Analyzing of the hydration crack evolution in compacted GMZ bentonite with consideration of technological void ratio

•Characteristics of hydration cracks were accurately quantified during the whole self-sealing process.•There is no obvious effect of technological void ratio on the cracking behavior in the kTV range of 10–25%.•A larger technological void ratio extends the time needed for both the cracks closing and technological voids self-sealing.•There exists a critical ΔkTV, below which the hydration cracking is suppressed and unable to develop completely. Technological voids widely exist around the bentonite blocks in the deep geological repository. After groundwater seeps into the voids, bentonite nearby expands to fill up the voids and completes the self-sealing gradually. The self-sealing ability is of highly importance, which prevents the technological voids from playing a preferential channel for groundwater migration and thus threatening the safety of the canister accommodating the high-level radioactive nuclear waste (HLW). So far, much of the research focus has been on the hydro-mechanical properties of bentonite after self-sealing, while the process itself is neglected. But actually, a cracking phenomenon will pop up in the interim of sealing process. In this paper, the dynamic evolution of self-sealing on compacted bentonite with different technological voids and sample thicknesses was observed by using a transparent observation apparatus. Digital image processing method combined with fractal theory was utilized to process, analyze and quantify the images captured. The results show that the self-sealing process contains the lifetime of the hydration crack, including its appearance, development and extinction. The technological void ratio evidently affects the time require for closing stage compared with the cracking stage. But in the range of 10–25%, there is no clear correlation between the technological void ratio and the geometry parameters or complexity of crack network. The fractal dimension introduced to quantify the complexity corresponds well with the visible image result, showing the feasibility and credibility of box-counting method. The cracking and closing mechanisms are also investigated.