Search

The objectivesof this investigation were to measure the effects of moderate heat treatments, below the dehydroxylation temperature, on physical and chemical properties of a calcium-montmorillonite clay. Previous workers have noted the reductions in cation exchange capacity and swelling property after heating in the range 200°–400°C, and have suggested several possible explanations, such as hysteresis effect, increased interlayer attractions owing to removal of interlayer water, or changes in the disposition of interlayer or layer-surface ions. The liquid limits of Ca-montmorillonite steadily decreased with increased treatment temperature, until they reached a constant level at 450° to 500°C. The plastic limit decreased slightly up to 350°C. Above this temperature samples could no longer be rolled into threads. The gradual change is in contrast with sudden major changes noted for weight loss (maximum rates of change at 100°C and 500°C), glycol retention surface area (maximum change at 520°C), and changes of peak intensity and breadth character after glycolation of the first diffraction maximum (530°C). Other properties showing more gradual reductions with heat treatment were: cation exchange capacity by NH4Ac method, d001intensity (21 Å spacing) after storing at 100 per cent r.h. one month and re-wetting with water, and amount of calcium exchangeable without water soaking. Previous water soaking allowed much greater release of fixed Ca++up to 450°C. Similar results were obtained with cation exchange capacities when samples were treated with In CaCl2solution. The 21.0 Å peak intensity curve showed close similarity to the liquid limit and plastic index curves in the low temperature range, and an explanation is suggested.

Zeta-potentials of Na- and Ca-bentonites treated with varying amounts of Ca(OH)2were measured by a Northrop-Kunitz horizontal cell and non-polarizing Zn-ZnSO4electrodes. 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)2is 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)2in 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), 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.