Comparison of microstructural features of three compacted and water-saturated swelling clays: MX-80 bentonite and Na- and Ca-purified bentonite

Abstract

The planned final disposal repositories of spent nuclear fuel in several countries, including Finland, pose significant scientific challenges due to their extremely long lifespan. One of the key materials proposed for use in Posiva Oy's repository in Finland is MX-80 bentonite in a compacted, water-saturated state. Border cases of calcium and sodium forms of purified bentonite were included in this study. The MX-80 in the repository is expected to undergo cation exchange due to the composition of the groundwater. The clays were studied at different dry densities between 0.7 and 1.6 g cm -3 . The microstructure of the water-saturated, compacted clays was investigated using small-angle X-ray scattering, nuclear magnetic resonance and transmission electron microscopy. Additionally, atomic force microscopy was used to characterize the shape and size of the fine-fraction clay platelets. As expected, the average shape of the fine fractions was smaller than the bulk material, but a more elongated shape was present in the purified material. Mainly due to sample density, the pore structure was noticeably different for the Na form of purified bentonite at 0.7 g cm -3 density, but at higher degrees of compaction, no significant differences were noted between the samples. The laboratory results obtained in this study could be useful for safety and performance analysis of this high-level waste repository where sodium bentonite is used and is expected to change its charge-compensating cation composition during the repository's lifetime. Microstructural data may be used in modelling of diffusion and sorption by surface complexation modelling, for example, or as a basis for mechanical and water transport models.

Polyetheramine as swelling‐inhibitor for expansive soil: Performance and mechanism

“…Therefore, the microcosmic physical–chemical properties truthfully reflect the macroscopic performance properties. Appropriate characterization techniques can effectively construct a reflection between micro and macro performances 39,45,46 . X‐ray diffraction is a powerful technique to indicate both qualitative and quantitative mineralogical results, especially the basal spacing of minerals as a function of angles.…”

Properties and mechanism of a poly(ionic liquid) inhibitor contained bi‐functional groups for bentonite hydration

“…According to the cation species, the bentonite can be classified as Na bentonite, Ca bentonite, and so on. 3,4 The properties of bentonite are dominated by montmorillonite crystal structure and interlayer cation. [5][6][7] Bentonite clay contains a myriad of properties including cation exchange, 8 expansibility, 9,10 stability, 11,12 suspension, 13 dispersion, 14 and hydrophilicity.…”

The Effect of Porosity Change in Bentonite Caused by Decay Heat on Radionuclide Transport through Buffer Material

“…In the past decades, numerous research projects and studies have been conducted worldwide to understand the complex behavior of bentonite as a buffer material more widely and deeply, in geological radioactive waste disposal. A wide range of aspects related to the thermal-hydraulic-mechanical-chemical processes of bentonite have been studied, including high temperature influences on bentonite performance [9][10][11][12], groundwater/gas flow characteristics within bentonite [13][14][15][16][17], swelling pressure and mechanical properties [18][19][20][21][22][23][24][25], and the mineral chemical evolution of bentonite [26][27][28]. Moreover, experimental analysis and numerical simulation have been employed to study the performance evaluation of bentonite in the thermal-hydraulic-mechanical-chemical coupling process under environmental evolution conditions [29][30][31].…”

“…Bentonite has been widely studied as a buffer material for the multibarrier system of radioactive waste disposal due to its low permeability, high water retention capacity, high swelling pressure and capacity, thermal characteristics, and microstructure and contaminant transport [20][21][22]30,[56][57][58][59]. In the performance analysis and evaluation of the buffer material, not only hydraulic characteristics but also thermal (T), hydraulic (H), mechanical (m), chemical (C), biological (B), and radiation (R) processes and properties need to be considered.…”