The tests results for bentonites from the localities of Jelsovy potok, Kuzmice and Lastovce are presented and analyzed. Laboratory investigations have been performed in order to obtain more detailed information of the sealant quality of bentonite, zeolite, diatomite and some of their mixtures. The purpose of this contribution is to suggest a guide-line for verification of bentonite-based materials conformable with the design engineer\'s requirements for environmental sealants with low permeability acceptable in landfill construction forwaste containment. Due to the results obtained, the assessment of the environmental sealants used in GCLs- Geosynthetic day liners containing bentonite mined in Slovakia is presented.