Rheology and Microstructure of Aqueous Clay Suspensions

The clay minerals kaolinite and montmorillonite are ubiquitous and abundant and have therefore been the focus of research since the early 20th century when Eugene C. Bingham used aqueous kaolinite suspensions in his seminal work that introduced the concept of yield stress, and Irving Langmuir used aqueous montmorillonite suspensions in his seminal work that laid the foundation of Onsager’s theory of the isotropic–nematic liquid crystal transition. Since then, an enormous amount of work has been carried out on the microstructure and rheological behaviour of these two clay minerals. However, the true microstructure and associated rheological response remain poorly understood. The present work focuses on understanding the microstructure of kaolinite and montmorillonite suspensions and their associated rheological behaviour. The microstructure of kaolinite suspensions was investigated as a function of pH and concentration using scattering and rheology experiments. The microstructure at pH 10—far above the iso-electric point (IEP) of the alumina basal plane—showed the absence of aggregation up to a clay concentration of 55 wt.%. Increasing the concentration further led to aggregation to form entities larger than 20 µm in size. The microstructure at pH values below and above the edge IEP did not change except in terms of attraction strength, signifying that the silica-alumina basal plane interaction was the main mode of aggregation. For montmorillonite, scattering experiments revealed objects in the microstructure that were at least an order larger than individual particles, along with the presence of order. Cryo-electron microscopy complemented the scattering results, showing the presence of domains having strong particle-particle ordering as well as regions of particle-particle aggregation. Thus, the presence of an assembly of particles was confirmed, which refuted a widely accepted model for a purely repulsive nature. The presence of nematic domains refuted a widely accepted model for a purely attractive nature. Rheology experiments revealed a pH-driven aging bifurcation. Furthermore, low ionic strength suspensions at clay concentrations above the sol–gel transition were shown to exhibit features of physical aging and thixotropy, suggesting an out-of-equilibrium behaviour, even within the limit of low ionic strength.Ph.D