Abstract: Zeta-potentials of Na- and Ca-bentonites treated with varying amounts of Ca(OH)2 were measured by a Northrop-Kunitz horizontal cell and non-polarizing Zn-ZnSO4 electrodes. Influences of other factors such as particle size, clay concentration, and salt concentrations were also studied.
The results indicate a four-stage change in zeta-potential and viscosity when Ca(OH)2 is added in increasing amounts to Na-bentonite suspensions:
In the first stage, the addition of small amounts of lime caused no change in zeta-potential suggesting a counteracting effect of Ca++ adsorbtion and reaction of OH− ions to increase the negative surface charge with only a slight change in viscosity. In the second stage, the addition of more lime resulted in a rapid decrease in zeta-potential accompanied by a rapid increase in viscosity and appearance of flocs. In the third stage, additional lime led to a slower decrease in zeta-potential but a continued rapid increase in viscosity and the formation of distinct large flocs. Apparently in all three stages the plentiful OH− ions caused dissociation of weakly acidic groups to provide additional negative sites for linking the clay particles by Ca++ ions. The dissociation of weakly acidic groups probably reached its full capacity by the end of the third stage, termed the “lime retention point”. In the fourth stage, the additional Ca(OH)2 in excess of the lime retention point caused a slight change in zeta-potential and a slight decrease in viscosity. In this stage free lime was available for slow pozzolanic reaction.
In the Ca-bentonite-Ca(OH)2 system, the first stage was an increase in zeta-potential due to dominant influence of OH− potential-determining ions. The second stage was a rapid decrease in zeta-potential and increase in viscosity, corresponding to the third stage in treatment in the Na+-clay. The end of the second stage is the lime retention point, after which excess lime is used for pozzolanic reaction.