Unfrozen Water Test And Molecular Simulation Study Of Typical Clays

China is the third largest country in the world for permafrost distribution,and the high-altitude permafrost area is the highest in the world.The area of permafrost and seasonal permafrost regions accounted for 21.5%and 53.5%of the country’s land area,respectively.The construction of a large number of projects in permafrost regions of our country is affected by the freezing of soil layers.The freezing effect of soil not only shortens the service life of buildings,deteriorates operating conditions,but also increases the operation and maintenance of many non-productive labor,materials and investment.It has affected the people’s living and economic construction in these areas and seriously restricted the development of the national economy.The essence of soil freezing is the physical process of intrinsic micro-water ice phase transformation.The liquid water migration in frozen soil is the growing source of frozen ice lens.Therefore,the study of the content of unfrozen water is the basis for studying the problem of frozen soil.Among them,the crystal layer structure of clay minerals is closely related to water,and the influence of clay minerals on the properties of frozen soil is worth exploring.Firstly,the nucleus magnetic resonance test system was used to test the unfrozen water content of montmorillonite,illite and kaolinite during freezing,and the change of unfrozen water content during freezing was analyzed.It was found that the freezing temperature of montmorillonite was the lowest,followed by illite,and the freezing temperature of kaolinite was the highest.During the freezing process,the unfrozen water content of montmorillonite decreased slowly and evenly,and its unfrozen water content was higher than illite and kaolinite under each negative temperature condition.The freezing of water in illite and kaolinite was mainly concentrated in the temperature range of 0^--2.5°C,the unfrozen water content in the initial stage of illite freezing is lower than that of kaolinite.With the decrease of temperature,the unfrozen water content of kaolinite is the lowest at about-12°C^-20°C.The ratio of unfrozen water content of montmorillonite,illite and kaolinite at 20°C is about 1:0.077:0.063.In addition,the hysteresis effect under the freeze-thaw cycle was analyzed.It was found that for the same soil,the higher the ice content of the soil during the freezing process,the more obvious the hysteresis effect.Then,the molecular dynamics simulation method was used to compare the structural properties,thermodynamic properties and some kinetic properties of montmorillonite,illite and kaolinite at ambient conditions.From the perspective of radial distribution function,the hydration characteristics of the montmorillonite hydrates show the two stages of crystal expansion and permeation expansion due to the hydration of the interlayer compensation cations with clay and interlayer water.The intrinsic mechanism of illite and kaolinite as non-swelling clay is explained from the radial distribution function and the number of hydrogen bonds:when the water content of illite increases,it promotes the special effect of interlayer cation K⁺and clay,and enhances the interlayer connection of illite,while the hydrogen bonding between the kaolinite layers is strong.which makes the illite and kaolinite repel water molecules into the crystal layer.Hydrogen bonding is stronger than intermolecular interactions,so the kaolinite interlayer structure is the most stable,and water molecules are the most difficult to enter.In addition,montmorillonite is intercalated with the outer spheres of water molecules at high water content,and the self-diffusion coefficient of water molecules shows that the material activity between montmorillonite layers is much higher than that of illite and kaolinite,both of these indicate the potential for further hydration of montmorillonite under conditions of high water content.Through the radial distribution function curve of interlayer water molecules,it is found that the degree of water polymerization between montmorillonite layers is higher than that of illite,and the degree of water polymerization between illite layers is higher than that of kaolinite.When the water content is 488mg_water/g_clay,the ratio of polymerization degree of water molecules between montmorillonite,illite and kaolinite is about 1:0.806:0.790.Through the self-diffusion coefficient of interlayer water molecules,it is found that the activity of water between montmorillonite layers is much higher than that of illite and kaolinite.When the water content is 488mg_water/g_clay,the ratio of self-diffusion coefficient of water molecules between montmorillonite,illite and kaolinite is about 1:0.060:0.046.This provides a molecular level explanation of the highest unfrost water content of montmorillonite and the least frozen water content of kaolinite at-20°C:The higher the degree of polymerization of water molecules,the greater the activity,the lower the freezing point,the more difficult it is for water molecules to freeze,and the water molecule activity has the greatest impact on the freezing process.In addition,when the water content of illite increases,it promotes the special role of interlayer cation K⁺and clay,and enhances the connection between illite layers.This may be the molecular reason that why the temperature of the illite does not decrease until the unfrozen water content is too low.The paper has 32 pictures,10 tables and 100 references.