As a major buffering backfill material for high-level radioactive waste disposal repository, bentonite is affected by soluble salts in groundwater and thermal energy generated by nuclear waste decay, leading to drying shrinkage. Studying the drying shrinkage characteristics of bentonite under the influence of salt solutions is crucial to rationally evaluate its engineering performance. Constant-temperature drying shrinkage tests were carried out on bentonite from Gaomiaozi under the exposure of NaCl and CaCl2 solutions with concentrations of 0.5, 1.0 and 2.0 mol/L, respectively. The influence of salt solutions on the drying shrinkage process of compacted bentonite was analyzed. Results show that the higher the salt solution concentration, the lower the initial saturated water content and the higher the residual water content of the sample, indicating a lower water evaporation rate. Meanwhile, the effect of Ca2+ on water evaporation is greater than that of Na+. During the drying process, the surface fissures of compacted bentonite exhibit four stages, namely fissure opening, fissure maintenance, fissure closure, and fissure stability. Salt solutions could inhibit the cracking of soil mass, and the higher the concentration, the smaller the surface crack ratio of the sample. Salt solutions can change the microstructure of bentonite from a dispersed viscous particle form to an aggregated structure, thereby reducing the shrinkage strain and weakening the anisotropy of the shrinkage. As the concentration and cation valence of salt solutions increase, the volume shrinkage strain εvf induced by the decrease of unit water content increases.
Key words
compacted bentonite /
salt solution /
water evaporation /
drying shrinkage /
fissures
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