[en] Sealing of deep boreholes in repository rock is planned to be made by installing dense smectite clay plugs where the rock is low-permeable and casting concrete where the holes intersect water-bearing fracture zones. Such zones have to be stabilized before sealing starts because fragments of rock can otherwise fall off and make it difficult to bring equipment for concrete casting and clay plug units down. These parts of the holes are filled with concrete and clay plugs are then inserted up to the nearest fracture zone where concrete is filled to the required level etc. The role of the concrete in the hole and in the closest part of the surrounding fracture zone is to provide stable parts that are sufficiently fine-porous to prevent clay particles from contacting clay plugs to migrate into the fractures and be lost by erosion. While the larger parts of long clay plugs are believed to stay largely intact chemically for hundreds of thousands of years, the parts adjacent to concrete plugs may undergo changes and so can the concrete plugs themselves. The objective of the presently reported project was to identify the detailed processes and quantify associated changes in physical properties by investigating samples of clay and concrete from a 2.5 m long plug of clay over which an equally long concrete plug had been cast and left to rest for 3 years. The outcome of the investigations was that significant chemically induced changes in mineralogy and physical performance had occurred within a few centimetres distance from the clay/concrete contact but that virtually no changes had taken place at larger distance. A comprehensive laboratory study including X-ray diffraction (XRD), X-ray fluorescence (XRF) and electron microscopy study (SEM and TEM) on the sample material was performed including also dual beam (combined ion and electron) microscopy. It was found that the clay had infiltrated the contacting concrete plug after filling of the borehole since clay was detected both along the contact between the plug and the borehole wall as well as interleaved within the bottom of the concrete fill. Chemical alteration of the cement mineral phases at the clay-concrete contact released some quantities of Ca and K that had partly replaced Na in the interlayer of the clay. Precipitation of gypsum and halite had occurred as well as chemical modifications of the clay. The cement has clearly been altered in contact with the clay plug and has lost part of the material strength. Neocrystallization of a fibrous Ca-Si phase had occurred along with the formation of some amorphous components. Dissolution in the saline water is the probable mechanism. The major practical consequence of the chemical interaction of contacting clay and concrete is that the clay becomes slightly less expandable and slightly more conductive within a few centimetres distance from the contact and that the concrete loses some of its strength in 3 years. The adhesion between concrete and rock appeared to be low and concrete plug segments could easily be extruded from the discs by hand