Experimental evidence on the cracking and sealing mechanisms of compacted bentonite by using microfocus X-ray computed tomography

Abstract

Compacted bentonite plays important roles in multi-barrier systems for the deep geological disposal of radioactive waste; one such role is to absorb groundwater infiltrating from the surrounding rock and to expand to seal technological voids. In the self-sealing process, the expansive deformation occurs along with a cracking phenomenon, which may lead to the performance degradation of engineered barrier. In this paper, Gaomiaozi bentonite was compacted into a dense sample to conduct a hydration test, accompanied with a simulated radial technological void. Microfocus X-ray computed tomography was used to scan the sample at various hydration moments to observe the evolutions of external appearance and internal structure. Consistent and significant results were obtained through qualitative observations and quantitative analyses. The self-sealing process underwent a gradual slowdown, manifested as a linear correlation between sample volume and hydration time on the semi-logarithmic coordinate. The sealing order of technological void was from bottom to top, consistent with the wetting path. Simultaneously, the hydration cracks developed following the same sequence. The maximum height of crack propagation was always less than that of the sample expansion. Moreover, the cracking direction of bentonite was always perpendicular to the tensile stress, resulting in a vertical strike for most cracks. However, there still existed a horizontal crack, which may be attributable to the weakness of the stratified interface. Under persistent hydration, the bentonite adjacent to the cracks was supplied with water, thereby strengthening the expansion and gradually closing the cracks. The hydration, expansion, and cracking were closely interrelated, and the relevant mechanisms are revealed in detail.