Formation of Arrested States in Natural Di- and Trioctahedral Smectite Dispersions Compared to Those in Synthetic Hectorite — A Macro- and Microrheological Study

The effect of natural clay-mineral properties on the rheological behavior of dispersion is very important in new geotechnical and industrial applications. The colloidal behavior of natural clay minerals with various octahedral structures was investigated using macro- and microrheological measurements and compared with the behavior of synthetic hectorite. In the present study montmorillonite (dioctahedral smectite of Volclay), natural hectorite (trioctahedral smectite of SHCa-1 Source Clay), and the synthetic trioctahedral smectite Laponite®, with lateral layer dimensions of 277, 100, and 30 nm, respectively, were used. The structure formation, kinetics of aging, and broad bandwidth viscoelastic response (10-2 — 106 rad/s) of their dispersions were obtained using mechanical shear and squeeze flow rheometry combined with diffusing wave spectroscopy (DWS) and multiple particle tracking (MPT) microrheology. State diagrams were determined at inherent pH considering the clay-mineral and NaCl concentrations as well as the kinetics of structure formation and sample aging. Due to the larger mean layer diameter and greater layer-charge density of natural clay-minerals, their sol—gel transitions occurred at higher solid and NaCl concentrations than those of Laponite®. Structure formation was faster at pH < pHPZC,edge than at pH > pHPZC,edge (point of zero charge at the edge). The long-term aging of natural clay-mineral samples was less pronounced in the glass state than in the gel state, in contrast to the findings for Laponite®. The storage modulus, G’, of clay-mineral dispersions in arrested states remained essentially constant in a wide frequency range (up to 100 rad/s), as expected. The corresponding plateau value of G’ depends on the number of particle contacts per volume and, hence, increased with decreasing particle size at a given concentration. The dissipation mechanisms determining the high-frequency loss modulus, G", however, are independent of particle size and, accordingly, the high-frequency crossover of G’ and G" shifted to higher values when the particle size decreased. The MPT data revealed structural refinement on the submicrometer length scale during the aging of weak hectorite gels, which was similar to the results for Laponite®. No refinement, however, occurred for montmorillonite in the glass or strong gel state.