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Pore fluid composition strongly influences the mechanical behavior of clays, impacting both on their volumetric and shear response. Accounting for this aspect is crucial for engineering applications where changes of the chemical composition of the pore fluid are anticipated, such as transport through engineered barriers for the containment of pollutants, or slope stability of natural formations rich of clay minerals subjected to freshwater infiltration. In this work, a chemo-mechanical model capable of reproducing the response of medium to low activity clays under both mechanical and chemical loading paths is presented. The model is developed starting from the interpretation of experimental evidences in an elastic-plastic framework. Chemo-mechanical coupling is introduced both in terms of stress variables and hardening law. In particular, the formulation is specialized to variations of salt concentration, introducing osmotic suction as a chemical stress variable. The model was implemented in a constitutive driver for the integration at the REV level of the incremental constitutive equations, thus allowing for its validation against literature data.