Abstract: Cement solidification/stabilization is an economical and efficient method to dispose hazardous waste. However, the leachates from portland cement stabilization/solidification product landfills generally contain high concentration of Ca²⁺, which may has detrimental effects on the long-term hydraulic performance of geosynthetic clay liners (GCLs), and cause a secondary pollution. Therefore, hydraulic conductivity tests are performed on the GCLs using two leachates and five effective stresses. The test results show that permeation of leachates results in an increase in the hydraulic conductivity of 179 to 721 times under the effective stress of 24 kPa, and the higher the concentration of Ca²⁺ in the leachates, the higher the hydraulic conductivities of the GCLs. The negative effects of the leachates can be offset by the effective stress applied on the GCLs. For example, when the effective stress increases to 438 kPa, the hydraulic conductivities to the leachates are comparable to those obtained for the controlled specimens (permeated with tap water), and the detrimental effects resulting from the permeation of the leachates are balanced completely by the effective stress.