Ageing and collapse of Bentonite gels — Effects of Mg(II), Ca(II) and Ba(II) ions

• Bentonite gels became unstable and collapse at > 0.02 M Mg2 +, Ca2 + and Ba2 +.
• Gel instability causing phase separation is undesirable as a nuclear storage barrier.
• Slurry wall performance for isolating contaminated water will also be affected.
• Stable gel displayed pronounced structural recovery or ageing behaviour.
• Hydrolysis products of metal ions affect the solid consolidation of the sediment.

The ageing and stability behaviour of Bentonite slurries were evaluated under the influence of Mg2 +, Ca2 + and Ba2 + ions at concentration ranging from 0.005 to 1 M. Stable gels were formed at low metal ion concentration of < 0.05 M. These gels displayed pronounced ageing or structural recovery behaviour. A well-defined initial state of ageing was employed. This was the surface-chemical equilibrium (SCE) state. The dissolved metal ion salt hastened the attainment of this SCE state of freshly prepared gel to less than 100 min. As the structure recovered during ageing, the yield stress increased. The temporal ageing yield stress displayed an initial period of rapid increase, followed by a period of gradual increase and then a period of no increase. The Leong model described this ageing behaviour well. At metal ion concentration of 0.05 M or greater, the gels became unstable and collapsed. In contrast, gel instability occurred at a much higher concentration of 0.5 M for Na+, K+ and Cs+ ions. Also the settling layer of divalent metal ion weakened gels did not consolidate as effectively. The stability time of these weakened gels was found to decrease with increasing hydration bond length of the divalent metal ions, i.e. Ba(II) < Ca(II) < Mg(II).

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