Progress in Understanding the Functional Properties of Superplasticizers in Fresh Concrete


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Title: Progress in Understanding the Functional Properties of Superplasticizers in Fresh Concrete

Author(s): C. Jolicoeur, P. C. Nkinamubanzi, M. A. Simard, and M. Piotte

Publication: Special Publication

Volume: 148


Appears on pages(s): 63-88

Keywords: adsorption; calorimeters; cement pastes; cements; heat of hydration; mineral admixtures; rheological properties; slurries; superplasticizers; Materials Research

Date: 9/1/1994

A variety of new literature and data on the properties of cement pastes and concentrated slurries of various types of mineral particles are examined to elucidate the origin of the fluidification of cement pastes by superplasticizers. The influence of sodium poly-¯-naphthalene sulfonate superplasticizers (NaPNS) of different molecular weights on the rheological properties of pastes and on the early heat of hydration of cement, together with results from other physicochemical measurements (adsorption, zeta potential), suggests that the unique fluidification effect of these admixtures depends on at least four distinct phenomena. With reference to fluidification of slurries of "inert" minerals, the superplasticizer effect in concrete can be understood in terms of nonspecific (physical) and specific (chemical) effects. The "physical" effects comprise: adsorption of the superplasticizer molecules by van der Waals and electrostatic forces (direct or assisted by cations); surface charging that induces long-range interparticle repulsive forces; steric hindrance between adsorbed polymer molecules on neighboring particles, leading to added short-range repulsive forces. The "chemical" effect involves a reaction of the PNS superplasticizer molecules with the most reactive sites of cement particles (particularly C 3A), substantially reducing the initial surface hydration rate. This description is largely based on data relevant to PNS-type superplasticizers, but with proper allowance for specific chemical effects, it should also be valid for other types of superplasticizer.