Chemically active saturated clays containing several cations are considered in a two-phase framework. The solid phase contains the negatively charged clay particles, absorbed water and ions. The fluid phase contains free water and ions. Electroneutrality is ensured in both phases. Water and ions can transfer between the two phases. Emphasis is laid on the electro-chemo-mechanical constitutive equations in an elastic-plastic setting. Elastic chemo-mechanical coupling is introduced through a potential, in such a way that the tangent elastic stiffness is symmetric. The elastic-plastic behaviour aims at reproducing typical experimental phenomena observed on natural clays during chemical and mixed chemo-mechanical loadings, including chemical consolidation and swelling already described in the simpler context of Na-Montmorillonite clays. Ionic replacements, e.g. Na{sup}+ by K{sup}+, is accompanied with important, reversible volume changes, in agreement with experimental data.
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