Rheological and zeta potential behaviour of kaolin and bentonite composite slurries

•Kaolin determines the location of the pH of maximum yield stress.•Bentonite content determines the magnitude of the yield stress.•The τymax (mixed clay)/τymax (kaolin) is the same for both low and high Ca(II) kaolins.•The yield stress–DLVO model is obeyed by low Ca(II) clay.•This model is not obeyed by high Ca(II) clays.

The results of the yield stress–pH and zeta potential–pH behaviour of pure kaolin and bentonite slurries, and mixed bentonite–kaolin slurries were evaluated. A high and low Ca(II) content kaolin clays were used. The location of the pH of maximum yield stress of the two pure kaolin slurries is very different, pH 9 for the high Ca(II) kaolin and 2.5 for the low. The trend of the variation of the yield stress with pH for the bentonite–kaolin composite slurries was similar to that of pure high Ca(II) kaolin irrespective of bentonite content. For the low Ca(II) kaolin, the maximum yield stress of the composite suspensions is located at the same low pH as that of pure low Ca(II) kaolin and bentonite suspensions, at pH ∼ 2.5. Bentonite content was found to determine the magnitude of the yield stress. The sequence of KOH addition, whether during or after the preparation stage, did have a significant effect on the yield stress but only for the composite slurries with a high bentonite content. A higher degree of swelling and delamination of the bentonite particles in one of the preparation methods was attributed as the cause. Zeta potential results showed that both the pure and mixed clays are negatively charged at all pH. However the magnitude is reduced by Ca (II). Low Ca(II) kaolin suspension was found to obey the yield stress–DLVO model. This study has provided an excellent approach of controlling and tailoring rheological properties of kaolin suspension with bentonite or vice versa for specific applications.

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