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Clays and clay stones comprise about 50 % of all sediments and sedimentary rocks on earth. However, due to their very small grain-sizes (usually less than 2 μm in diameter) as well as the sensitivity of many clay minerals towards water, their direct microstructural characterization is still challenging.The Boom Clay and the Ypresian clays are the two reference materials for research on the long-term disposal of high- and medium-level, long-lived radioactive waste in a deep geological formation in Belgium (Van Marcke et al., 2005; ONDRAF/NIRAS, 2013a; Verhoef and Schroeder, 2011; Verhoef et al., 2011). Therefore, the detailed characterization of their pore space is indispensable to gain a full understanding of the relevant processes, governing the transport of radionuclides in these materials (i.e. diffusion, adsorption and migration of radionuclides as well as fluid flow).In the present study, research was carried out on representative samples of the Boom Clay at Mol-1 (Mol-Dessel research site, Belgium; Zeelmaekers, 2011 sample series, chapters 4, 5 and 7) and the Ypresian clays at Kallo-1 (Belgium, chapters 6 and 7), covering end members with regard to mineralogical compositions and grain-size distributions. Recent progress in ion-beam milling tools has revolutionized the microstructural analysis of fine-grained geomaterials. To characterize the microstructure and pore space in Boom Clay and the Ypresian clays, down to the nm-scale resolution in 2D and in 3D, in the present study, focused- (FIB) and broad-ion-beam (BIB) milling were combined with high resolution scanning electron microscopy (SEM), X-ray μ-CT and Mercury Intrusion Porosimetry (MIP).Direct insights to pore space morphologies in 2D were obtained by BIB-milling in combination with high resolution SEM (chapters 4, 6 and 7), whereas a combination of FIB serial cross-sectioning with simultaneous SEM-imaging, X-ray μ-CT and pore network extraction modeling (PNE; Dong and Blunt, 2009) delivered insight to pore space morphologies and the connectivity of the pore space in 3D (chapter 5). MIP was used to relate direct microstructural observations within limited areas or volumes of investigation to bulk sample porosity measurements (chapter 4). Moreover, Wood’s metal injection in combination with cooled BIB-milling and high resolution SEM-imaging was used to analyze the connectivity of the pore space and visualize the impact of high pressure liquid metal injection on the porosity and microstructures of fine-grained, argillaceous geomaterials (chapter 7).Results show characteristic pore morphologies and total observed 2D porosities within different mineral phases of the Boom Clay (chapter 4). These are similar for samples of different mineralogical composition and grain-size distribution and irrespective of the depth of origin of the samples. Largest pores were found to occur preferentially at the boundaries between clay-matrix and non-clay mineral (NCM) – mostly quartz – grains and their sizes seem to depend on the size of the adjacent NCM-grains. Therefore, total BIB-SEM observed porosities are suggested to depend significantly on a sample’s mineralogical composition and grain-size distribution. The overall distribution of porosity within a sample can be modeled based on the distribution of different mineral phases within the sample and the grain-size distribution of the material.In fine-grained, clay-rich (> 50 dry wt.-% clay minerals) Boom Clay the intra-clay matrix porosity contributes to the major fraction of the total porosity and moreover, controls the overall connectivity of the pore space. In more coarse-grained, non-clay mineral-rich (> 50 dry wt.-% non-clay minerals) samples on the contrary, larger inter-aggregate pores account for the major fraction of the total resolved porosity and also significantly contribute to the overall pore space connectivity. Once these larger inter-aggregate pores are directly connected to each other, this might increase the permeability of the material to a large extent. In general it was distinguished between two different porosity and pore space connectivity regimes: (1) a highly connected pore space within the clay-matrix with pore throats mostly below 100 nm in diameter and (2) larger inter-aggregate pores at the boundaries between clay-matrix and non-clay mineral grains with pore throats > 1 μm in diameter, but with a significantly lower interconnectivity at the scales of observation of FIB-SEM, BIB-SEM and X-ray μ-CT.Pore-size distribution analyses in 2D and 3D reveal power-law distributions of pore-sizes in Boom Clay over about 6 orders of magnitude, hinting towards scale invariance and self-similarity of the pore space characteristics and towards the possibility of up-scaling of our nm- to μm-scale observations to much larger scale characteristics of the formation (chapters 4 and 5). 2D BIB-SEM is essential to bridge the gap in observational scales between 3D X-ray μ-CT and FIB-SEM tomography and moreover, to select suitable spots for FIB-SEM porosity analyses (chapter 5).Comparing the results of porosity analyses on the Boom Clay and the Ypresian clays shows similar microstructures and pore space morphologies within the clay-matrix of the fine-grained, clay-rich samples, whereas in more coarse-grained, NCM-rich Ypresian clay samples, significantly different pore space morphologies were observed (chapters 4 and 6). Porosity distributions in the Ypresian clays seem to be bi-modal, whereas in the Boom Clay unimodal porosity distributions were observed.Results of Wood’s metal injection substantiate the high connectivity of the pore space in Boom Clay and the Ypresian clays (chapter 7).In the future, BIB-SEM observed porosities, pore morphologies, pore-size distributions and information on the pore space connectivity should be used to model bulk sample physical properties, such as permeability, diffusivity and electrical resistivity as well as single and multi-phase fluid flow and the migration of radionuclides in the analyzed materials.

The application of ion-beam milling techniques to clays allows investigation of the porosity at nm resolution using scanning electron microscopy (SEM). Imaging of pores by SEM of surfaces prepared by broad ion beam (BIB) gives both qualitative and quantitative insights into the porosity and mineral fabrics in 2D representative cross-sections. The combination of cryogenic techniques with ion-beam milling preparation (BIB and FIB, focused ion beam) allows the study of pore fluids in preserved clay-rich samples. Characterization of the pore network is achieved, first, using X-ray computed tomography to provide insights into the largest pore bodies only, which are generally not connected at the resolution achieved. Secondly, access to 3D pore connectivity is achieved by FIB-SEM tomography and the results are compared with 2D porosity analysis (BIBSEM) and correlated with bulk porosity measurements (e.g. mercury injection porosimetry, MIP). Effective pore connectivity was investigated with an analog of MIP based on Wood’s metal (WM), which is solid at room temperature and allows microstructural investigation of WM-filled pores with BIB-SEM after injection. Combination of these microstructural investigations at scales of ,1 mm with conventional stressstrain data, and strain localization characterized by strain-fields measurement (DIC – digital image correlation) on the same sample offers a unique opportunity to answer the fundamental questions: (1) when, (2) where, and (3) how the sample was deformed in the laboratory. All the methods above were combined to study the microstructures in naturally and experimentally deformed argillites. Preliminary results are promising and leading toward better understanding of the deformation behavior displayed by argillites in the transition between rocks and soils.

Mc 2009. Microscopy Conference, Graz, Austria. 30 August - 4 September 2009. First Joint Meeting Of Dreiländertagung And Multinational Congress On Microscopy.