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SABIR, B. B.; WILD, S.; KHATIB, J. M. Strength development and workability of concrete blended with metakaolin and fly ash. In: INTERNATIONAL CONFERENCE ON HIGH- PERFORMANCE CONCRETE, AND PERFORMANCE AND QUALITY OF CONCRETE STRUCTURES, 2., 1999, Gramado. Anais… Gramado, 1999.
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Abstract

The work reported in this paper investigates two key properties, compressive strength and workability, in concretes made from normal portland cement blended with metakaolin (MK) and fly ash. Twelve concrete mixtures with varying MK and fly ash contents were examined. The percentage replacements of normal portland cement were in the ranges of 0-20 for MK and 0-30 for fly ash. All the mixtures were superplasticised and subjected to workability tests. The compressive strength was determined after curing in water for 1, 7, 14, 28, 98 and 365 days. At the very early ages, MK-fly ash blended cements displayed lower strength than the control at all replacement levels. This is attributed to the dilution effect. Cements blended with MK only gave increased strengths at 7 days of curing and beyond. This is attributed to the pozzolanic reaction with the CH produced by the hydration of the cement. A peak in relative strength was achieved at 14 days. Smaller strength gains, relative to the control strength, were obtained when the binder (cement + MK) is further blended with fly ash. Marked reductions in the long-term strength, relative to the control, was obtained when the cement is blended with the higher levels of MK and fly ash. The optimum replacement levels for portland cement concrete with water/binder ratio of 0.45 were found to be 10% MK and 10% fly ash. It is suggested the reduced CH content and the dilution effect reduce the pozzolanic contribution of fly ash above a critical MK content.
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