The viscoelastic behavior of very concentrated and electrostatically stabilized suspensions of kaolinite particles has been investigated in the linear and nonlinear regime as a function of volume fraction, ionic strength and in the presence of polymer at various concentrations. Material properties such as linear viscoelastic moduli and cohesive energy density are extensively enhanced by either increasing volume fraction or decreasing ionic strength. Attention has been paid to the large amplitude oscillatory shear behavior of concentrated suspensions of plate-like particles, characterized by a hump in G '' curves. Rheological investigation shows the extreme sensitivity of the intensity of the strain hardening in G '' to excluded volume, electrostatic and steric interactions. A physical interpretation of this nonlinear behavior has been proposed. (c) 2007 The Society of Rheology.