The purpose of hydraulic barriers, such as geosynthetic clay liners (GCLs), is to isolate waste liquids from the environment. Bentonite clay is widely used in GCLs because of its elevated sealing capacity in the presence of water and its ability to restrict the migration of solutes (chemico-osmotic efficiency or semi-permeable membrane behaviour). However, exposure to high concentrations of inorganic solutions can change the clay fabric increasing its hydraulic conductivity and degrading its membrane behaviour, with a consequent harm to the environment. The aim of this research was to study the hydraulic and chemico-osmotic performance of amended clays. For this purpose, an engineered clay (HYPER clay) was developed through treatment of a natural bentonite with an anionic polymer and the results were compared with two amended clay materials (multi-swellable bentonite (MSB) and a dense prehydrated GCL (DPH GCL)). To demonstrate the potential benefits of polymer treatment, material characterisation through X-ray diffraction analysis, density of solid particles, Atterberg limits, and swelling tests was performed on treated and untreated samples. Subsequently, hydraulic conductivity and chemico-osmotic tests were performed with CaCl₂ solutions on treated and untreated clays, to evaluate the modified clays resistance to chemical attack. The results of this research showed that the present amendment technology has a great potential for future GCL applications. X-ray diffraction analysis demonstrated the intercalation of the polymer in the interlayer region of the clay inducing a dispersed clay structure. The swell index and the liquid limit of the clay increased with increasing polymer dosage suggesting a potential benefit of the polymer on preserving the hydraulic performance of the clay. Unlike the untreated clay, HYPER clay treatment maintained low hydraulic conductivity of the clay to CaCl₂ even in the long term and protected the clay against the destructive role of diffusion, maintaining the initial osmotic efficiency in the long term. Test results were also compared with other amended clays MSB and DPH GCL. These two amended clay materials displayed a chemico-osmotic behaviour at the steady state similar to that observed on untreated clay. On the other hand, the preservation of the chemico-osmotic efficiency of the HYPER clay with time suggests that the carboxymethyl cellulose was not flushed out during the long period of permeation with deionised water.