更为突出。低剪切应力下的应力-应变关系接近线弹性,随后便进入到应变软化阶段,而在高剪切应力下,不同水化量蒙脱石会在极限应力后,产生较大的应变变形和滑移错动;相较于接近线性和难以发生变形的干燥状态,水化黏土矿物的剪切现象更明显;剪切应力下的主要破坏机制是夹层的脱聚和失效破坏,从而导致黏土片层间沿界面方向剪切滑移,或者沿侧面方向剪切错动;剪应力的施加减少了体系中的氢键作用;平行于黏土层的界面方向,更容易发生滑移错动,其剪切模量和剪切强度比侧面方向低一个数量级;水化和拉压应力作用下,主要发生晶格长度c与晶格角β的变化;界面剪切应力τ_zx主要造成晶格长度a与晶格角β的变化;侧面剪切应力τ_xy则主要造成晶格长度c、晶格角α和晶格角β的变化,并且晶格角的明显改变,主要发生在极限应力前后。相关工作获得的纳米级力学性质,是开展膨胀黏土力学特性研究及建模的基础。（4）研究了PHPA聚合物在膨胀性黏土矿物表面的流变及黏土结合能力,基于MD方法分析了水化蒙脱石-聚合物复合体系在不同剪切速率、温度、聚合物浓度下的剪切流变行为与相互作用机制,并开展了混合流体的剪切流变试验和环境扫描电镜观测。MD研究表明,PHPA会与蒙脱石表面发生相互作用,吸附在其上形成粘性薄膜,并抑制其溶胀;随着剪切速率升高,剪切应力τ增大,粘度η减小,且具有明显的剪切稀化行为;τ和η会随着温度的升高而减小,随着PHPA浓度的增加而增大。而试验研究表明,随着剪切速率增加,剪切应力非线性增大,粘度非线性减小;聚合物浓度越高,剪切应力越大,整体粘度越大;低浓度范围内（0 kg/m³至0.05 kg/m³）,屈服应力随浓度近似线性增长,但浓度继续提高,斜率会放缓;体系触变性随浓度非线性增长。环境扫描电镜成像结果表明,混合后样品完整度和粘聚程度更高,其表面相较于单独的蒙脱石颗粒也更为光滑、规整和圆润;聚合物以蓬松网状结构的形态吸附到黏土矿物表面,形成一层包裹作用的粘性薄膜,减缓水分子的层间渗入。相关工作揭示了蒙脱石与聚合物间的相互作用机制,确定了复合体系工程行为的影响因素及作用模式。本文立足学科重难点问题,提出利用MD小尺度预测岩土物理力学行为的优势,确定膨胀性黏土矿物的基本物理力学性质及其与层间水、聚合物间的相互作用,为岩土灾害宏（微）观防治及机理解释、土体性质及工程适应性评价预测、膨胀黏土力学特性研究及建模、蒙脱石-聚合物支撑性流体的充分使用开展基础性的探索。本文分析方法通过编写Perl脚本,开展过去方法难以实现的MD计算模拟,并提取所需的物性参数,实现岩土基本性质和内在机理的定量分析,为解释岩土材料中的膨胀、变形和失效现象及机理提供技术支持。蒙脱石-聚合物复合体系的推广使用,也有助于废弃膨胀黏土矿物的循环利用。更多还原

【Abstract】 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 stressτ_zxmainly causes the changes of lattice length a and lattice angleβ;the lateral shear stressτ_xy 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.更多还原