Effect of pH on rheological properties of purified sodium bentonite suspensions

The effect of pH on the rheological behavior of three purified sodium bento nite (HP, GP, and WP) suspensions is studied. Each clay is studied as a fun ction of pH, at a chosen constant concentration. These concentrations corre spond to the lower concentrations at which the yield stress can be detected . They are 12.5, 8.5, and 5.5% w/w, respectively, for the three clay suspen sions. The natural pH of these samples measured by a pH meter or by a color imetric test paper are usually neutral or weakly basic. To obtain a large r ange of pH values, HCl or NaOH solutions are used in the preparation of the suspensions. From the natural pH and when pH values become more basic, the yield stress measured, using the vane method, decreases. It reaches a mini mum before it increases sharply, at very high basic medium. When the pH dec reases, the yield stress increases and reaches a maximum for pH in weakly a cidic media before decreasing again, in a very highly acidic medium where t he structure of the clay is probably attacked. The pH where the maximum app ears depends on the nature of the clay. At the same time, the conductivity and the electrophoretic mobility are measured, at the different pH values, on the supernatants obtained after centrifugation of the previous suspensio ns. The isoelectric points of the edges of the three studied purified sodiu m bentonites, which are of a great importance for the understanding of the interactions between the clay particles, have been determined. In summary, a correlation between rheological and electrokinetic results, is shown. And moreover, different types of interactions between clay particles are deduc ed, depending on the pH value of the medium. At acidic medium, there is a d ominance of the attractive forces between particles. At high basic medium, interparticle interactions lead to a card-house-like structure based on edg e to edge, edge to face, and face to face repulsion, instead of attraction. (C) 1999 Academic Press.