Yield stress of water-bentonite dispersions

Yield stress of aqueous bentonite dispersions was determined at two concentrations, with two bentonites, over a range of pH values, with the vane technique and by extrapolation of the full rheograms, derived with concentric cylinder viscometer, fitted to Herschel-Bulkley and to Casson models. All samples exhibited a yield stress and gave very similar yield stress values determined by the three techniques and hence, any of the techniques can be used for measurement of the yield stress. Data extrapolation using either the Herschel-Bulkley or the Casson model would be favoured, though, because it gives, in addition to the yield stress, the theological model parameters. The close matching observed for all three techniques is attributed to preparation and intensive preshearing procedures, similar to ones experienced by fluids in flow situations. pH of dispersions affected their yield stress but the effect was different for the two bentonites and the two concentrations tested. Measurement time at each rotational speed should be kept at a minimum of 60s. Bentonite dispersions build continuously structure over time and the yield stress evolution with time could be well described by power law. A model to predict yield stress, previously suggested for suspensions at the isoelectric point, could be a good starting point for yield stress prediction of bentonite dispersions. (C) 2008 Elsevier B.V. All rights reserved.