Pore Structure, Permeability, and Chloride Diffusion in Fly Ash-and Slag-Containing Pastes and Mortars
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PORE STRUCTURE, PERMEABILITY, AND CHLORIDE DIFFUSION IN FLY ASH-AND SLAG-CONTAINING PASTES AND MORTARS R.I.A. MALEK, D.M. ROY and Y. FANG Materials Research Laboratory, The Pennsylvania State University, University Park, PA 16802 Received 30 November, 1988; refereed ABSTRACT The transport of ions through cement pastes and mortars with variable contents of fly ashes and granulated blast-furnace slag from different sources and with variable composition has been investigated. The research included the determination of chloride diffusion rate and chloride permeability in relation to microstructure development. The median pore size generally was much diminished in mature blended material compared with Portland cement (PC) pastes and mortars. It appears that, at the same age, a finer microstructure is generally developed in blended specimens compared to PC specimens. Also, it was found that the microstructure approaches a limiting value at longer ages of hydration. That limiting value may be reached at earlier ages with the blends. The chloride diffusion rates and permeabilities in the blends were significantly lower than PC mixes. A comparison between the blends containing fly ashes and those containing slag was made. INTRODUCTION The addition of slag and fly ash as blending materials gives rise to different characteristics of the hardened materials due to both chemical reactions and physicochemical effects. The chemical composition of the C-S-H formed in blended hardened cement pastes has been reported to differ from that of Portland cement hydration products [1]. The physicochemical effects are apparent at different stages in the hydration process. During mixing, the addition of blending material may significantly affect the size, size distribution and structure of the individual flocs comprising of fresh cement paste. The changed floc structure affects the rheological properties of the fresh paste, and ordinarily has important consequences for the microstructure of the hardened paste, including its pore size distribution, permeability, and related properties. Numerous investigators have found that the pore size distribution varies with the material as well as the stage of reaction. The median pore size generally is much diminished in mature blended materials compared with Portland cement paste. The reduced value of the mean pore size results in lower ionic diffusion rates [2,31. It also results in lower water permeabilities [4,5, 61. Uchikawa [1] also reported from determinations by mercury porosimetry that there was a larger proportion of fine pores in blended pastes, compared with portland cements. Nitrogen adsorption techniques were used to detect a larger proportion of even finer pores. EXPERIMENTAL Materials The materials used were Type I cement, three slags and three fly ashes. The slags were from three different sources: one U.S. and two English. The fly ashes, supplied by the American Fly Ash Company, were one Class C and two Class F ashes. Chemical compositions of these materials are given in Table I. Additi
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