<p>Comprehensively understanding the self-sealing behavior of technological voids is essential for modeling the hydromechanical performance of the bentonite engineering barrier. In this study, a series of hydration tests on compacted bentonite blocks containing varying volume ratios (7.6%, 3.9%, and 0%) of annular technological voids were performed by a self-developed visualization apparatus. Both macroscopic morphology observations and microscopic postmortem analyses (physical state and pore structure) were employed to elucidate the mechanisms underlying the self-sealing behavior of technological voids between compacted bentonite blocks. Results revealed that the macroscopic self-sealing dynamics primarily entailed three phenomena: filling, cracking, and homogenizing. Filling denoted that hydrated and swollen bentonite filled technological voids, analogous to unconfined swelling, forming a distinct “sealing belt.” Cracking involved cracks in the inner cylindrical specimen induced by differential swelling and cracks in the technological void zone caused by dehydration. Homogenizing referred to a transformation from heterogeneity to homogeneity. Nevertheless, complete homogeneity was not expected to be achieved, and the specimen eventually stabilized in a saturated but non-homogeneous state. From the viewpoint of microscopic structure, the zone containing initial technological voids underwent swelling and compression deformations successively, while zones away from initial technological void only underwent swelling deformation. Swelling deformation was characterized by an increase in inter-aggregate void ratio and undetected void ratio, while compression deformation exhibited an opposite trend. No clear changes in intra-aggregate void ratio were observed during these deformations. Finally, a homogeneity factor based on dry density distribution was introduced to quantitatively investigate the correlations between specimen homogeneity and hydration time as well as volume ratio of technological voids.</p>