| Nuclear energy is a clean,safe and reliable energy source.It is widely used in medical,industrial,and national defense fields.However,the development of nuclear energy has also resulted in a large amount of nuclear waste.At present,the most feasible treatment measures are deep geological disposal methods in China and overseas.It is based on the design concept of multiple barriers（engineering barriers and natural barriers）and aims to achieve high levels of nuclear waste.In this way,the waste is permanently isolated from the ecological environment.In China,the research work on the deep geological disposal of high-level radioactive waste started in the 1980s.After more than 30 years of researching,the Beishan area of Gansu Province was selected as the first choice for China’s high-level radioactive waste deep geological repository.Inner Mongolia Gaomiaozi（GMZ）Bentonite is the preferred buffer backfill material.Based on previous studies,this paper studies the coupling effect of sand content,temperature and chemical solution on the swelling properties of GMZ bentonite-sand mixture,and provides theoretical data for underground tests and basement tests.In this paper,the experiment of vertical load-free expansion are carried out under different temperature gradients by selecting different concentrations of chemical solutions as pore fluids and by adding bentonite-sand mixtures with different sand ratios.The effects of NaCl-Na2SO4 solution and temperature on the swelling deformation properties of compacted GMZ bentonite-sand mixtures are analyzed.At the same time,the experiment of mineral composition and microstructure is carried out before and after the test,revealing the microscopic mechanism of the influence of temperature,solution and sand content on the swelling deformation properties.Some results are showed as follows:（1）With the increase of temperature,the expansion deformation process of the specimen is accelerated,the maximum expansion rate increases,and both of them tend to be stable with time.As the temperature increases,the expansion process accelerates.The increase of dry density corresponds to an increase the content of bentonite,so that the expansion deformation is enhanced.However,the effect of temperature on the swelling of specimens with different initial dry densities was no significant difference.The addition of quartz sand will reduce the relative content of bentonite in the sample,thus reducing its swelling.（2）By adding quartz sand to bentonite,the integrity of bentonite specimen will be destroyed,which will increase the cracks between bentonite and sand produce and the quantity of micro-cracks in bentonite.What’s more,with the increase of the quantity of micro-cracks in bentonitethe,ratio of large porosity of bentonite-sand mixture will be enlarged,which is beneficial to the heat transfer and internal fluid flow in the sample.Therefore,the quartz sand will improve the effect of temperature on the swelling.And with the increase of sand content,the effect is more significant.（3）With the increase of solution concentration,the maximum expansion rate of the specimen decreases.However,as the increase of temperature,the reduction rate of the maximum expansion rate has been reduced,which indicates that the temperature can counteract the inhibition effect of some chemical solution on the swelling property.（4）For low temperature,NaCl-Na2SO4 solution or high temperature,distilled water,there is no change in the mineral composition of the sample;while under high temperature and high concentration of NaCl-Na2SO4 solution,the structure of montmorillonite may be changed,but new minerals are not producing.（5）What’s more,adding quartz sand to bentonite will reduce the total void ratio of compacted mixture specimen,but the number of larger pores increases.The expansion test ends and the porosity ratio of the shrinkage stabilized specimen is much smaller than that of the compacted specimen before the expansion test,which shows that the shrinkage of the sample can be more dense than the original specimen,but the increase of sand content increase the porosity ratio of the stabilized soil and increase the number of large pores. |
