Delamination of Na-montmorillonite particles in aqueous solutions and isopropanol under shear forces

Salt coagulation or flocculation? In situ zeta potential study on ion correlation and slime coating with the presence of clay: A case of coal slurry aggregation

The aggregation state of mineral slurry by coagulant with the presence of clay nanosheets appears similar to a flocculation gel with the absence of flocculant. The interactions between particles in mixed clay and quartz minerals systems are influenced by the interactions of ion correlation and slime coating, which creates zeta potential variation. Particle concentration has a substantial effect on zeta potential, and coal slurry has a relative high particle concentration. To realize an in situ aggregation study, zeta potential measurement of coal slurry was performed using electrokinetic sonic amplitude (ESA) without dilution at different calcium ion concentrations and pH values. The zeta potentials of three minerals commonly occurring in coal slurry (quartz, kaolinite and montmorillonite) with similar particle concentrations were also measured. The result suggests that aggregation of coal slurry by calcium ions in the presence of clay minerals belongs to fluctuation. The ion correlation and slime coating could be detected by ESA with a decrease in zeta potential under high calcium ion concentration conditions. The face-face coagulated clay nanosheets were formed by ion correlation, acting as a "flocculant chain". The adsorption force between the "flocculant" and particles is described as a slime coating. This flocculation process is referred to as "salt-clay-coagulation-flocculation" (SCCF). During ion correlation, rearrangement of the electric double layer between face-face spaces causes a reduction of the zeta potential. The super-fine negatively charged clay nanosheets might coat onto the quartz particle surfaces under the effect of electrostatic attraction force and Ca²⁺ ion correlation. Quartz and clay minerals were oppositely charged because the special adsorption of calcium ions on the clay basal face was restricted in hexatomic rings.

Modelling Hydration Swelling and Weakening of Montmorillonite Particles in Mudstone

It is of paramount importance to understand the hydration swelling and weakening properties of clay minerals, such as montmorillonite, to determine their mechanical responses during deep underground argillaceous engineering. In this study, the mineral components and microscopic structure of mudstone were characterised using X-ray powder diffraction and field-emission scanning electron microscopy. Experimental schemes were devised to determine the properties of mudstone under the influence of underground water and stress; these involved compacting montmorillonite particles with various water contents and conducting uniaxial compression tests. Experimental results demonstrated that compaction stress changes the microscopic structure of the montmorillonite matrix and affects its properties, and stress independency was found at particular water and stress conditions. Two equations were then obtained to describe the swelling and weakening properties of the montmorillonite matrix based on the discrete element method; further, the hydration swelling equation represents the linear decrease in the density of the montmorillonite matrix with an increase in the water content. It was also determined that the water dependency of uniaxial compressive strength can be described by negative quartic equations, and the uniaxial compressive strength of the montmorillonite matrix is just 0.04 MPa with a water content of 0.6. The experimental results are in good agreement with the calculated solutions and provide an important experimental basis to the understanding of the mechanical properties of montmorillonite-rich mudstones under the influence of underground water and stress.