Three-dimensional characterization and quantification of hydration-induced cracking process of bentonite

Abstract:

To elucidate the hydration-induced cracking phenomenon and the spatiotemporal evolution of crack network in bentonite during the self-sealing process of technological voids in deep geological repository of high-level radioactive wastes, a hydration test was conducted on the compacted bentonite with a radial technological void. Microfocus X-ray computed tomography was utilized for detecting the bentonite non-destructively at different hydration time. Qualitative and quantitative characterizations of the crack network were performed after the three-dimensional reconstruction. The results demonstrate that the bentonite sample experienced an evolvement from crack initiation and propagation to crack closure.Statistical analysis of morphological and geometric parameters revealed that Feret diameter and sphericity were respectively distributed within the ranges of 0.1-0.2 mm and 0.6~0.8, indicating that the crack network was composed of a large number of dotted cracks in isolation (>95%) and a small number of flaky cracks of interconnection. Notably, the latter accountted for over 80% of the total volume and surface area.The linear relation between the logarithm of hydration time and both the sample and technological void volume suggested that the expansive deformation of bentonite gradually slowed down in the hydration process. Furthermore, the linear relations of swelling rate, cracking rate and closing rate versus time in double logarithmic coordinates, as well as the persistent dominance of swelling rate over cracking and closure rates, reflected an intrinsic connection between expansion deformation and crack evolution. Specifically, the expansive deformation served as both a prerequisite for cracking and a necessary condition for crack closure.Through three-dimensional characterization and quantitative analysis, this study revealed the evolutionary regularity and underlying mechanism of hydration-induced cracks, thereby providing critical references for the design and optimization of the buffer/backfill materials.