(326h) Temperature-Induced Aggregation and Rheological Response of Concentrated Portlandite Suspensions

Temperature is well known to affect the aggregation behavior of colloidal suspensions. While processing concentrated cementitious suspensions and other particulate mineral suspensions in highly alkaline media, which are subjected to varying processing temperatures, the temperature-induced modification in the suspension rheology can be crucial and can impact processability. This study provides insights into aggregation processes that affect the temperature-dependent rheology of suspensions that feature strong charge screening behavior. We elaborate the mechanisms that control the temperature-dependent rheological behavior of portlandite (calcium hydroxide: Ca(OH)₂) suspensions that feature high ionic strength and strong charge screening behaviors. Unlike the saturated Ca(OH)₂ solution suspending medium, the viscosity of dense portlandite suspensions increases with increasing temperature, featuring an anomalous “negative” effective activation energy for viscous flow. This behavior is shown to arise from the temperature-induced aggregation of portlandite particulates caused by the diminution of electrostatic repulsive forces with increasing temperature. Increasing temperature weakens the electrostatic repulsion, leading to faster aggregation and the formation of larger aggregates, which manifests itself in the unusual rheological response of these portlandite suspensions with increasing temperature. The temperature dependence of the suspension viscosity is further found to diminish with increasing particle volume fraction as a result of volumetric crowding and the formation of denser fractal structures in the suspension. The temperature-dependent rheological response of the suspensions is found to be strongly affected by the suspending medium’s properties, including ionic strength and ion valence, which affect aggregation kinetics. Further, in comb polyelectrolyte dispersant-based electro-sterically stabilized portlandite suspensions, the dispersant concentration is found to strongly influence the temperature-induced aggregation kinetics. The optimum dispersant dosage to mitigate the temperature-induced aggregation varies with the temperature. Insights from the present study offer guidance toward the design of suspensions with tailored temperature-dependent and temperature-responsive rheological responses.