Interaction of low pH cement solutions and Friedland Ton for 5 months showed that the hydraulic conductivity of the clay had not increased, which is most important for its use as backfill in a repository where concrete is a construction component. Chemical analyses of the cement solutions and XRD investigations showed that the only change of the mixed layer muscovite–montmorillonite clay minerals was some slight dissolution. A very small change in potassium content of the solutions indicates that illitization was insignificant. With the selected technique, 15 cm3 of cement water, corresponding to about 150 cm3 of fully reacted concrete, interacted with 15 g of solid clay. The conclusion is that complete reaction between a 1-cm layer of Friedland Ton and a 10-cm concrete wall made of low pH cement causes insignificant changes of the clay. Hence, concrete plugs can be used in repositories without significantly altering the isolating properties of such backfill in any time perspective.