Chemical Interaction of Clay Buffer Materials and Concrete

摘要

Various types of repositories for low-,medium-,and high level radioactive wastes require concrete for plugging or construction purposes, the concrete being in direct contact with clay buffer materials. In the case of Na montmorillonite, two main chemical processes can affect this type of clay, namely pH-induced attack on the crystal lattices, and exchange of initially adsorbed Na to Ca. The ion exchange is of minor importance while lattice destruction may be serious. Theoretically, it would lead to the formation of zeolites. Three experimental studies were conducted to find out the type and extent of chemical reaction. One concerned a close investigation of a 70 year old concrete/clay foundation, which showed that alterations of the chemical composition could not be found at larger distances than about 50 mu m from the interface. The second study was an accelerated 3-month test in a borehole in the Stripa Mine, which indicated no mineralogical alterations whatsoever, even at the interface, and no sign of a calcium diffusion profile into the clay. The third study involved percolation of a Na bentonite disc with NaOH solution for one year. No mineralogical alterations, such as zeolite formation, could be identified in this case either, but some dissolution of quartz and possibly feldspars was observed. The permeability was typical of that of untreated Na bentonite while the swelling. (Atomindex citation 13:709017)