Hydro-mechanical Properties Of Bentonite After Heating With Supercritical Temperature

How to dispose of the nuclear waste generated from the application of nuclear energy has attracted the attention of scholars and governments worldwide.Bentonite was recognized as a preferred buffer material for deep geological repositories due to its good swelling ability and low permeability.The bentonite buffer layer will be subjected to high temperature and groundwater infiltration for a long time.This is because the waste tank releases large amounts of decay heat and there exists geothermal gradient in the geological repositories.Many scholars have studied the hydro-mechanical properties of bentonite after heating,and discovered the illitization phenomenon.However,in actual engineering,the low-saturated bentonite buffer layer will be affected by the critical temperature(105℃)and groundwater for a long time.It has not been agreed whether its long-term hydro-mechanical performance can be maintained.Therefore,it is essential to study the hydro-mechanical deterioration mechanism of bentonite subjected to supercritical temperature under different heating history and temperatures conditions.Bentonite is a special kind of soil,which contains a small amount of illite,kaolinite,zeolite,quartz and so on.Through the existing research basis and literatures,the history of supercritical temperature action was similated,and free expansion rate,constant volume expansion force-permeability coefficient and specific surface area tests were carried out to study the evolution of hydro-mechnical performance of bentonite with temperature and steam.The degradation mechanism of hydro-mechanical performance was revealed from a microscopic perspective.And from A microscopic perspective reveals the degradation mechanism of its hydraulic performance.The main research contents and conclusions are as follows:1)A muffle furnace was used to heat the powdered bentonite at 105℃.After heating to different ages,it was taken out and sealed for subsequent tests.Samples with the same initial water content were compacted statically to same dry density,and expansion force and permeability coefficient tests were conducted using a constant volume expansion permeameter.It was found that the expansion force decreased with increasing heating history,and the permeability coefficient increased with increasing heating duration.2)With reference to relevant specifications and literature,use powdered bentonite with different heating ages to conduct tests on free expansion rate and specific surface area.The results show that with the increase of heating duration,both the free expansion rate and the specific surface area of the bentonite decrease rapidly at 15 days and then tend to be constant.The study shows that after long-term heating at 105℃,the free expansion rate and constant volume expansion force of the expansion performance of the bentonite were reduced,the result indicates that the saturation permeability coefficient of its isolation and impermeability gradually increased,the specific surface area of its adsorption performance was reduced and the amplitude is small.3)Using X-ray diffraction,scanning electron microscopy,Fourier infrared spectroscopy,thermogravimetry,and laser particle size analysis to explore the evolution law of hydromechanical performance with heating age.The results show that during the heating cycle,by deionizing hydrated water,sodium and magnesium ions escape with the evaporation of water,and at the same time,siliceous oxide cement is formed,and then the bentonite particles are aggregated to form large particles,which affects the self-healing ability of the bentonite.4)Use a humidifier to arrange bentonite with different water contents,place it in the heating chamber of a high-pressure reactor,set it to 105℃ for 15 days,take it out,and seal it for use.The research on the hydro-mechanical properties of samples with different water content at supercritical temperature was carried out.It is found that after the action of steam,the free expansion rate of bentonite decreases,the expansion force decreases,and the permeability coefficient increases.When the bentonite buffer layer is in a water vapor unsaturated state,especially when the water content is 15%,the swelling performance is affected by the decay heat,the swelling performance is the worst,and the permeability coefficient is the largest,which will accelerate the movement of the nuclide and weaken the self-healing ability of the bentonite.5)Bentonite samples with different water contents were selected for thermogravimetric and X-ray diffraction tests.It was found that the samples subjected to steam had no obvious structural changes,and no new silica-oxygen conjugates were produced during the change of water content.Combined with the experimental results of relevant literature and the scanning electron microscope test,it can be known that the aggregation of bentonite particles with a water content of 0% and a moisture content of 30% are caused by physical aggregation.The aggregation of bentonite particles with a water content of 15% is not only caused by the physical aggregation of particles,but also other reasons need further research.