Effects of pH on the gel properties of montmorillonite, palygorskite and montmorillonite-palygorskite composite clay

The effects of pH on the structures, compositions, morphologies, Zeta potential, and gel properties of montmorillonite, palygorskite and their composite clays were investigated in the study. With the increase in pH, the above properties of montmorillonite, palygorskite and their composite clays showed different changing trends. The compositions of montmorillonite and palygorskite basically remained unchanged in the pH range of 1-13. The dissociation and dispersion degrees of montmorillonite sheets, the separation and dispersion degrees of palygorskite fiber cluster and the absolute value of Zeta potential increased. The viscosity of palygorskite sample firstly increased and then decreased. The viscosity of montmorillonite and composite clay samples increased and the increase was rapid especially at pH above 12. The viscosity of montmorillonite and composite clay samples reached 9151 mPa.s at pH 13, but the viscosity of composite clay sample was lower than that of montmorillonite at the same pH value. The influencing factors of viscosity mainly included the dissociation degree of montmorillonite sheets, the separation degree of palygorskite fibers and the Zeta potential. The higher degrees of dissociation and dispersion corresponded to the higher viscosity. With the increase in the absolute value of Zeta potential, the viscosity of montmorillonite and composite samples increased, but the viscosity of palygorskite increased firstly and then decreased. The potential mismatch between palygorskite and montmorillonite was responsible for the lower actual viscosity of the composite sample compared to the calculated viscosity.