The Diffusion of Chloride Ions in Fly Ash/Cement Pastes and Mortars
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THE DIFFUSION OF CHLORIDE IONS IN FLY ASH/CEMENT PASTES AND MORTARS R.I.A. MALEK, D.M. ROY* and P.H. LICASTRO Materials Research Laboratory, The Pennsylvania State University, University Park, PA 16802 *Also affiliated with the Department of Materials Science and Engineering
Received 12 November,
1986; refereed
ABSTRACT Fly ashes having three distinctly different levels of calcium, designated low-calcium (Class F), intermediate-calcium (Class F/C), and high-calcium (Class C) comprised the basic material for the present study. Pastes and mortars were made out of the three types of fly ashes and one type of cement (Type I) at various levels of replacement as well as different water/solid ratios (w/c). Chloride ion diffusion was measured by applying an electrical potential across cured cylindrical samples and measuring the amount of current passed in a certain period of time (proportional to amount of Cl- passed in this time). Other supportive measurements were made, e.g., porosity and pore size distribution, water permeability and surface area. The Cl- ion diffusivities were correlated with the chemical composition of fly ash (FA), mix proportioning, and water permeabilities of the tested hardened pastes or mortars. INTRODUCTION The diffusion of Cl- ions has been found to be strongly influenced by the type of cement, and type and proportions of blending materials [1]. Blending cements (with 30% fly ash or 65% slag) results in lower diffusion rates than neat cement pastes [2,3]. Chloride diffusion follows Fick's law [4-9] and is lower in blast furnace
slag cements than in portland cements. The reduction in diffusion rate in fly ash cements is significant [4,6,7,10] though variable, while sulfate-resistant portland cements may have higher diffusion rates. Silica fume has been shown to decrease the diffusivity [4,11], even at relatively high w/c ratios [4,7, 11]. In an attempt to predict time-Cl- penetration depth plots in concrete, Pereira and Hegedus [12] developed a diffusion-reaction-equilibration model in which Langmuir adsorption and Fickian diffusion were coupled. Uchikawa et al. [13] showed that Na+ diffusion was markedly decreased in slag and fly ashcontaining pastes, which they related to the zeta-potential. Pore solution compositions may provide a key to transport inhibition mechanisms and enable comparisons among the effectiveness of various materials. It has been observed [14-16] that the chloride content of pore solutions of fly ash-containing and slag-containing cement pastes is diminished by up to 80% over a 3-month period, compared with 40% reduction in a neat portland cement paste [17,18]. This study examines the effect of various factors on chloride ion diffussivity in cement/fly ash blends. MATERIALS AND TECHNIQUE Materials
An ASTM Type I cement (with 12.3% C3 A content) and two different classes
of fly ashes were used in this investigation. Oxide compositions are presented in Table I. Two Class F fly ashes were used; the Baldwin ash having higher
Mat. Res. Soc. Symp. Proc. Vol. 86. ' 1987 Mat
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