Influence mechanism of quartz sand content on drying shrinkage and crack of paste-like bentonite-sand mixtures as buffer/backfill materials

The shrinkage and crack characteristics of bentonite-sand mixtures as buffer/backfill materials play an important role in the security of high-level radioactive waste (HLW) geological disposal. In this investigation, the paste-like bentonite-sand mixtures with sand ratios of 0% to 50% in dry mass are prepared and subjected to indoor thermostatic drying in order to study their volumetric shrinkage and desiccated crack behaviors. The tests results indicate that when the sand ratio is less than 30%, the soil shrinkage characteristic curves (SSCC) and radial shrinkage strain curves are almost identical, respectively, indicating that the shrinkage and crack characteristics of the mixtures are dominated by bentonite because sand particles are suspended in bentonite powders without interaction. When the sand ratio is larger than 30%, the sand particles gradually contact to increase friction, and larger pores are formed by sand mixing, a phenomenon which restrains the shrinkage and crack development of the paste-like mixtures. The amount of air entry remarkably increases, and the shrinkage limit slightly increases because of the increased contact of sand particles, indicating the restraint of shrinkage of bentonite. Larger pores weaken the capillary force, and a higher fraction reinforces the cap ability of the mixtures to resist fractures, a phenomenon which directly restrains the extension of drying cracks. The minimum sand ratio to restrain shrinkage and desiccated crack propagations of the paste-like bentonite-sand mixtures is 30%.