The Application Of Metakaolin In Concrete

Metakaolin is produced by kaolinite at a proper temperature. As a mineral admixture, metakaolin has a similar activity with silica fume. The application of metakaolin in concrete can lead to a significant enhancement in the performance of concrete including reducing the consumption of cement, improving the physical property and durability of concrete. The impaction of metakaolin on the course of hydration recation, physcical properties and durability is discussed in this paper. According to the experimental design,0～15% of cement is replaced by metakaolin, and the concretes of C30, C40, C60 strength grades are prepared. Further researches on the hydration products, microstructure of the concrete using metakaolin are studied by XRD, TG-DSC, SEM-EDAX and MIR In order to investigate the compatibility of metakaolin with other mineral admixtures, multi-admixtures with slag and fly ash are also prepared. The main conclusions of the experiments are as follows:The application of metakaolin will increase the standard consistency water demand of Portland cement,24% more water is required in compared with the plain cement mortar when the content of metakaolin reaches to 15%. Metakaolin can significantly shorten the setting time of cement, and acclerate the porcess of hydration reaction while decreasing the total heat release of cement. Metakaolin has a similar activity with silica fume, which leads to a splendid ameliorative effect in the mechanical property of concrete when it is used as a mineral admixture. The compressive strength, flexural strength, and splitting tensile strength of concrete at 56 days respectively improved 17.3%,15.1%, and 9.2% when the dosage of metakaolin was 15%. The strength effect coefficient indicated the improvement of early strength still existed when adding metakaolin at a high replacement. The compatibility of metakaolin with slag and fly ash shows a positive result, and the multi-admixtures are proved to have a better mechanical property and durability.Metakaolin can improve the durability of concrete, the concrete admixing metakaolin shows better resistance to freezing-thawing destruction and chloride ion permeability. The incorporation of metakaolin has no obvious influence on the durability property of carbonization.Composition of hydration products, microstructure and pore structure of concrete admixing with metakaolin were analyzed by XRD, TG-DSC, SEM-EDAX, MIP. The results indicated that, metakaolin can consume great amount of Ca(OH)2 decreasing the size and Ca/Si ratio of CSH gel, making the morphology more dense. The Ca/Si declined 39% contrasting with the plain concrete when the application of metakaolin came to 15%. Metakaolin can reduce the porosity of the concrete, refining the average pore diameter, and increase the content of harmless pore.