Influência dos óxidos de ferro na reação pozolânica da metacaulinita

Abstract

Pozzolans are materials that react with calcium hydroxide in the presence of water, forming secondary hydrated products and promoting gains in strength and durability in cementitious products. These materials must be composed mainly of silicon, aluminum and iron oxides, which, in amorphous phases, will provide the pozzolanic reaction. The objective of this work was to study the reactivity of iron oxide and its influence on the pozzolanic reaction of metakaolinite. Kaolin, with high purity, was calcined to obtain metakaolin with 97,17% dehydroxylation. The study was conducted in white cement paste, as it is Fe-free, with 25% replacement of the cement mass by compositions of the materials tested. The iron oxide contents used in relation to the total mass were 1,5%, 3%, 4,5%, 6%, 9% and 25%. To compare pastes with metakaolin and without iron oxide, but which had the same cement consumption, samples were produced with compositions of metakaolin and quartz powder (considered inert in this research) to evaluate hydration at 3, 7, 28 and 63 days. The samples produced with 25% metakaolin had a mechanical performance index at 63 days of 141,17% compared to the standard sample with 100% cement, while the samples with 25% iron oxide and 25% quartz powder obtained indexes of 82,11 and 83,86% respectively; while the samples with compositions of metakaolin with quartz powder or iron oxide showed an increase in the indexes as the metakaolin content increased, with no significant difference being observed when comparing pastes with the same iron oxide content and quartz powder. There was a reduction in the content of portlandite and growth in the formation of other hydrated products with the increase in the content of metakaolin in the compositions, verified by the areas of DTG curves and by the increase in the combined water; a method for calculating effective combined water (without portlandite) is also proposed. It is noteworthy that in XRD diffractograms it was not possible to identify the formation of hydrated products containing iron, implying that the oxide remained inert during the cement hydration process and during the pozzolanic reaction of metakaolinite. In view of the study carried out, it is understood that the only iron oxide present in calcined clays is hematite, observed in XRD analysis, which does not have pozzolanic activity and does not participate in the hydration of cement, and cannot be classified as pozzolanic oxide.