Effect of pH on rheological properties of purified sodium bentonite suspensions

The effect of pH on the rheological behavior of three purified sodium bentonite (HP, GP, and WP) suspensions is studied. Each clay is studied as a function of pH, at a chosen constant concentration. These concentrations correspond 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 suspensions. The natural pH of these samples measured by a pH meter or by a colorimetric test paper are usually neutral or weakly basic. To obtain a large range 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 minimum before it increases sharply, at very high basic medium. When the pH decreases, the yield stress increases and reaches a maximum for pH in weakly acidic media before decreasing again, in a very highly acidic medium where the structure of the clay is probably attacked. The pH where the maximum appears 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 suspensions. The isoelectric points of the edges of the three studied purified sodium 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 deduced, depending on the pH value of the medium. At acidic medium, there is a dominance of the attractive forces between particles. At high basic medium, interparticle interactions lead to a card-house-like structure based on edge to edge, edge to face, and face to face repulsion, instead of attraction. (C) 1999 Academic Press.