5:15 PM - 7:15 PM
[HCG20-P08] Microstructural investigation of cementation effects on bentonite swelling using electron microscopy
Keywords:Scanning electron microscope (SEM), Transmission electron microscope (TEM), Clay mineral, Smectite, Geological disposal, Long-term alteration
In the geological disposal of high-level radioactive waste, bentonite, a swelling clay, will be used as a buffer material. The swelling property of bentonite is vital to maintain low permeability and inhibit the migration of radionuclides. However, in the long term, cementation of bentonite may occur, where secondary minerals precipitate in pores and adhere to montmorillonite, leading to a degradation of its swelling capacity[1]. To predict the long-term swelling performance of bentonite, it is necessary to understand the interaction between montmorillonite and secondary minerals at the nano- to microscale. However, conventional electron microscopy requires vacuum conditions, making it difficult to observe bentonite in its swelling state. In this study, we employed a specialized method that enables observation while maintaining the swelling state, using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to investigate the microstructural effects of cementation on swelling properties.
The samples were collected from Tsukinuno bentonite mine in Yamagata, Japan. We compared cemented bentonite ore with reconstituted samples, in which cementation effects were eliminated by grinding and reshaping. Previous studies on Tsukinuno bentonite have reported that cemented ore exhibits lower swelling pressure than reconstituted samples[1] [2]. To prepare samples for observation, we embedded millimeter-sized cut specimens in agar, immersed them in ultrapure water for swelling, and sequentially replaced water with ethanol and resin to preserve the swelling state[3]. SEM observations revealed regions where resin permeated montmorillonite and regions where it did not. In particular, at the interfaces with coexisting minerals such as silica and feldspar, montmorillonite was less permeable to resin, and montmorillonite was denser within a few hundred nanometers to 1 µm from these interfaces. A comparison between cemented ore and reconstituted samples indicated a higher proportion of coexisting minerals with densely distributed montmorillonite in the cemented ore. Subsequently, TEM analysis was conducted on ultrathin sliced samples prepared using a microtome and basal spacing of montmorillonite were measured. The results indicated that the basal spacing of montmorillonite near the coexisting mineral interfaces was smaller than other area. Furthermore, a comparison between cemented ore and reconstituted samples revealed that the basal spacing was consistently smaller in the cemented ore, regardless of whether montmorillonite was adjacent to coexisting minerals. These findings suggest that the swelling of montmorillonite is heterogeneous within the sample, and cementation particularly inhibits swelling near coexisting mineral interfaces. This inhibition is likely caused by the adhesion of coexisting minerals to montmorillonite during the formation of bentonite, which restricts water access and prevents full swelling in certain area. Consequently, this incomplete swelling may lead to a decrease in the overall swelling pressure of bentonite. In this presentation, we will extend our observations, statistically compare a larger dataset, and further discuss the implications of cementation on long-term bentonite performance in geological disposal.
References
[1] Japan Atomic Energy Agency & Radioactive Waste Management Funding and Research Center (2019) The project for validating near-field assessment methodology in geological disposal, H30 Annual Report (in Japanese).
[2] Ito, D., and H. Komine. 2023. Smart Geotechnics for Smart Societies, 1834–38. London: CRC Press.
[3] Bozolla, J.J. (2007), Electron microscopy: methods and protocols (pp.1–20). New York: Springer.
The samples were collected from Tsukinuno bentonite mine in Yamagata, Japan. We compared cemented bentonite ore with reconstituted samples, in which cementation effects were eliminated by grinding and reshaping. Previous studies on Tsukinuno bentonite have reported that cemented ore exhibits lower swelling pressure than reconstituted samples[1] [2]. To prepare samples for observation, we embedded millimeter-sized cut specimens in agar, immersed them in ultrapure water for swelling, and sequentially replaced water with ethanol and resin to preserve the swelling state[3]. SEM observations revealed regions where resin permeated montmorillonite and regions where it did not. In particular, at the interfaces with coexisting minerals such as silica and feldspar, montmorillonite was less permeable to resin, and montmorillonite was denser within a few hundred nanometers to 1 µm from these interfaces. A comparison between cemented ore and reconstituted samples indicated a higher proportion of coexisting minerals with densely distributed montmorillonite in the cemented ore. Subsequently, TEM analysis was conducted on ultrathin sliced samples prepared using a microtome and basal spacing of montmorillonite were measured. The results indicated that the basal spacing of montmorillonite near the coexisting mineral interfaces was smaller than other area. Furthermore, a comparison between cemented ore and reconstituted samples revealed that the basal spacing was consistently smaller in the cemented ore, regardless of whether montmorillonite was adjacent to coexisting minerals. These findings suggest that the swelling of montmorillonite is heterogeneous within the sample, and cementation particularly inhibits swelling near coexisting mineral interfaces. This inhibition is likely caused by the adhesion of coexisting minerals to montmorillonite during the formation of bentonite, which restricts water access and prevents full swelling in certain area. Consequently, this incomplete swelling may lead to a decrease in the overall swelling pressure of bentonite. In this presentation, we will extend our observations, statistically compare a larger dataset, and further discuss the implications of cementation on long-term bentonite performance in geological disposal.
References
[1] Japan Atomic Energy Agency & Radioactive Waste Management Funding and Research Center (2019) The project for validating near-field assessment methodology in geological disposal, H30 Annual Report (in Japanese).
[2] Ito, D., and H. Komine. 2023. Smart Geotechnics for Smart Societies, 1834–38. London: CRC Press.
[3] Bozolla, J.J. (2007), Electron microscopy: methods and protocols (pp.1–20). New York: Springer.