Microstructure and time-dependent behavior of composite NaMnt-kaolin gels: Rigid - flexible platelet interactions and configurations

Composite NaMnt (SWy-2)-kaolin (KGa-2) gels at more than 10 wt% solids with the NaMnt fraction ≥ 20% formed an open cellular microstructure. The flexible, nanosized NaMnt platelets formed the continuous structure with kaolin particles dispersed within it bonding with the NaMnt platelets. The morphology adopted by the single layer NaMnt platelets is varied and complex producing a wide range of unique particle interaction configurations. Many acquired a curved shape surface with curled edges. Some of the platelets developed elongated string-like features made by the curling of edges and may have 3–5 of these strings for bonding. Kaolin particles are rigid and multilayered, and most have a platelet morphology. A common NaMnt-kaolin bonding configuration observed was the face at the curled-edge of NaMnt platelet resting on the face of the kaolin particle. A 15.5 wt% gel with a 1:2 mass ratio of NaMnt to kaolin was demonstrated to display both thixotropic and rheopectic behavior. A stepdown shear rate method was used to evaluate the structural development kinetics at 10 s⁻¹ of this gel commencing with the structural state sheared to equilibrium at 1000 s⁻¹. The Leong model described this kinetics well and found that the structural development characteristic time to be very fast, in seconds. The ageing behavior was also evaluated without the imposition of a constant shear. The yield stress increase continued for weeks and months for the 2:1 kaolin to NaMnt composite gels at 15.5 and 10.7 wt% solids. The characteristic time of the structural development process was much longer, in days. Upon reducing the NaMnt content from 33.3% to 20%, the 10.8% wt% gel became weakly time-dependent with a marginal yield stress. A continuous structure cannot be formed immediately upon the cessation of shear due to an inadequate NaMnt platelet concentration.