Thermo-chemo-mechanical Characteristics And Microstructure Evolution Of Compacted Bentonite

The application of bentonite is involved in many environment engineering fields such as the deep geological disposal of high-level radioactive waste and the construction of waste landfills.In these projects,bentonite is subjected to complex environmental loads,including the combined effects of temperature,humidity,chemical,and mechanical.Under various environmental loads,the microscopic structure and pore water state of bentonite,as well as the experimental and theoretical of the macroscopic physical and mechanical behavior,are currently an urgent and very difficult problem in the geotechnical engineering.Therefore,this study took two typical bentonites（Na-and Ca-bentonite）,carried out a series of micro-analysis tests and expansion/compression tests under complex environmental loads,trying to analyze the swelling mechanism of bentonite.And then,based on the concept of intergranular stress,the macro-mechanical behavior under complex environmental loads was described.The main research results are as follows:（1）Using mercury intrusion porosity（MIP）and scanning electron microscopy（SEM）tests,the microscopic pore structure changes of compacted bentonite were analyzed.The microscopic analysis test shows that the pore distribution of the bentonite in the compacted and saturation state have a bimodal distribution.Both compaction and saturation affect only the large pores,while the concentration affects the size of the large pores and the density of small pores;the lamella structure of Na-bentonite is thin and has large curvature,while the lamella structure of Ca-bentonite is thick and relatively broken.（2）The X-ray diffraction（XRD）technique was used to analyze the changes of interlayer water during the water absorption process and concentration changes of compacted bentonite.Under confined conditions,with the increase of humidity,the basal spacing of Na-bentonite increases from 0 to 2 layers of water,while Ca-bentonite remains at 2 layers of water.In the saturated state,there are 3 layers of water between the crystal structure,as the concentration increases,the basal spacing decreases at low densities,and the basal spacing does not change much at high densities.（3）Through the swelling pressure and swelling deformation test,the water type in the saturation bentonite was divided into interlayer water,double layer water and osmotic water.The T₂ distribution curve was used to calculate the water content of three types.It was found that as the pressure increased,the interlayer water content between the Na-bentonite increased,and the interlayer water content between the Ca-bentonite decreased,and the double layer water and osmotic water content of the two bentonite decreased;As the salt solution concentration increases,the interlayer water content changes little,and the double layer water and osmotic water content decreases;with the increase in temperature,the interlayer water content decreases,and the double layer and osmotic water content increase.（4）Under the e-lgσ’_v coordinate,the saturated void ratio of the two bentonites has a linear relationship with the pressure,and the slope of the line decreases as the concentration increases.With the increase of temperature and concentration,the swelling pressure decreases,and the influence of temperature and concentration on Na-bentonite is greater than that of Ca-bentonite;the effect of concentration on swelling deformation is greater than that of temperature,and Ca-bentonite can better resist temperature and chemical weakening.The effect of temperature on the swelling characteristics is affected by the stress level.The swelling deformation increases with the increase of temperature under low pressure,but the inverse under high pressure.By analyzing the influence of temperature on the state of water in the soil,it is found that the difference in the amount of change of different types of water driven by temperature is the main cause of temperature expansion variation.（5）The compressibility of bentonite strongly depends on the concentration of pore water,and the temperature effect is not significant.The temperature volume change behavior of bentonite is controlled by the stress state.The soil exhibits thermal expansion in the over-consolidated state,and thermally shrinks in the normal-consolidated state.The soil with salt is more likely to shrink.The compacted bentonite exhibits swelling behavior inconsistent with low-plasticity soil during the cooling stage.（6）According to the change of s osmotic pressure and surface force potential of saturated soil,the formula of intergranular stress was derived.Similar to the traditional effective stress,the deformation characteristics at different concentrations can be expressed by the intergranular stress formula.And by calculating the swelling pressure under different dry densities,the main factors of the simulation parameterαare analyzed,and the membrane effect and the variability of basic physical properties in the compacted bentonite are revealed.