Abstract text (incl. figure legends and references)

According to the multi-barrier concept, radioactive waste is isolated in geological repositories by the host rock and engineered barriers (seals). One group of sealing materials are concretes. During the emplacement and hardening of the concrete, processes can occur that could impair the effectiveness of the seals. As long as the mixture is fluid, the components can separate and air bubbles can escape. The heat of the chemical reactions results in a temperature rise and fall that could lead to volume changes, stresses, and cracking. Moreover, stresses and volume changes can be the result of mineral transformations and the build-up of a grain structure.

During the closure of the Morsleben repository drift seals will be erected. When using salt concrete, the contact zones to the host rock will be grouted. A pilot seal was built to investigate the spatial distribution of the sealing materials, their structure as well as the occurrence of cracks and their spatial distribution. The investigations included microscopic analyzes of the core seal, the contact zone to the host rock and the adjacent rock salt. Thick and thin sections were prepared from drill cores from all areas. For better visualization of cracks, the specimens were vacuum impregnated with blue colored resin and the surfaces of the specimens were polished. All specimens were examined with digital light microscopes. The non-transparent grout and concrete were viewed in reflected light. Moreover, thin section analysis and computed tomographic analysis were carried out. Rock salt could be viewed in reflected and transmitted light.

The work can be divided into an image analysis and a crack analysis. The aim of the image analysis was, for example, to describe the distribution of aggregates in the salt concrete (concrete edge zone), the shape of the contact area with the host rock and the spatial distribution of the grout. The blue-colored resin made it possible to draw conclusions about the depth of damage to the rock salt. In order to draw conclusions about the permeability of the damaged area, information on the extent, thickness, spatial position and cross-linking of the cracks is required. This task included the crack analysis. The basis of the analysis were sets of photos. The technique of image stacking was used to improve the depth of field (cf. Engelhardt 2016).

Measurement values of crack orientations were analyzed by the the numeric computing platform MATLAB. Photo stitching was used to get an overview of larger sample areas. Anaglyphs were made to get a three-dimensional impression of the structure of the rock salt. The ImageJ/Fiji software package was used for image and crack analysis.

The application is an example of the combined and successful use of microscopic methods in the field of structural diagnosis.

Engelhardt, H.-J. (2016) Der Einsatz von Focus-Stacking bei der Untersuchung von Flüssigkeitseinschlüssen im Steinsalz. Mikroskopie, 3 (07): 166-172.

Von Borstel, L.E.; Engelhardt, H.-J. & Köhler, M. (2012) Präparationstechnik zur Visualisierung des Porenraumes von kompaktiertem Salzgrus mittels Dünnschliffen. Schriftenreihe der Deutschen Gesellschaft für Geowissenschaften (SDGG), Heft 80, GeoHannover 2012, 1.–3.10.2012, p. 552.