Rheology of Suspensions and Pastes for Processing Applications

Concentrated colloidal suspensions are important components in many industrial and consumer applications but their complex flow behavior creates difficulties for processing. In particular, cementitious suspensions experience processing complications due to particle aggregation of the micron-scale particles. Suspensions of magnesium oxide powder are used to model cement pastes in order to understand the rheology and mitigate flow instabilities. Various approaches were undertaken to lower viscosity, reduce particle aggregation, and improve flow resistance with emphasis on the role of polymer superplasticizers on particle-particle and particle-polymer interactions. Particle jamming was found to be dependent on particle size distribution and superplasticizer concentration with shear start-up tests used to analyze the degree of flow hindrance. Particle networks and hydroclusters were hypothesized to form for narrow and wide particle size distributions, respectively, based on effective side-chain density and side-chain overlap. Furthermore, the impact of non-adsorbed polymers on superplasticizer-stabilized suspensions was examined in terms of depletion flocculation and depletion stabilization based on concentration. Flow curves demonstrated a viscosity increase at low concentrations (10-20% based on weight percent of adsorbed polymer) while viscosity increases were seen for depletion-aggregating high concentrations (30 – 50% based on weight percent of adsorbed polymer). Additionally, a new class of lignin-based polymer, lignopolymers, was studied for their viscosity-reducing potential as alternatives to the commercially available comb-polymer. Concentrations as low as 0.25 mg/mL of the polyacrylamide lignopolymer were shown to lower viscosity as much as the 2.7 mg/mL industrial comb-polymer. The findings from all of these projects may be applied to the construction industry to improve the mixing, pouring, and placement of cements and other concentrated suspensions.