Adsorbed sodium fraction's effect on rheology of montmorillonite-kaolinite suspensions
1991
Keren, R.
This study was conducted to determine whether the adsorbed cation composition (Na-Ca) and pH affect the rheological behavior of montmorillonite-kaolinite suspensions. The rheological characteristics mixed kaolinite-montimorillonite suspensions were studied in aqueous medium at a total clay concentration of 50 g kg-1 and a temperature of 25 degrees 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. Conversely to kaolinite suspension, non-Newtonian rheology was observed when Na/Ca montmorillonite was introduced into the kaolinite suspensions at pH 4 or 7. At these two pHs, the deviation from Newtonian flow increased with the exchangeable sodium percentage (ESP) and it was greater at pH 4 than at pH 7. For the clay suspensions at pH 10, however, the deviation from Newtonian flow was much smaller for all ESP levels. When the pH increases well above the point of zero charge (PZC) of the pH-dependent charge surfaces, the association between kaolinite and montmorillonite platelets is negligible. The concept of electrostatic attractive forces between the positively charged surfaces and the negatively charged surfaces (the planar surfaces in montmorillonite), at pH values below the PZC, can be applied successfully to explain the influence of montmorillonite on the rheology behavior of mixed kaolinite and montmorillonite suspension. The increase in the extrapolated shear-stress values with increasing ESP suggests that the number of linkages between montmorillonite and kaolinite particles increases with the sodicity level, since the number of montmorillonite particles per unit weight of clay in suspension increases with the ESP.
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