Mica slurries were found to be flocculated over the whole pH range of between 2 and 12. These slurries of different solid loading appeared to obey the yield stress-DLVO force model producing a single value for the deflocculation point zeta potential or the critical zeta potential (at point of flocculated-disperse state transition) of similar to 48 mV. Such a high value indicates the presence of an additional attractive force in addition to the van der Waals force. This force is attributed to edge-face unlike charge attraction. The use of pyrophosphate additive to minimize edge-face interactions via positive edge charge neutralization caused a large reduction in the critical zeta potential, to 22 my. With this new value, the Hamaker constant of mica in water was determined to be similar to 13 zJ. This falls within the range of values reported for mica. Low Ca(II) kaolin suspension displayed a lower critical zeta potential of 41 mV and the use of pyrophosphate additive at 0.2 and 0.4 dwb% appeared not to decrease the magnitude of the critical zeta potential. This pyrophosphate-influence critical zeta potential is also independent of Ca(II) content in the kaolin. The established model of edge-face attraction for kaolin suspension at low pH may not be completely correct. A face-face interaction model may be more appropriate. A power law model relating the maximum yield stress with volume fraction showed a very high exponent value of 8 for mica slurries, only 3.1 for the low Ca(II) kaolin and 3.6-3.9 for the high Ca(II) kaolin suspensions. This exponent value may reflect the predominant particle-particle interaction configuration in the clay suspensions. (C) 2014 Elsevier B.V. All rights reserved.