Study On Dry-crack Characteristics And Shrinkage Cracking Mechanism Of Bentonite Cushion Backfill Material

High level radioactive waste?HLRW?needs to be isolated from living environment,because it contains strong radionuclides that are high toxic and have a very long half-life.The disposal of HLRW is very difficult.The deep geological disposal method is currently the most feasible and accomplished with the deep burial of HLRW underground.A multiple barrier system is accepted as the repository,in which the buffer backfilling materials are the most important engineering barriers to block the migration of radionuclides from high-level radioactive waste to human living environment.Much research work has been done on the selection of China buffer backfill materials in the past 20 years.So far,Na-bentonite compacted to very high density materials has been considered as ideal buffer backfilling material in HLW repositories due to its high absorption,high swelling property and low permeability.However,the thermal conductivity of pure bentonite materials is very low.If the heat produced by radioactive waste is not able to diffuse out of the surrounding rock promptly,the temperature of the buffer layer would rise correspondingly.When the temperature reaches at 100^oC,liquid gasification will produce enormous water vapor pressure that will destroy the buffer layer structure and the integrity of the buffer backfilling material.Because of the very high plasticity,pure bentonite tends to be pelletoid during bentonite-water mixing process and may lead to inhomogeneous wetting of dry bentonite powder.It has poor workability that hinders the pure bentonite to be compacted to a high density.However,putting some additional material,such as sand into bentonite,can not only improve its mechanical strength,heat conductivity but also minimize long-term creep deformation without obvious high absorption,high swelling property and impermeability.The bentonite-sand mixture developed as a mixed buffer is expected to deform and even crack in the process of contraction,which seriously affects the normal operation of the project.Therefore,shrinkage and cracking characteristics of bentonite-sand mixture during water loss are the innovative research directions of this research field,and also the mainstream research direction of buffer backfill materials in the field of geological disposal of HLRW in the world.In this study,bentonite sand-adding mixture and different cationic montmorillonites were used as research objects to conduct laboratory tests.The dry-wet cycle tests under temperature control?40^oC?and high suction control were carried out to study the water holding characteristics and the characteristics of swelling and shrinkage deformation.At the same time in the process of dry-wet circulation,for Na-montmorillonite and Ca-montmorillonite were carried out by scanning electron microscopy?SEM?and mercury injection test?MIP?test.The analysis of saturated compacted bentonite samples at different levels under different dry density and shrinkage cracking characteristics,combining previous research results to choose the suitable for different volume contractions model evolution rule is described,preliminary discussion on the sample in the contraction-fine microscopic mechanism in the process of cracking.The main contents and research results of this paper are as follows:The main contents and research results of this paper are as follows:1.Basic physical and chemical properties of montmorilloniteTemperature control experiments were conducted on Shandong Montmorillonite?referred to as SMT?,and on quartz sands produced in Fengyang,Anhui province,China.The basic physical and chemical properties of clay minerals such as composition,chemical composition,crystal structure,electrochemical properties,specific surface area and boundary water content were analyzed by laboratory tests.The results showed that montmorillonite has a 2:1 crystal structure,which is linked by oxygen-oxygen bond between layers.The bonding force was weak and prone to the phenomenon of isomorphism replacement.Montmorillonite exhibited an intragranular structure,which absorbed more cations with a large specific surface area.2.Drying-shrinkage-cracking characteristics of bentonite mixtureThe compacted samples of bentonite sand mixture showed two stages of deceleration and residual.With the increase of bentonite content,water holding capacity of the sample was stronger and the energy required for water molecules to escape to by evaporation was high,so that the deceleration period of water evaporation lasted longer.The volume shrinkage process of the sample also showed obvious stages normal shrinkage,residual shrinkage and zero shrinkage.As the content of bentonite determines the pore distribution in the aggregate,the normal shrinkage stage of the sample with higher bentonite content lasts longer.The specimen shrinkage crack developed with the bentonite content.With the increase in the degree of structure compactness,water evaporation,volume change and fracture development of the samples showed different characteristics.With the increase of the initial dry density,pore connectivity and shrinkage space decreased,and the evaporation rate and shrinkage of the samples decreased.A specific shrinkage model was selected to describe different shrinkage processes under different structural compactness conditions.Considering the influence of bentonite content and structural compactness,the relationship between two factors and the final pore ratio of shrinkage was established.In the process of shrinkage,development characteristics of surface cracks were greatly affected by structural compactness.3.Mechanical mechanism study on the drying and cracking process of bentonite mixture materialsWater and air balance method?humectant bar?and temperature control were used to test bentonite mixture material and different cation exchange of montmorillonite.The results showed that the bentonite water retention curve with different initial dry density could be divided into three stages:rapid change stage?s=7.06⁴4.9 MPa?,transition stage?s=44.9¹38 MPa?and slow change stage?s=138³31 MPa?.The hysteresis effect in dehumidification and hygroscopic process is explained reasonably.At the same time,it wasfound that the soil-water characteristic curve without considering volume change has little influence on the initial pore ratio,while the soil-water characteristic curve with considering volume change is affected by the initial pore ratio,so the influence of volume change should be considered when studying the water-holding property of expansive soil.The water-holding curves of different cationic montmorillonite can be divided into two stages:rapid change?s=7.06⁴4.9 MPa?and slow change?s=44.9³31MPa?.It is considered that the interlayer cation is the main reason for the difference of moisture holding capacity and hysteresis of montmorillonite during desiccation and hygroscopic process in the high suction section.The drying process of bentonite and sand mixture can be divided into deceleration stage and residual stage.The evaporation rate of water in the initial stage is affected by the initial dry density,while the evaporation rate in the residual stage is not affected by the dry density and bentonite content,but the final residual water content is affected by the bentonite content.Increasing the initial dry density and decreasing the bentonite content can make the mixture deform harmonized and reduce the anisotropy of shrinkage deformation,thus inhibiting the development and expansion of cracks.Cornelis shrinkage model is used to describe the shrinkage process of bentonite and sand mixture,among which model parameters a,b,c and final pore ratio e₀ have good correlation with bentonite content and initial dry density,indicating that these two indexes are important parameters affecting the drying and shrinkage process.4.Study on the structural evolution of the fracture process of bentonite mixtureThrough MIP,nitrogen adsorption test?N₂?and SEM test,the structural evolution analysis and evaluation of the crack process of bentonite mixture materials were systematically studied.The results showed that the pore distribution characteristics of the mixture of Na-montmorillonite,Ca-montmorillonite and bentonite in the process of dehumidification were different,which was attributed to the different structure of soil.In addition,in the process of dry-wet cycle,it was found that the soil pore structure had irreversibility of different degrees.When the pore diameter is larger than 1000 nm,the variation rule of pore diameter is only related to the size of suction;when the pore diameter is smaller than 1000 nm,the variation rule of pore diameter is determined by the path of dry-wet cycle and the size of suction.SEM experiment was carried out,the Na-montmorillonite,Ca-montmorillonite found after cation exchange of montmorillonite found obvious differences in microstructure,which caused the specimen in the process of dehydrated show different structural characteristics,but what is certain is the structure of montmorillonite and composition sheet of montmorillonite mineral crimp contraction is caused by montmorillonite the root cause of the cracks in the process of water loss contraction.In the range of high bentonite content,quartz sand is not enough to form an overall framework support,but the sand content is appropriately increased.For example,when the bentonite content is 70%montmorillonite,after shrinkage,has a more uniform and compact aggregate structure.In addition,with the increase in dry density,the thickness of lamellar aggregates increased significantly,and the anti-crimp stiffness of clay mineral particles increased,which effectively inhibited the development and evolution of fractures.5.Study on the micro-mechanism of dry-shrinkage crack process of bentonite mixtureThe adsorbed water state of bentonite mineral surface at different stages of evaporation was obtained by theoretical calculation.When the swell-sand mixture sample entered the residual stage from the deceleration stage,1 layer of strong binding water molecular membrane was uniformly adsorbed on the surface and between layers of clay minerals,that was,the water content in the residual stage was only controlled by the content of montmorillonite minerals in the sample and the specific surface area.According to the different content of bentonite sample shrinkage process of the micro images,put forward the microscopic shrinkage-cracking model according to the different characteristics of saturated compacting sample shrinkage cracking,fine microscopic mechanics on the explanation,considering internal pore can be equivalent to the internal microcracks of mixture material.