Effect of Clay Charge Density and Adsorbed Ions on the Rheology of Montmorillonite Suspension

AbstractThe rheological characteristics of montmorillonite suspension as a function of exchangeable sodium percentage (ESP), (when the complementary cation is either Ca2+ or Mg2+) and specific electrostatic charge density were studied in aqueous media at a clay concentration of 25 g kg−1, at pH 7 and temperature of 25 °C. The suspension showed a Newtonian rheology when the clay was saturated by either Ca2+ or Mg2+, but the differential viscosity (the derivative of shear stress with respect to shear rate at a given shear rate) of Mg‐clay (2.3 mPa s) was higher than that of Ca‐clay (1.5 mPa s). The higher value for the Mg‐clay suspension is due to the smaller average number of platelets in a tactoid of Mg‐montmorillonite than in the Ca‐montmorillonite suspensions. Introducing Na+ at a low percentage (10%) into the exchange complex of Mg tactoid was enough to break down some of the tactoids to smaller ones, whereas the Ca tactoids remained stable. The differential viscosity at high shear stresses increases with increases in ESP, showing higher values for Na/Mg systems at a low ESP range. At higher ESPs, no difference in differential viscosity between Ca and Mg systems was observed. The higher extrapolated shear stress values for Na/Mg‐montmorillonite suspensions suggest that the number of linkages between clay particles per unit weight of clay is higher than that found in Na/Ca‐montmorillonite for the same ESP and clay concentrations. The differential viscosity and the extrapolated shear stress values for the various smectites (the electric charge density varied between 0.91 and 1.22 molc kg−1) suggest that the extent of particle‐particle interaction in clay suspension decreases with increases in electrostatic charge density of smectites.