The Permeability of Chloride Ions in Fly Ash-Cement Pastes, Mortars and Concrete
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THE PERMEABILITY OF CHLORIDE IONS IN FLY ASH-CEMENT PASTES, MORTARS AND CONCRETE R.I.A. MALEK and D.M. ROY Materials Research Laboratory, The Pennsylvania State University, University Park, PA 16802 Received 20 November, 1987, refereed ABSTRACT Fly ash-cement pastes are known to develop fine pore structures that may retard the transport of ionic species. The rapid chloride permeability technique for studying the Cl- ion diffusion in hydrated fly ash/cement pastes, mortars and concrete was used. The technique applies an electrical potential across a cylindrical sample and measures the charge passed in a certain period of time. The results obtained on pastes and mortars cured for 28 days were reported previously and contrasted with those of neat cement pastes and mortars. The present paper reports more extensive studies made to examine the chloride permeabilities of pastes and mortars cured for up to 90 days. In addition, the effect of variable fly ash contents was examined. Concrete samples were included in the test scheme and the data were compared with pastes and mortars. Two important factors controlling the test results are discussed: first the mix design and curing conditions; second the experimental conditions during C1- permeability measurements. In the second factor, the amount and rate of heat build up and the chloride ion concentrations are compared with the current passed. In addition, measuring current versus resistivity are critically discussed in terms of the voltage-current varistic characteristics of cement matrices. INTRODUCTION The high alkalinity of concrete pore fluids [1-4] provides the reinforcing steel with adequate corrosion resistance by forming a passivation layer on its surface. This layer can be destroyed by the presence of small amounts of chlorides in concrete pore fluids. To ensure durability of reinforced concrete structures exposed to high chloride levels, a minimum permeability of chloride ions should be maintained. This could be achieved by reducing the porosity of the concrete and/or placing an overlay of low-permeability concretes. Fly ash cements are known to develop fine pore structure that leads to a significant reduction in the diffusion of chloride ions compared with portland cements [5-8]. It has also been found that Na+-diffusion is markedly decreased in fly ash-containing pastes [9]. In addition, it is observed that the chloride content of fly ash-containing cement pastes is diminished by up to 80% over a three-month period [10], compared to
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