Experimental study on shrinkage characteristics of compacted bentonite–sand mixtures

Bentonite–sand mixtures have been proposed as the potential engineered barrier materials because of their low permeability and strong sealing ability under hydrated repository conditions. However, the mixtures may gradually lose water and shrink, which could compromise the engineering integrity of the barrier, under the environmental conditions of the repository where there is a continuous release of radiant heat from nuclear waste. To investigate the shrinkage characteristics of compacted bentonite–sand mixtures, we carry out laboratory drying tests by preparing samples with varying initial dry densities, water contents, and sand contents. The fabric of sand particles is characterized through the quantitative analysis of micro-CT scanning results. Experimental results reveal that both initial compaction condition and sand content significantly influence the shrinkage behaviors of bentonite–sand mixtures. Both shrinkage strain and shrinkage coefficient of the specimens decrease with the increasing initial dry density and the decreasing initial water content. Generally, the development of suction during drying is responsible for the volume shrinkage. The shrinkage and water retention capacities reduce with the increasing sand content in the mixture. Moreover, micro-CT results reveal that, with higher amount of sand inclusions, the mixture presents higher sand particle contact density. When sand content exceeds 30%, these sand particles tend to form sand skeletons that provide high inter-particle friction forces and limit the further shrinkage of mixture. This study is expected to improve the basic understanding of the underlying shrinkage mechanisms of bentonite–sand mixtures and help guide the future design of nuclear waste repository.