Influence of High Silica Fume and High Fly Ash Contents on Alkalinity of Pore Solution and Protection of Steel Against Corrosion

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Title: Influence of High Silica Fume and High Fly Ash Contents on Alkalinity of Pore Solution and Protection of Steel Against Corrosion

Author(s): U. Wiens, W. Breit, and P. Schiessl

Publication: Special Publication

Volume: 153

Issue:

Appears on pages(s): 741-762

Keywords: alkalinity; blast furnace slag; chlorides; corrosion; fly ash; high-strength concretes; portland cement; reinforcing steels; silica fume; Materials Research

Date: 6/1/1995

Abstract:
To assess the risk of corrosion due to high silica fume or fly ash content, hardened cement paste and concrete tests were performed at the Institute for Building Materials Research at the Aachen University of Technology to determine the influence of these concrete admixtures on the alkalinity of the pore solution and on chloride-induced corrosion of the reinforcing steel in the concrete. The fly ash content in the tests was up to 60 percent by mass and the silica fume content up to 25 percent by mass of total binder content. The mixtures were made up with a portland cement and a portland blast furnace slag cement (50 percent by mass blast furnace slag) at varying water-binder ratio. A combination of 45 percent by mass portland cement, 15 percent by mass silica fume, and 40 percent by mass fly ash was also included in the program. Reducing the portland cement clinker content in mixtures with high silica fume contents by the use of blast furnace slag or by the substitution of high amounts of fly ash leads to a rapid exhaustion of calcium hydroxide. Substantial quantities of alkalies are bound to reaction products, resulting in a dramatic drop of pH value in pore solution (below pH = 12.0) and, hence, increasing the risk of depassivation of the steel surface. The reduced alkalinity must be weighed against a significant refinement of pore structure through the rapid pozzolanic reaction of silica fume, clearly increasing the electrolytic resistance of concrete and reducing the corrosion rates to possibly negligible values.