Water-Reducing Properties of Fly Ash in Cement Pastes, Mortars, and Concretes: Causes and Test Methods

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Title: Water-Reducing Properties of Fly Ash in Cement Pastes, Mortars, and Concretes: Causes and Test Methods

Author(s): R. A. Helmuth

Publication: Special Publication

Volume: 91

Issue:

Appears on pages(s): 723-740

Keywords: cement pastes; concretes; fly ash; mortars (material); portland cements; tests; water content; water-reducing admixtures.

Date: 2/1/1986

Abstract:
Specifications for fly ashes should be based on a proper understanding of the factors controlling, and test methods for, water requirements for flow. The reduction of the water requirement for flow of concretes which results from partial replacement of portland cements by certain fly ashes is commonly attri-buted to the spherical shape of many of the fly ash particles. Critical re-examination of the literature does not support that conclusion. An alternative hypothesis is proposed: the water reduction is a result of adsorption of very fine fly ash particles on portions of the cement particle surfaces, with result-ing dispersion of the cement particles, similar to the action of organic water-reducing admixtures. This hypothesis is consistent with published data on fly ash-cement-admixture interactions. This effect should be separated in the test methods and specifications from that of the amount of fly ash coarser than 45 um, but is not in the present ASTM C 311 test for water requirement.