On the application of focused ion beam nanotomography in characterizing the 3D pore space geometry of Opalinus clay

The evaluation and optimization of radioactive disposal systems requires a comprehensive understanding of mass transport processes. Among others, mass transport in porous geomaterials depends crucially on the topology and geometry of the pore space. Thus, understanding the mechanism of mass transport processes ultimately requires a 3D characterization of the pore structure. Here, we demonstrate the potential of focused ion beam nanotomography (FIB-nT) in characterizing the 3D geometry of pore space in clay rocks, i.e. Opalinus clay. In order to preserve the microstructure and to reduce sample preparation artefacts we used high pressure freezing and subsequent freeze drying to prepare the samples. Resolution limitations placed the lower limit in pore radii that can be analyzed by FIB-nT to about 10-15nm. Image analysis and the calculation of pore size distribution revealed that pores with radii larger than 15nm are related to a porosity of about 3vol.%. To validate the method, we compared the pores size distribution obtained by FIB-nT with the one obtained by N adsorption analysis. The latter yielded a porosity of about 13vol.%. This means that FIB-nT can describe around 20-30% of the total pore space. For pore radii larger than 15nm the pore size distribution obtained by FIB-nT and N adsorption analysis were in good agreement. This suggests that FIB-nT can provide representative data on the spatial distribution of pores for pore sizes in the range of about 10-100nm. Based on the spatial analysis of 3D data we extracted information on the spatial distribution of pore space geometrical properties.

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