An experimental investigation on the interaction between a contaminant and cement-bentonite mixtures is described and discussed. The best mixture had to be selected to construct a self-hardening cut-off wall to isolate a hazardous waste landfill. The contaminant (i.e. the landfill leachate) is a very acidic inorganic solution which could adversely affect the hydraulic performance of cement mixtures. Immersion and permeation tests were performed to investigate the mixture performance under very aggressive environmental conditions in order to accelerate the effects of chemical reactions. The results of the tests are discussed, showing that the mixture-leachate interaction does not result in a worsen hydraulic performance. An assessment of the durability of the mixture is given.