Experimental study on the drying shrinkage characteristics of a modified bentonite-based sealing slurry for buffer barrier gaps in HLW

The sealing performance of modified bentonite slurry used in repository construction joints is strongly influenced by temperature and humidity conditions. Understanding the cracking mechanism of such modified slurries is therefore essential for both the construction and long-term operation of a high-level radioactive waste repository. In this study, the nano-bentonite (Na-b), sodium polyacrylate (PAA), and carboxymethyl cellulose (CMC) were selected as modifiers. Based on the apparent viscosity of the modified slurries, the optimal soil–water ratio and modifier dosages were first determined. Drying tests were then conducted to monitor evaporation rates and crack evolution in the optimized slurries, thereby analyzing the dominant characteristics of crack development and drying–shrinkage behavior. Finally, the modification and anti-shrinkage mechanisms of the additives were further investigated using Electron microscope scanning(SEM)and Infrared Spectroscopy(FTIR)analyses. The results show that a soil–water ratio of 1:4 is optimal for injectable modified bentonite slurry. The Na-b fills the inter-particle voids, and the overall pore volume of the soil structure was reduced. CMC interacts with free water through hydration, and its polar groups (–OH and –CH₂COO⁻) form a coating film around bentonite particles, yielding the densest structure with the smallest pore volume. Both Na-b and CMC significantly increase slurry viscosity and effectively suppress drying shrinkage and cracking. Cracking is dominated by circumferential shrinkage patterns, and the maximum reduction in surface crack ratio reaches 62.3% and 68.4% for Na-b and CMC, respectively. In contrast, PAA forms a hydrogel-like network with free water, dispersing bentonite particles; although this greatly enhances slurry fluidity, it also accelerates crack formation and enlarges the cracking area. The maximum reduction in surface crack ratio for PAA-modified slurry is approximately 34.6%. Considering viscosity, free-swelling behavior, and drying–shrinkage resistance comprehensively, the optimal dosages of Na-b, PAA, and CMC are 10%, 15%, and 1%.