Swelling Characteristics Of Different Cationic Bentonites Under Salt Solutions

Bentonite is used as the main material of buffer and backfill materials in the repository because of its high swelling property,low permeability and strong nuclide absorption.However,the environment of the repository under the strata is relatively complex,and its ofen affected by the chemical substance of groundwater.When the chemical composition of groundwater is different from the cation composition of bentonite,the ion exchange reaction will occur.The swelling properties are different for the bentonite containing different cations.Some will promote the swelling of bentonite,while some will inhibit swelling.As a result,the safety of the high-level waste repository cannot be guaranteed.Therefore,it is necessary to study the swelling properties of different ion bentonites in different salt solutions.In this study,the expansion characteristics of different cationic bentonites in different salt solutions were studied by theoretical analysis and laboratory tests.The study is divided into three parts:（1）Three methods were used to test the CEC of clayey soil,and the applicability of MB method was explored;（2）Three kinds of bentonites,K-MX80,Ca-MX80,Na-MX80,were prepared from MX-80 bentonite,and the swelling properties of the three kinds of soil were preliminaries determined by MB method.（3）Using the three kinds of bentonite as the objects,the expansion index and expansion force tests were designed to explore the expansion characteristics of the three kinds of bentonite in different salt solution concentrations.The main conclusions are as follows:（1）The CEC of clayey soils was measured by ammonia method,barium method and MB method,the results show that the ammonia method and barium method are consistent;But MB method is slightly different from ammonia method and barium method,the CEC was lower at low CEC,and higher when the CEC was high.The MB method is modified by the expansion multiple of layer spacing d₀₀₁,and the results show that the accuracy of the MB method is improved obviously after the correction.（2）MX-80 bentonite was purified by immersion method to prepare three kinds of single ion bentonite,K-MX80,Ca-MX80 and Na-MX80 bentonite,and dialysis was used to remove excess salt from bentonite surfaces.XRF chemical composition analysis was carried out on the prepared soil samples,the result shows:The corresponding cation content of the three kinds of single ion bentonite is increased,indicating that the immersion method has a better purification effect.（3）The expansion characteristics of three kinds of bentonites in different salt solutions were studied by expansion index and expansion force test.The results show that the order of expansion index is Na-MX80>K-MX80>Ca-MX80 bentonite,and the inhibition effect of salt solution on bentonite expansion index is Ca Cl₂ solution>KCl solution>Na Cl sution;The order of swelling force is Na-MX80>Ca-MX80>K-MX80 bentonite,and the inhibiting effect of salt solution on the swelling force of bentonite is KCl solution>CaCl₂ solution>NaCl solution.