Dispersing nano- and micro-sized portlandite particulates via electrosteric exclusion at short screening lengths

Jason Timmons; Iman Mehdipour; Shang Gao; Hakan Atahan; Narayanan Neithalath; Mathieu Bauchy; Edward Garboczi; Samanvaya Srivastava; Gaurav Sant

doi:10.1039/D0SM00045K

Dispersing nano- and micro-sized portlandite particulates via electrosteric exclusion at short screening lengths†

Jason Timmons,‡^ab Iman Mehdipour,

‡^b Shang Gao,^c Hakan Atahan,^bd Narayanan Neithalath,^e Mathieu Bauchy,^fg Edward Garboczi,^h Samanvaya Srivastava

*^cg and Gaurav Sant*^abgi

Author affiliations

* Corresponding authors

^a Department of Materials Science and Engineering, University of California, Los Angeles, CA 90095, USA
E-mail: gsant@ucla.edu

^b Laboratory for the Chemistry of Construction Materials (LC2), Department of Civil and Environmental Engineering, University of California, Los Angeles, CA 90095, USA

^c Department of Chemical and Biomolecular Engineering, University of California, Los Angeles, CA 90095, USA
E-mail: samsri@ucla.edu

^d Department of Civil Engineering, Istanbul Technical University, Istanbul, Turkey

^e School of Sustainable Engineering and the Built Environment, Arizona State University, Tempe, AZ 86587, USA

^f Laboratory for the Physics of Amorphous and Inorganic Solids (PARISlab), Department of Civil and Environmental Engineering, University of California, Los Angeles, CA 90095, USA

^g Institute for Carbon Management, University of California, Los Angeles, CA 90095, USA

^h Applied Chemicals and Materials Division, Material Measurement Laboratory, National Institute of Standards and Technology, Boulder, CO 80305, USA

ⁱ California Nanosystems Institute (CNSI), University of California, Los Angeles, CA 90095, USA

Abstract

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 5× at similar levels of adsorption as compared to linear dispersants, thus enhancing the critical solid loading (i.e., at which jamming occurs) by 1.4×. 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.

Soft Matter

Dispersing nano- and micro-sized portlandite particulates via electrosteric exclusion at short screening lengths†

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