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
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 using cement and one of three types of fly ashes at various levels of replacement and water-solid ratios. 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 CE" passed in this time). Other supportive measurements were made, e.g. porosity, pore size distribution, water permeability and surface area. The CE" ion diffusivity was correlated with the chemical composition of fly ash, mix proportion, and water permeability of the hardened paste or mortar.
INTRODUCTION The diffusion of CI" ions has been found to be strongly influenced by the type of cement, and type and proportions of blending materials [1]. Blended cements (with 30% fly ash or 65% slag) produce in lower diffusion rates than neat cement pastes [2,3]. The reduction in diffusion rate in fly ash cements is significant [2,4-6] though variable. Silica fume has been shown to decrease the diffusivity [4,7], even at relatively high watercement
ratios
[4,5,7].
Chloride diffusion follows Fick's law [2,4-5,8-10]. In an attempt to predict time-Cl" penetration depth plots in concrete, Pereira and Hegedus [11] developed a diffusion-reaction equilibration model in which Langmuir adsorption and Fickian diffusion were coupled. Uchikawa et al. [12) showed that Na+ diffusion was markedly decreased in slag and fly ash-containing 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 [13-15] 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 [16,17]. This study examines the effect of various factors on chloride ion diffusivity in cement/fly ash blends.
MATERIALS AND TECHNIQUE Materials An ASTM Type I cement (with 12.3% C 3 A content) and three fly ashes were used in this investigation. Oxide compositions are presented in Table I. Two Class F fly ashes, the Baldwin ash having higher Fe 2 0 3 and lower CaO and alkali contents than the Crawford ash, and one Class C fly ash were used.
Mat. Res. Soc. Sympo. Proc. Vol. 85. -1987 Malerials Research Society
224
Table I Chemical composition and other parameters of cement and fly ashes Cementa Sio 2 (%) A1 2 0 3 (%) Fe 2 0 3 (%) CaO (%) MgO (%)
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