Soil composition and moisture content have significant effects on the resulting ulk density of clayey soils because of the rearrangement of the solid particles and chemical stabilization. In this study, kaolinite and kaolinite with calcium carbonate (KC), silica gel (KS), and both calcium carbonate and silica gel (KSC) were compacted at different moisture contents according to ASTM standards. These mixtures of compacted clays were then subjected to leaching by distilled water, followed by a solution of heavy metals. Experimental results indicate that the dry density and coefficient of hydraulic conductivity are significantly influenced by the soil constituents. Specifically, kaolinite exhibits a low dry density and a high coefficient of permeability when compared to the other soils (KS, KC and KSC). As permeant flow approaches a stationary regime, greater pore volumes of effluents result, and an increase in the coefficient of permeability is observed in all types of soils. Tests confirm that chemical reactions are responsible for permeability increases up to a constant value. Expressly, kaolinite mixtures with silica gel or calcium carbonate exhibit a coefficient of permeability almost one order less than that of kaolinite. Results indicate the importance of silica gel as an additive for the reduction of the permeability in clay soils.