594921 Dispersing Nano- and Micro-Sized Portlandite Particulates Via Electrosteric Exclusion at Short Screening Lengths

Friday, November 20, 2020
Particle Technology Forum (03) (PreRecorded+)
Sharu Bhagavathi Kandy1, Jason Timmons2, Iman Mehdipour3, Shang Gao4, Hakan Atahan5, Narayanan Neithalath6, Mathieu Bauchy7, Edward Garboczi8, Samanvaya Srivastava4 and Gaurav Sant9, (1)University of California, Los Angeles, Los Angeles, CA, (2)Materials Science and Engineering, University of California Los Angeles, Los Angeles, CA, (3)Civil & Environmental Engineering, Laboratory for the Chemistry of Construction Materials (LC2), UCLA, Los Angeles, CA, (4)Department of Chemical and Biomolecular Engineering, University of California, Los Angeles, Los Angeles, CA, (5)Department of Civil and Environmental Engineering, Laboratory for the Chemistry of Construction Materials, Los Angeles, CA, (6)School of Sustainable Engineering and the Built Environment, Arizona State University, Tempe, AZ, (7)Civil and Environmental Engineering, Laboratory for the Physics of Amorphous and Inorganic Solids, Los Angeles, CA, (8)Applied Chemicals and Materials Division, Material Measurement Laboratory, National Institute of Standards and Technology, Boulder, CO, (9)California NanoSystems Institute, University of California, Los Angeles, Los Angeles, CA

In spite of their high surface charge (zeta potential ζ = + 34 mV), aqueous suspensions of portlandite (calcium hydroxide: Ca(OH)2) exhibit a strong tendency to aggregate, and thereby present unstable suspensions. While a variety of commercial dispersants seek to modify the suspension stability and rheology (e.g., yield stress, viscosity), it remains unclear how the performance of electrostatically and/or electrosterically based additives is affected in aqueous environments having either a high ionic strength and/or a pH close to the particle’s isoelectric point (IEP). We show that the high native ionic strength (pH ≈ 12.6, IEP: pH ≈ 13) of saturated portlandite suspensions strongly screens electrostatic forces (Debye length: κ-1 = 1.2 nm). As a result, Coulombic repulsion alone is insufficient to mitigate particle aggregation and affect rheology. However, a longer-range geometrical particle-particle exclusion that arises from electrosteric hindrance caused by the introduction of comb polyelectrolyte dispersants is very effective at altering the rheological properties and fractal structuring of suspensions. As a result, comb-like dispersants that stretch into the solvent reduce the suspension’s yield stress by 5x at similar levels of adsorption as compared to linear dispersants, thus enhancing the critical solid loading (i.e., at which jamming occurs) by 1.4x. Significantly, the behavior of diverse dispersants is found to be inherently related to the thickness of the adsorbed polymer layer on particle surfaces. These outcomes inform the design of dispersants for concentrated suspensions that present strong charge screening behavior.

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