The present invention relates to improved clayey barriers, such as compacted clay liners or geosynthetic clay liners, which can be used, among other uses, to isolate waste liquids from the environment. More in particular, the present invention relates to clay which is treated with an anionic polymer and is subsequently dehydrated before it is used as a barrier. The latter improved clays are surprisingly well, and for a long term, protected from chemical attack by aggressive electrolyte solutions present in, for example, sea water or waste liquids.