FRACTAL MODEL FOR YIELD STRESS OF BENTONITE COLLOIDS

The performance of the bentonite buffer in nuclear waste repository concept relies to a great extent on the buffer surrounding the canister having sufficient dry density. Loss of buffer material caused by erosion remains as the most significant process reducing the density of the buffer. Yield stress of bentonite colloids is a key parameter to express the erosion process of bentonite aqueous solution. A model for the yield stress of aggregates is presented. It incorporates fractal dimension taking into account the solid volume fraction and the aggregate diameter. The model shows that the yield stress(σ_y) of aggregates increases with the solid volume fraction (φ_s) as a power law, and is given by σ_y=σ_y0φ_s^m, where the exponent(m) is related to fractal dimension(D), and σ_y0 is a referenced parameter. The relationship between exponent(m) and fractal dimension is validated by published data of aggregates and represents the measured data very well, over a wide range of the solid volume fractions. The referenced parameter(σ_y0) is calibrated from experiments of yield stress using power law fittings. The agreement between theory and experiments supports the idea that yielding is ultimately caused by the rupture of a few interparticle bonds within aggregates.