Molecular Dynamics Simulation Of Water Absorption Characteristics Of Kaolinite

Coal mining environment is sedimentary rock.In the sedimentary rock of coal measure strata,mudstone is a main rock formation,including carbonaceous mudstone,sandy mudstone,shale and other weak rock bodies.Mudstone is mainly composed of clay minerals,quartz and other minerals.The proportion of clay minerals is as high as 40%?90%.The common clay minerals are kaolinite,illite,montmorillonite and illite/montmorillonite mixed layer.Due to the special weathering and water swelling characteristics of clay minerals,they often have destructive effects on the s Tableility of roadways.After the roadway is excavated,the surrounding rock will undergo hydration expansion,strength reduction,softening and disintegration or even sliming due to unloading and weathering,especially the influence of water soaking,thus inducing engineering disasters such as roof caving,floor heave and even collapse of soft rock roadway.Compared with various instrumental analysis techniques,molecular simulation techniques based on molecular mechanics,molecular dynamics and quantum chemistry are powerful tools for revealing the relationship between the structure and properties of substances and understanding the interaction mechanism of substances in physical and chemical systems.In this paper,kaolinite is taken as the research object,and Materials studio software is used to illustrate the essence of water absorption of coal measures mudstone from a microscopic point of view by means of molecular simulation.Considering the fact that the depth of burial is 800 m as the critical value,the stress of the original rock is approximately 20 MPa,and the temperature of some high-temperature mines can reach about 70 ?,the changes of kaolinite after contacting with water under two factors of different temperatures and anddifferent pressures are simulated.The adsorption amount,heat,energy and swelling rate of kaolinite to water under different conditions are analyzed.The main reasons for kaolinite to absorb water and the adsorption sites of water on the surface of kaolinite are also analyzed.Combined with electrochemical knowledge,the migration of water molecules in kaolinite crystal under external electric field is understood from the microscopic view.The specific conclusionsare as follows:(1)The adsorption capacity of kaolinite to water decreases with the increase of temperature,that is,the increase of temperature is not conducive to the adsorption of kaolinite to water;However,the logarithmic law increases with the increase of pressure,which indicates that the increase of pressure will promote the adsorption of more water molecules on the surface of kaolinite crystal layer.(2)Under different pressure and temperature conditions,the maximum adsorption heat of kaolinite to water is 2.133 k J/mol,which is much lower than the threshold value of 42 k J/mol for chemical adsorption.It can be seen that kaolinite mainly absorbs water physically.(3)Kaolinite crystals will expand in volume after absorbing water.Analysis shows that with the increase of pressure,the expansion rate gradually increases and then tends to be s Tablele.At the same time,the decrease of temperature will also increase the expansion rate.(4)The peak value and distance of radial distribution function of different atoms in water molecules and kaolinite can qualitatively reflect the order of hydrogen bonds formed after water adsorption on different surfaces of kaolinite and the bond length of hydrogen bonds.The adsorption distance between the two peaks of water molecules and hydrogen atoms in kaolinite in the radial distribution function is less than the threshold value of hydrogen bond linkage of0.3 nm,which indicates that hydrogen bonds are formed between water molecules and kaolinite surface atoms in the process of kaolinite adsorbing water.(5)Kaolinite undergoes great changes in energy during water absorption,of which hydrogen bond energy contributes the most,followed by van der Waals energy,indicating that hydrogen bond is the main bonding type during adsorption,followed by van der Waals force.Therefore,it can be concluded that kaolinite water absorption coexists with physical adsorption and chemical adsorption.(6)There are a large number of hydroxyl groups on the surface of kaolinite crystal.When water molecules enter,hydrogen bonds are formed with hydroxyl groups,making water molecules adsorb on its surface.A large number of hydroxyl groups provide a large number of adsorption sites for watermolecules,which is the main reason for kaolinite to absorb water.(7)There are a large number of free-moving charged ions in kaolinite-water system,which will undergo directional migration under the action of electric field.