Kaolinite clays from two sources were found to display different rheological and electrokinetic behaviour despite having similar mineral composition. This difference is mainly due to the surface chemistry at the edges of the clay particles. Making the edge surface chemistry similar with an adsorbed small anionic additive such as citrate, produced a similar yield stress-pH behaviour for the different kaolin clay slurries. Heterogeneous attraction between positively charged edges and negatively charged faces of the clay particles is responsible for the yield stress behaviour of the clay slurry. However, the different points of zero charge of the edge caused the clay slurries to display different yield stress-pH behaviour. Adsorption of anionic citrate on negatively charged kaolin clay particles is reflected in a significant increase in the magnitude of the negative zeta potential. The adsorption is likely to be on the edge of the clay particles. Adsorbed citrate generally reduces or eliminates heterogeneous charge attraction leading to a much denser sediment being formed in the kaolin slurries with a coarser particle size distribution. (C) 2009 Elsevier Ltd. All rights reserved.