Abstract <p>Thanks to theoretical works on the use of low-pressure underground fresh water, it has become advisable to build artificial storage reservoirs for efficient use of water resources in the irrigation of arid lands. Problems arise in the construction of reservoirs with regard to the external and internal waterproofing of the structures. The most widely recognized materials for this purpose are the waterproofing materials based on bitumen, synthetic resin polymers, and mineral binders. They are distinguished by their water impermeability, water resistance, durability, and mechanical strength due to their chemical composition. Today, there is a particular need for new, more durable materials that meet these requirements and are affordable. With these features in mind, we conducted studies on a highly effective composite waterproofing material made from bentonite clays. We present the results of laboratory tests on the chemical and mineralogical composition and thermal analysis of bentonite clay from the Levashi deposit in the mountains of Dagestan. Experimental studies have established the optimal compositions based on bentonite clay and Portland cement clinker with the addition of a modifier, nanodispersed sodium polysilicate, ground together in a ball mill to a specific surface area of 2500–3000 g/cm². The technological and physicomechanical properties of the composite paste and the composition as a whole after hardening have been determined both under natural conditions and under optimal heat and moisture conditions of the arid regions in southern Russia.</p>