Study On Water Retention Characteristics And Micro Mechanism Of Buffer Backfill Materials

With China’s carbon peak in 2030 and carbon neutrality in 2060,the development and utilization of clean energy has attracted increasing attention.Nuclear energy as one of the clean energy,the waste disposal after its utilization becomes a practical engineering problem to be solved urgently.Bentonite is the main component of buffer backfill material for high radioactive waste disposal repository.The research on its physical properties has great theoretical reference value for practical engineering application.In the study of unsaturated soils,the suction of soil is a very important physical reference value.Bentonite,as a buffer backfill material,has a strong ability to absorb water,and its suction force can reach tens of hundreds of MPa.In this study,GMZ bentonite from Inner Mongolia was used as the research object to conduct laboratory test and software simulation analysis of water holding characteristics curve of compacted sample of buffer backfill material in a wider range of suction force,and analyze and discuss the water holding characteristics of buffer backfill material from macro and micro levels.Based on adsorption thermodynamics,capillary model,Metadynamics and other related theories,the influence of GMZ bentonite on SWCC in high suction（bound water）and low suction（capillary water and gravity water）was investigated.Based on the results of macro test and micro simulation,the relationship between the maximum suction and water content of GMZ bentonite was obtained,and the relationship between the microstructure characteristics and water holding characteristics of buffer backfill material was discussed,and the internal mechanism of suction generation and action was analyzed and revealed in essence.Use MS software of montmorillonite bentonite main mineral components of molecular cell,build and carry out the model optimization and the water molecules adsorbed simulation work,from the aspects of structure,energy,layer spacing changes such as discussion analysis and preliminary findings molecular layer spacing and increasing with the increase of water content,its change law can approximate function as the number of water molecules,Its density is negatively correlated with the number of water molecules.Before and after the optimization of molecular structure model,the configuration of tetrahedron and octahedron,the positions of water molecules and cations and hydroxyl groups between cell layers changed greatly.The van der Waals force contributes most to the total energy before configuration optimization,but after optimization,it is mainly composed of the electrostatic action of non-valence bond and the Angle bending of valence bond.Dialysis method and saturated salt solution method is used to test for the high temple son of bentonite in different suction and water content curve found that test results are consistent with simulation part of the trend of the qualitative analysis,when the soil is in a high moisture content（> 40%）,under the condition of freedom while suction changes are very small,only in 1 mpa changes will take place great change when the soil water content,However,the change of confining condition is relatively moderate.When the soil is at low water content（< 40%）,the variation of suction shows a geometric increase,but the variation of water content is relatively peaceful regardless of free condition or confined condition.Furthermore,the action mechanism and correlation between the water holding characteristics and its microscopic mechanism at the actual test and microscopic level are analyzed.The water holding characteristics of bentonite under two different limiting conditions were compared and analyzed,and the influence of different limiting factors on water holding characteristics was further understood.LAMMPS software was used to simulate and calculate the relationship between soil suction and water content of NA-montmorillonite,which was difficult to be realized in macroscopic test,and SWCC of NA-montmorillonite with low water content（< 30%）was obtained.In this part of simulation calculation,the soil moisture content still decreases with the increase of suction,and according to SWCC,when the water content increases,the soil suction tends to decline exponentially,which is consistent with the definition of suction.Although there are some errors in the cohesion of SWCC images obtained by combining experimental and simulation results,the overall trend is consistent.And further expansion found that the maximum suction reaches 4188 MPa.