Study On The Corrosion Of GMZ Bentonite By Strong Alkaline Solution

In the underground repository of high-level wastes,lining concrete is an effective measure to seal the wall of the repository.However,during the long-term operation of the repository,the concrete lining will be corroded by groundwater in the surrounding rocks,generate the cement hydrate with strong alkalinity,and then penetrate into the bentonite buffer barrier.At the same time,the decay of radionuclides in waste tank will release a large amount of radiant heat.The superposition of chemical and temperature fields will cause the degradation of the performance of the buffer barrier,which will affect the safety of the barrier.The GMZ bentonite was studied by laboratory test and theoretical analysis.First of all,the batch test of bentonite’s hydraulic index was conducted to measure the liquid-plastic limit and free swelling rate of bentonite under different strong alkaline solutions,and quickly select the chemical solutions and the combinations,which the properties of bentonite changes.Then,the contact diffusion test between GMZ bentonite and KOH solution with different pH values was carried out under a certain temperature,simulating the corrosion of bentonite under the combined action of alkaline cementitious materials and radiant heat.After,using X-ray diffraction(XRD),nitrogen adsorption and scanning electron microscopy(SEM)to investigate the possible changes in clay minerals and microstructure of the bentonite,a comprehensive evaluation was made on the performance degradation of bentonite buffer backfill under the combined action of chemical and radiant heat.Through the batch tests of the liquid-plastic limit and free swelling rate of GMZ bentonite under different alkaline solutions,it was found that the liquid limit of bentonite decreases greatly with the increase of the pH value of alkaline solution,and the change in plastic limit is not obvious,resulting in the plasticity index decreased significantly.The free swelling rate of bentonite also decreases with the increase of the pH value.And it’s found that the alkali metal ion K~+ion has a significantly greater influence on bentonite hydraulic index than Na~+.In the contact diffusion test,the pH value of KOH solution was set at 12.6,13.0and 13.5,the temperature was 30℃and 60℃,and the reaction time was 7,14,21 and28 days,respectively.In order to reduce the disturbance to the sample,bentonite was pressed into the reaction device by the static pressure method at one time.As soon as diffusion tests terminated,it was found that with the increase of the pH value of alkaline solution and the temperature,the amount of bentonite squeezed into the tube gradually decreased,and the swelling performance of bentonite was greatly inhibited.The influence of temperature on bentonite swelling performance is less than the pH value of alkaline solution.The bentonite sample of layer A was taken for XRD,it was found that the pH value of the alkaline solution,the test temperature and the reaction time were important parameter that affects the dissolution of montmorillonite.The content of montmorillonite gradually decreases with the increase of the pH value,the test temperature and the reaction time.The dissolution mainly occurs on the contact surface between the bentonite and the alkaline solution,the content of montmorillonite decreased from original 44.4%to 25.9%.Nitrogen adsorption test found that with the increase of the pH value of KOH solution and the test temperature,the BET surface area of bentonite increased gradually,and the BJH average pore width decreased gradually,however,along the direction of diffusion layer,the result is opposite.The pore width of the bentonite mainly concentrated between 2~5nm and 10~100nm from the pore width distribution curves by BJH method.As the pH value increases,the volume of small pores in the sample does not change significantly,and the volume of large pores gradually increases.With the increase of the test temperature,the volume of the pores increases;along the direction of the diffusion layer,the volume of the pores gradually decreases.At the interface between bentonite and alkaline solution produce many visible cracks,which provides a convenient way for the infiltration of the alkaline solution,and accelerates the reaction of internal bentonite.The microstructure of bentonite was observed by SEM,it can be seen the wing-like gels produced by the hydration of bentonite are basically dissolved within 0-2mm of the surface,and become more fragmented and scattered,with many fissures and holes.Further analysis of SEM photo by Image J,it was found that the apparent porosity of bentonite increased with the increase of the pH value of alkali solution and the test temperature.