Effects of Saline Solutions on the Desiccation Cracking and Shrinkage Behavior of Gaomiaozi Bentonite

Abstract

Bentonite, when used as buffer/backfill material in the deep disposal of high-level radioactive waste (HLW), could undergo desiccation shrinkage or even cracking due to the heat released from HLW, impairing the efficiency of the barrier system. Furthermore, in-service buffer materials are inevitably in contact with the groundwater, which sometimes contain high salt concentrations. The groundwater salinity may modify the properties of bentonite and hence affect the process of desiccation and its performance. To investigate this effect, in this study, a series of temperature-controlled desiccation tests was conducted on compacted specimens of Gaomiaozi (GMZ) bentonite preliminarily saturated with two different saline solutions (NaCl and CaCl2) at the concentration varying from 0.5 to 2.0 mol/L. The experimental results indicated that, as the concentration of saline solution increases, the initial saturated water content of bentonite decreases, whereas the residual water content at the completion of the desiccation test increases. The water evaporation rate is reduced for the specimens saturated with a high-concentration saline solution, and CaCl2 has a more significant influence on water evaporation than NaCl. The evolution of cracks on the sample surface during the desiccation process can be divided into four stages: crack growth, maintenance, closure, and stabilization; an increase in the salt concentration effectively inhibits crack development. It was shown that the infiltration of saline solutions alters the microstructure of bentonite by changing the arrangement of clay particles from a dispersed pattern to more aggregate state, which results in a decrease in shrinkage strain and shrinkage anisotropy.

Effect of initial water content and dry density on the self-healing of desiccation cracks in compacted hipparion red clay

“…Du et al (2019) reported the cracking behavior of compacted soil not only depends on the tensile stress and tensile strength of cracking regions, but also depends on the overall shrinkage of the lower parts under cracking regions closely. However, there seems to be no fundamental explanation available for the phenomenon of crack closure that was observed in present experiments (Tan et al, 2020). Therefore, the self-healing of desiccation cracks is important for mechanism revelation of cracking in compacted soils and needs examination studies.…”

“…However, from the works of Day, (1997, Day, 1998, Miller et al (1998), Rao et al (2000), Yesiller et al (2000) and Auvray et al (2014), it is evident that not all the desiccation cracks run through the entire compacted specimens due to the high tensile strength of soli specimen. Thus, some complexity cracking behaviors have been observed (Tang et al, 2011b;Tabassum and Bheemasetti, 2020), such as the self-healing behavior of cracks within compacted clay specimen (Chertkov et al, 2004;Boivin, 2007;Du et al, 2019;Julina and Thyagaraj, 2019Li et al, 2020;Tan et al, 2020). In particular, the desiccation cracks in compacted specimen tended to close at the end of shrinkage stage during drying (Chertkov et al, 2004;Boivin, 2007).…”

“…In particular, the desiccation cracks in compacted specimen tended to close at the end of shrinkage stage during drying (Chertkov et al, 2004;Boivin, 2007). Du et al (2019) and Tan et al (2020) used the photography technology to capture the self-healing behaviors of desiccation cracks in compacted cohesive soils and bentonite soils during drying, and they divided the evolution of soil cracking upon drying into four stages including crack growth, maintenance, closure and stabilization stage. Thyagaraj (2019, 2021) introduced the XCT imaging and vernier caliper height method for tracing the shrinkage behavior of compacted expansive soil specimen, and they also found the phenomenon that the cracks area and cracks volume first increased and then decreased during drying.…”

Evaluation of Saline Solutions and Organic Compounds as Displacement Fluids of Bentonite Pellets for Application in Abandonment of Offshore Wells

“…Compared with Figure 2, in which the pellets are presented before contact with the brines, the images in Figures 3 and 4 show that all brines promoted the degradation of the compacted form of bentonite, causing it to take the form of a powder or very small pieces. This may occur due to changes in the microstructure of compacted bentonite promoted by the exposure to saline solutions once the concentrated saline media promotes an osmotic suction, which can act as an internal compaction tension, causing the particles to approach each other [21]. There is a minimization of the repulsion of the double layer that is associated with this mechanism, caused by the presence of the high cation content in the solution, resulting in an increase in the attraction between the lamellae and the consequent aggregation of the clay particles.…”

Volume change in compacted claystone-bentonite mixtures as affected by the swamp acidic water

“…Even at a bentonite content of less than 15%, the sample tends to compress. This compression is problematic when occurs horizontally as it results in cracks [42,43]. During lateral compression, the shear strength of the soil decreases and its permeability increases.…”