This paper presents an extensive study of a variable model clayey material, made of mixtures (in different proportions) of two mineralogically different clays. The basic clays are montmorillonite (Greek clay) characterised by a high plasticity, and kaolin P300, which is a more stable clay with low plasticity. The results show a significant variation in the mechanical properties of the mixtures, especially from 35% of montmorillonite. The behaviour exhibits an increase in strength of the material on the isotropic (and also the oedometric) path, and a marked decrease in the internal friction angle down to that of the montmorillonite. Microstructural investigation provided an explanation of these phenomena through a mechanism activated at the scale of particles.