Permeability and expansibility of natural bentonite MX-80 in distilled water

Abstract

Natural bentonite MX-80 differs from the purified and fully Na-exchanged bentonite in that it contains approximately 20.0% accessory minerals, in addition to the montmorillonite particles. Since the accessory minerals and montmorillonite particles have very different physical and chemical properties, natural bentonite MX-80 is found to expand much more slowly in distilled water, leading actually to a three-component system that has very different hydraulic properties from that of the fully Na-exchanged bentonite. To better understand and simulate the special features of expansion of natural bentonite MX-80 in distilled water, the focus is put primarily on the development of a Kozeny–Carman-like equation for its hydraulic permeability in the same way as it was done for Na-exchanged bentonite. With this permeability model, the dynamic force balance model that was originally developed for colloidal expansion of montmorillonite in a two-component system is applied to the natural MX-80 system. Without making any changes to the model, however, two strategies are used to account for both physical and chemical effects of the accessory minerals. The “lumped” strategy assumes that the accessory minerals are stuck onto the montmorillonite particles in such a way that they behave just like one solid component. The “stepwise” strategy changes the pore water chemistry gradually from initially distilled water to eventually achievement of the equilibrium condition. These strategies are simple but proved to function well. The agreement between the simulations and the experimental results indicates that the two-component dynamic force balance model works well in predicting the general features and the behavior of upward expansion of natural bentonite MX-80 in distilled water in a vertical test tube.