摘要
高铁路基膨胀灾害工程实例发现,部分膨胀是由叶绿泥石水化膨胀引起的,但目前国内外学者对叶绿泥石膨胀特性研究较少,相关机理尚未明确。采用多尺度表征的方法研究了宏观和纳观尺度下叶绿泥石的膨胀特性,运用分子动力学模拟方法研究了叶绿泥石水化机理及动力学响应下的键能变化。结果表明:叶绿泥石具有一定的膨胀潜力。通过不同数量水分子的吸附试验,得到层间域极限数量为32个,吸附位置为硅氧骨干形成的六方空穴,且仅能形成1层饱和水分子层。随着水分子增多,膨胀加剧,直至达到极限,极限状态的晶格常数为a=21.52A,b=18.61A,c=14.54A。水化模型经室温恒压的动力学模拟,发现模拟过程中静电作用能贡献最多,van der Waals作用能次之,键伸缩能最少。随着水化的加剧,水分子之间的相互作用变弱,径向分布函数峰值右移,层间水分子扩散性增强,且表现出靠近滑石层的倾向,水氧与滑石层氢原子形成的氢键键长1.8A,层间水与黏土矿物的径向分布函数峰值右移,同时氢键中的氢原子与受体原子之间的距离增加,氢氧配位数增加,这导致氢键键长增加,键角减小,黏土层开始产生膨胀。
Introduction Clay mineral crystals are mostly formed by stacking layers of silicon-oxygen tetrahedral sheets and aluminum–oxygen octahedral sheets.They basically have water absorption and expansion because of their unique layered crystal structure.Clay minerals after hydration and expansion usually cause serious engineering problems,which have a negative impact on engineering construction.They have received extensive research attention.It is generally believed that the hydration expansion of clay minerals is mainly divided into two stages,i.e.,crystal expansion and osmotic expansion.Crystal expansion is caused by the expansion of water molecules into the interlayer of clay minerals.Permeation expansion is due to the lattice substitution of clay minerals,which leads to the imbalance of valence and electricity of the crystal.Massive exchangeable ions are gathered on the surface of the crystal.After these ions are dissociated in water,they will repel each other with negative charge under the action of the diffusion double layer,resulting in expansion.Compared with osmotic expansion,the degree of crystal expansion is small,kaolinite,illite,pennine and other interlayer cation-free clay minerals do not have an expansibility.However,the problem of expansion disaster occurs under the working condition rich in pennine,indicating the research value of pennine hydration expansion.However,the water absorption characteristics,expansion characteristics and hydration mechanism of pennine in nano-scale are not yet clear,and related research needs to be carried out.In this paper,a molecular dynamics study on water absorption of mesophyll pennine was carried out to explain the hydration characteristics of mesophyll pennine nanocrystal structure.The changes of energy,structure and chemical bond of mesophyll pennine in the process of hydration expansion were discussed.Materials and method The ring cutter samples with different pennine contents (i.e.,0%,20%and 100%) were prepared with pennine mixed with quartz.The no-load expansion rate was measured.The particle size of the soil sample was controlled to be 200 mesh (0.075μm),the density of the soil sample was 1.8 g/cm^(3),and the initial water content was 10%.The hydration degree of pennine with different water contents was analyzed by thermogravimetric analyzer,and the water content controlled was 0%,5% and 10%.The microstructure and crystal parameters of pennine samples were analyzed by X-ray diffractometer and scanning electron microscope.The crystal model was established based on the results of the microscopic test.The sorption module was used to carry out the adsorption test.A hydration model containing different amounts of water molecules was established through the adsorption test.The geometry optimization task under the Forcite module was used to optimize the structure of the model.The molecular dynamics simulation of the model was carried out using the Dynamics task under the Forcite module,and the expansion deformation of pennine was simulated via dynamic calculation under the NPT ensemble.The Clayff force field suitable for clay minerals was used in the simulation process.The simulated temperature is 298 K and the simulated pressure is 0.001 GPa.The time step is 1 fs,the number of iteration steps is 100000,and the truncation radius is 12.5A.The charge calculation method is the Forcefield Assigned.The non-bond energy Coulomb interaction was calculated by the Ewald sum method.The van der Waals interaction energy was calculated by an atom-based method.Results and discussion The experimental results of the no-load expansion rate show that the no-load expansion rate of the ring knife sample increases with the increase of the content of pennine,indicating that the pennine has an expansibility.The thermogravimetric analysis shows that the higher the initial water content is,the higher the degree of hydration of the pennine will be.In addition to the free water adsorbed between the particles,some water molecules are bonded between the crystal layers via hydrogen bonding.The results of molecular dynamics simulation show that pennine is electrically neutral due to the simultaneous substitution of high-valence cations and low-valence cations,so the crystal expansion occurs.After the water molecules enter the interlayer,they are embedded in the hexagonal holes of the silicon-oxygen backbone layer.The oxygen atoms in the water molecules form a hydrogen bond connection with the hydroxide ions in the talc layer,which controls the hydration limit of the pennine crystal.In the process of hydration and expansion of pennine,it is dominated by electrostatic interaction energy,and followed by van der Waals force,and the bond stretching energy is the smallest.Compared with montmorillonite,pennine hydration is more difficult.The peak of the radial distribution function of water molecules shifts to the right as the degree of hydration increases,indicating that the spacing of water molecules and the diffusivity of water molecules increase.After massive water molecules enter the crystal,the total number of hydrogen bonds increases,the average coordination number of oxygen atoms and hydrogen atoms increases.However,the hydrogen bond length increases,the bond angle decreases,and the crystal expands.Conclusions The experimental and simulation results showed that when water molecules entered the interlayer of the crystal,pennine expanded slightly,which was consistent with kaolinite and illite without cations in the interlayer.The hydration expansion of pennine conformed to the linear growth law as a whole.Pennine crystals continued to expand until the limit as the hydration degree increased.The lattice constants in the limit state were a of 21.52A,b of 18.61A,and c of 14.54A.The limit adsorption amount of the crystal was controlled by the crystal structure.After water molecules entered the interlayer of the crystal,the adsorption site was in the hexagonal holes of the silica backbone layer,and each hexagonal hole could only adsorb one water molecule.When the water-absorbed pennine expanded under dynamic conditions,the electrostatic interaction energy was the largest,the van der Waals force was the second,and the bond stretching energy was the smallest.When water molecules entered the interlayer of pennine,hydrogen bonds could be formed or strengthened.The coordination number of hydrogen and oxygen and the length of hydrogen bond formed increased,but the bond angle decreased with the increase of the number of water molecules.
作者
张瑞
余飞
佟凯文
黄康
张伟
戴张俊
陈善雄
ZHANG Rui;YU Fei;TONG Kaiwen;HUANG Kang;ZAHNG Wei;DAI Zhangjun;CHEN Shanxiong(Institute of Rock and Soil Mechanics,Chinese Academy of Sciences,Wuhan 430071,Hubei,China;University of Chinese Academy of Sciences,Beijing 100049)
出处
《硅酸盐学报》
EI
CAS
CSCD
北大核心
2024年第7期2350-2363,共14页
Journal of The Chinese Ceramic Society
基金
国家自然科学基金(42172308)
中国科学院青年创新促进会(2022331)。
关键词
叶绿泥石
水化膨胀
分子模拟
径向分布函数
pennine
hydration expansion
molecular simulation
radial distribution function
