Study On Swelling And Permeability Evolution Of Bentonite-sand Mixture By Alkali Heat Aging

In the deep geological disposal project of high-level nuclear waste,the incorporation of a certain proportion of quartz sand into bentonite can help to improve the permeability,thermal conductivity and workability of the buffer layer.Currently,the time scale of buffer materials is relatively short,and it is necessary to Study on the expansion and permeability evolution of mixed buffer/backfill materials with multi-factor coupling under long-term scale.In this paper,a complex chemical-thermal coupling environment for buffer materials is built,and a platform for simulating the environment of the repository and accelerating aging is built.With the increase of aging time and aging temperature,the liquid plastic limit,plasticity index and free expansion rate of bentonite decreased,the main mineral composition did not change,and the mass percentage of montmorillonite gradually decreased to 61.2%.The p Hysical,mechanical and permeation properties of the bentonite-sand mixture are important parameters for evaluating the hybrid buffer material.Designing a sample of statically co MPacted mixture with different sand mixing rates,water content and dry density,through triaxial compression,expansion force and penetration test,the shear strength of the mixture increases with the increase of confining pressure.The stress-displacement curve changes from strain softening type to strain hardening type;when the sand mixing rate is 0%~50% and the confining pressure is 0~1.2MPa,the strain softening type transitions to strain hardening type with the increase of sand mixing rate;The maximum expansion force of the mixture decreases exponentially with the sand-mixing rate;when the sand-mixing rate is between 0% and 50%,the permeability coefficient and sand-mixing ratio of the mixture are linear,and the permeability coefficient is increased by one to two orders of magnitude.The fuzzy theory was used to comprehensively evaluate the buffering performance of different proportions of bentonite-sand mixture to determine the optimal sand mixing rate of 30%.The long-term stability of the bentonite-sand mixture is an important factor in the design of the buffer layer.The expansion force and permeability coefficient of the mixture with the optimum aging time and different aging temperatures are obtained through experiments.The permeability coefficient and expansion force of the mixture decrease with the aging time and aging temperature,and the permeability coefficient is on the order of 10-13~10-12m/s.According to Arrhenius' s law of temperature and time,it is a function of the vertical maximum expansion force and the permeability coefficient of the mixture.Under the safety requirement of 10000 years,the permeability coefficient meets the requirements.Co MPared with pure bentonite,the vertical expansion force decreased from 4.27 MPa to 1.39 MPa.