The paper presents results of a laboratory study on the compacted mixture of clayey silt with 1–8% of CaO. Adding of CaO increased the maximum dry density and decreased the optimum water content. In the case of 4–8 % of added CaO the size of the pores kept decreasing until 360 of curing days, while with 2% of CaO the decrease stopped at 120 days and no further pore size changes were detected with 1% of added CaO after 28 curing days. The hydraulic conductivity of the treated soil increased from ca 10⁻⁹ ms⁻¹ to 10⁻⁷ ms⁻¹ on adding CaO, but then decreased back to the order of 10⁻⁹ ms⁻¹ during several days of carrying out the permeability tests. At pH higher than 11.7 the pozzolanic reaction occurred and the soil voids changed due to the new mineral phases, hydrated calcium aluminates especially.