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[MIS17-08] Pore space property of clayey materials in the forms of gelled to compacted: a strategy for designing buffer materials
Keywords:clay, pore space, electric double layer: EDL, frequency mopdulation atomic force microscope: FM-AFM, microfocus X-ray computed tomography: micro XCT
Context: Rock and clay are still used as major constructive materials in the modern ages, even for the underground facilities such as radioactive waste repository due to their long-term stability. The chemical and mineralogical robustness of these materials is promised by their low reactivity. Until recently, there are many studies developing the geochemical models to validate the safety of waste disposal, because the repository uses buffer clay barrier and dissolution information of clay is top priority. Direct measurement revealed that dissolution rate of clay is lowered by suppressed surface area as a function of compaction density [1]. Recently, we have successfully formulated the effective ESA as the function of density involving isotropic-nematic transition [2]. Step-interaction also affects the dissolution kinetics [1, 3] by the step-dynamic law [4, 1]. Very recently, we have developed an atomistic scale method to evaluate the surface property of hydration structure on clay in groundwater [5]. These knowledges need to be confirmed for the actual bentonite system.
Methods: Direct comparison between observation and measurement is the most persuasive approach. We used FESEM to observe pores in a clay rock from Tsukinuno mine (Kunimine Industries Co.). The tomographic observation was also made on this clay with micro focus X-ray CT (Fig. 1). Direct dissolution measurement with interferometer and AFM was carried out on a purified separate from same clay rock (Kunipia P) under compaction [4]. We newly applied FM-AFM to visualize electric double layer (EDL) above Stern layer. [6].
Results and Discussion: When we consider that groundwater passes though these pores of the clay-rock underlain at the mine, the pore spaces between clay grains are not only the flow-path but also hydration volume with swelling pressure. The actual space can be simply regarded as restricted by the EDL as well as clay density. Generally, it is understood that the hydraulic conductivity varies as the function of clay density. Recent FM-AFM exhibits that the water-path in the compacted clay is prepared by free space excluded from the EDL [5]. This approach realized that dissolution of clay mineral at the pore space filled with equilibrium groundwater (pH 8) is restricted by the pore size that affects the curvature made of elemental steps, unless the pore size is below ~20 nm. Based on the step-dynamics and hydration structure with EDL, we can give a constraint on evolution of actual pore size during alteration [5]. Clay density can affect not only the exposed area of the grain edge at which dissolution mainly occurs [2], but also the hydraulic property by the variety of EDL in the compacted clay. If the clay contains other secondary minerals such as zeolite or silica (see Fig. 1a), the pore structure can be developed as mixture of inter and intra-grains. Furthermore, from the scope of elemental migration by groundwater like pollution, hydration of the minerals changes their sorption property appeared as partition coefficient, Kd (ml/g) that suppresses contamination.
Consequently, in order to design the clayey buffer materials, (1) grain structure from gel to compacted states, (2) electrochemical property as the EDL, (3) diffusion and dissolution, (4) alteration by nucleation of secondary zeolite, and (5) elemental migration by Kd need to be focused to evaluate water-stopping behavior and elemental retardation in case of damage of the barrier. Such a bottom-up approach with focusing pore space science is necessary for repository safety.
References: [1] Satoh et al. (2013) Clay Min., 48, 285-294.; [2] Terada et al. (2017) Mat. Res. Exp. 6 (3) 035514.; [3] Satoh et al. (2007) Am. Min., 92, 503-509.; [4] Burton et al. (1951) Phil. Trans. Royal Soc. London, 243, 299; [5] Satoh et al. (2019), Abst. Migration 2019; [6] Araki et al. (2017) Surf. Sci, 665 32-36.
Figure 1. Micro XCT images of gelled (a) and compacted (b) montmorillonite clay-stone.
Methods: Direct comparison between observation and measurement is the most persuasive approach. We used FESEM to observe pores in a clay rock from Tsukinuno mine (Kunimine Industries Co.). The tomographic observation was also made on this clay with micro focus X-ray CT (Fig. 1). Direct dissolution measurement with interferometer and AFM was carried out on a purified separate from same clay rock (Kunipia P) under compaction [4]. We newly applied FM-AFM to visualize electric double layer (EDL) above Stern layer. [6].
Results and Discussion: When we consider that groundwater passes though these pores of the clay-rock underlain at the mine, the pore spaces between clay grains are not only the flow-path but also hydration volume with swelling pressure. The actual space can be simply regarded as restricted by the EDL as well as clay density. Generally, it is understood that the hydraulic conductivity varies as the function of clay density. Recent FM-AFM exhibits that the water-path in the compacted clay is prepared by free space excluded from the EDL [5]. This approach realized that dissolution of clay mineral at the pore space filled with equilibrium groundwater (pH 8) is restricted by the pore size that affects the curvature made of elemental steps, unless the pore size is below ~20 nm. Based on the step-dynamics and hydration structure with EDL, we can give a constraint on evolution of actual pore size during alteration [5]. Clay density can affect not only the exposed area of the grain edge at which dissolution mainly occurs [2], but also the hydraulic property by the variety of EDL in the compacted clay. If the clay contains other secondary minerals such as zeolite or silica (see Fig. 1a), the pore structure can be developed as mixture of inter and intra-grains. Furthermore, from the scope of elemental migration by groundwater like pollution, hydration of the minerals changes their sorption property appeared as partition coefficient, Kd (ml/g) that suppresses contamination.
Consequently, in order to design the clayey buffer materials, (1) grain structure from gel to compacted states, (2) electrochemical property as the EDL, (3) diffusion and dissolution, (4) alteration by nucleation of secondary zeolite, and (5) elemental migration by Kd need to be focused to evaluate water-stopping behavior and elemental retardation in case of damage of the barrier. Such a bottom-up approach with focusing pore space science is necessary for repository safety.
References: [1] Satoh et al. (2013) Clay Min., 48, 285-294.; [2] Terada et al. (2017) Mat. Res. Exp. 6 (3) 035514.; [3] Satoh et al. (2007) Am. Min., 92, 503-509.; [4] Burton et al. (1951) Phil. Trans. Royal Soc. London, 243, 299; [5] Satoh et al. (2019), Abst. Migration 2019; [6] Araki et al. (2017) Surf. Sci, 665 32-36.
Figure 1. Micro XCT images of gelled (a) and compacted (b) montmorillonite clay-stone.