A NMR-Based Study On The Effect Of Temperature And Salt Solution On Microstructure Of Clay

With the construction of nuclear waste disposal warehouse and municipal landfill,the engineering properties of the cushion backfill material and the main material of the liner in the landfill are gradually paid attention to.For example,the salt content of groundwater in the area where the nuclear waste repository is built is often very high.With the gradual infiltration of groundwater and the release of a large amount of heat in the decay process of nuclear waste,the change of temperature and pore salt solution will cause the change of the microstructure of clay in the buffer backfill material,which will deteriorate the performance of the buffer backfill material,and eventually lead to major safety accidents such as nuclear waste leakage.Therefore,a correct understanding of the influence of temperature and salt solution on the microstructure of clay has important scientific significance and engineering value for optimizing the engineering barrier performance and evaluating the long-term stability of the disposal reservoir and the landfill site.Based on NMR technology,the pore distribution of clay is measured.In addition,the effects of temperature and salt solution on the pore water state of clay are studied.The main research results are as follows:1.Based on the theory of NMR,a fast and accurate method for measuring the relaxation rate of soil is proposed,and the pore distribution of soil is calculated by combining the relaxation rate of soil obtained by this method with the relaxation time distribution of soil.The optimal pore size obtained by NMR method is nearly an order of magnitude smaller than that obtained by mercury injection method,which may be related to the fact that the influence of soil aggregate structure on the relaxation process is not taken into account.If the influence of soil aggregate structure on the relaxation process is taken into account,the effective relaxation rate can be calculated by reducing the relative surface area.The pore size distribution of NMR obtained by effective relaxation rate conversion is close to that obtained by mercury injection method,indicating that the pore size distribution obtained by optimized NMR method is acceptable.2.Based on NMR technology and two-dimensional diffusion theory,the influence of temperature on the state of pore water in different clays is studied.The results showed that the T₁/T₂ values of the three clays were different.The higher the T₁/T₂ ratio was,indicating the higher the affinity between water molecules and soil particles,the lower the mobility of surface water molecules.The clay with the largest specific surface area and cation exchange capacity has the largest apparent activation energy of pore water.For Soil E,△E<0.This anomaly may be due to the high content of kaolinite and illite in clay minerals of soil E.Positive△E indicates that the permeability coefficient is proportional to temperature;Negative△E indicates that the permeability coefficient is inversely proportional to temperature.Larger△E means that the permeability coefficient is more sensitive to temperature.3.Based on NMR technology and two-dimensional diffusion theory,the effect of salt solution on pore water state in different clays is studied.The results show that T₁/T₂ values of expansive clay with high clay mineral content increase to a certain extent,and they are about 2.The results show that salt solution can inhibit the mobility of pore water in this kind of clay,and it has nothing to do with the type of salt solution.In addition,the cation type has an effect on the△E of pore water in different clays.When the clay with higher montmorillonite content is saturated with distilled water,Na Cl,KCl and Ca Cl₂ solution,the△E of pore water in clay decreases in turn.After saturated with Na Cl and KCl solutions,we think that the reason is that the ions in the pores not only inhibit the diffusion of electric double layer,but also reduce the mobility of hydration layer around the ions,resulting in the increase of△E of pore water.