Synergistic effect of metakaolin and fly ash on properties of concrete

This paper reports the effects of the interaction between metakaolin and fly ash on the microstructure and property development of concrete. X-ray diffraction analysis revealed unstable hemicarboaluminate and calcium monocarboaluminate compounds in most mixtures during 7-28 day curing periods, the relative amounts depending on the metakaolin to fly ash ratio. The mix with the highest peaks of the monocarboaluminate phase yielded the highest long-term strength. Significant improvements in terms of durability, abrasion resistance, chloride permeability and steel corrosion risk were observed. A proportion of cement:metakaolin:fly ash as 80:10:10 yielded marked improvements on slump, slump loss and long- term strength. Synergistic action in the ternary blend significantly improved the workability of fresh concrete and yielded a more uniform mix, denser microstructure and better performance of the hardened concrete.