On the rheological characteristics of GMZ bentonite suspension

Abstract

At the interface where the buffer bentonite contacts with the surrounding rock of the HLW disposal repository, the bentonite is likely to extrude into the fractures under swelling pressure and hydrate completely, which will as a result bring about looser bentonite suspension that might face the risk of being brought away easily by ground water flow. In this paper, the rheological characteristics of GMZ bentonite suspensions under different water–solid ratios and different types of saline solutions in different concentrations were studied through a rheology experiment using a “Brookfield DV-III Ultra” rheometer. The yield stresses of these GMZ bentonite suspensions were found from the corresponding rheological curves obtained from the rheology experiment. Then, the equation estimating the critical flow velocity was derived based on the Stokes law, and the critical flow velocities corresponding to these yield stresses were calculated. The results show that the rheological curve moves lower with an increase in water–solid ratio, when the water–solid ratio is below 5.0, and meanwhile, the suspension shows a characteristic of Newtonian fluids under lower shear stresses and a characteristic of nonlinear Bingham fluids under higher shear stresses; but when water–solid ratio goes beyond 5.0, the suspension shows a characteristic of pseudo-plastic fluids; when NaCl concentration increases in bentonite suspension, bentonite suspension’s yield stress decreases firstly and then increases, while bentonite suspension’s yield stress decreases when CaCl₂ concentration increases; the critical flow velocities range from 10⁻⁴ to 10⁻² m/s under the conditions of this paper.