<p>Compacted Clay Liners are widely used as barriers to prevent landfill leachate percolation. While Compacted Clay Liners in the field are exposed to leachate, when investigated in the laboratory, they are usually tested with water percolation. Disregarding leachate complexity and its interaction with Compacted Clay Liners overlooks changes in soil microstructure and permeability. This study aims to analyse the hydraulic behaviour and microstructural alterations of silty sand-bentonite mixtures used in a Compacted Clay Liners for landfill. The samples tested were composed of a mixture of local silty sand soil and bentonite. The fluids used were distilled water and landfill leachate. A factorial design of the experiments was carried out, evaluating two independent variables: water content (7%, 11% and 15%) and bentonite content (5%, 15% and 25%). The response variable was the permeability coefficient, both for distilled water and leachate. Then, the samples were analysed under optical microscopy and scanning electron microscopy. The permeability of SB mixtures to landfill leachate was up to one order of magnitude higher than their permeability to distilled water. The increase in permeability is attributed to contaminants in the leachate that cause a reduction in the diffuse double layer, expansive potential of the soil, and clay flocculation as observed in optical microscopy and scanning electron microscopy images. This study found a significant change in the hydraulic behaviour and soil structure of SBs in CCL. This highlights the need to reassess design criteria for these liners with more appropriate parameters than just water permeability.</p>