Keren, R., Inst. of Soils & Water, ARO, The Volcani Ctr., Bet Dagan, Israel.

The rheological characteristics of mixed kaolinite-montmorillonite suspensions as a function of adsorbed ion (Na+ or Ca2+), montmorillonite concentration and suspension pH were studied in aqueous medium at a total clay concentration of 50 g kg-1 and 25°C. The suspensions of kaolinite showed a Newtonian rheology regardless of the adsorbed ion and pH, indicating that neither structural breakdown nor orientation occurs during the flow. Non-Newtonian rheology was observed when small amounts of montmorillonite (saturated by Na+ or Ca2+ ions) were introduced into the kaolinite suspensions at pH values of 4.0 and 7.0. An association between kaolinite and montmorillonite platelets appears to take place, with the intensity (as inferred from the extrapolated shear stress and viscosity) higher at pH 4.0 than at pH 7.0. This association is probably due to the attraction between the positive charge on the pH-dependent charge surfaces of kaolinite and the negative charge associated with the planar surfaces of montmorillonite. When the pH increases well above the point of zero charge of the pH-dependent charge surfaces, the association between kaolinite and montmorillonite platelets is negligible. -Author