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Over the last few decades cementitious materials have been increasingly used in the construction of hazardous-waste barriers, against the release of a variety of contaminants, such as toxic, radioactive and chemical wastes, that are first encapsulated in metallic drums, to be later embedded in reinforced- concrete caissons, generally buried in controlled landfills. The long-term efficacy of this treatment largely depends on the durability of the treated material and on that of the cementitious embedment. Hence, understanding the relationship between the contaminant release and the durability of the cementitious embedment is a must. Similar problems occur in R/C foundation mats whenever the water table is higher than the top surface of the mat and concrete permeability becomes the controlling factor of water seepage. Since 2001 the Department of Structural Engineering and the Department of Electrochemistry have cooperated with the Italian Authority for Energy and Environmental Issues (ENEA) and with a couple of well-known laboratories within a joint project concerning the site identification for the final repository of low-activity radioactive waste materials. Within the aforementioned project the various institutions involved investigated concrete leaching, i.e. the selective transport of particles occurring in a material once water seepage is stabilized (leaching may be activated by long-term concrete weathering). The characterization of suitable, low-permeability concrete mixes was performed by means of standard leakage tests carried out on concrete slabs. Then four full-scale prototype tanks (20 m3 each) were built with the same concrete mixes, and waterproofing tests were carried out. As expected, two weak points became very clear: the construction joints (that allow some leakage even when the joints are hardly noticeable) and the local lack of homogeneity within the concrete mass, amplified by the thinness of the wall and of the steel congestion (vibrating the concrete was impossible and plasticizers had to be used to guarantee concrete workability). A sonic survey was instrumental in pin-pointing the local sand pockets and porosity in the concrete wall and slabs. The research is still in progress, and new information is expected soon.