Influence of metakaolin on temperature rise and delay ettringite formation in cement composites

Several concrete structures in Brazil are suffering prematurely from delayed ettringite formation (DEF). There are still many gaps and about the behavior of supplementary cementitious materials and their effectiveness in mitigating DEF expansions. Furthermore, monitoring and controlling on site often bring future concern after placing concrete in situ given the high temperatures reached by the hydration heat. Given the above, this research presents two case studies of concrete cast with pozzolanic cement and incorporation of metakaolin in which temperatures achieved 70 degrees C. In order to progress the understanding of this deleterious process in the presence of metakaolin and to contribute with the technical community, an experimental research was performed in laboratory to evaluate DEF in cementitious composites with and without of metakaolin at various levels. The data observed in the field showed that temperatures surpassed the preliminary thermal previsions after casting concretes, bringing risks of DEF over time, despite of the previous warnings regarding to the mixtures and generation of heat. Furthermore, laboratory studies carried out up to 365 days have indicated that the mortar tests take a long time to signal deleterious expansion process of DEF. Thus, if ages up to one year are adopted, there are risks of considering levels of metakaolin proper for use that, in fact, are classified as false negatives. Ettringite formations were identified by microstructural analyses in all tested mortars. In this regard, complimentary concrete studies are necessary to better understand and delineation of the expansive behavior in the presence of mineral admixtures against the DEF process during service life of structures, besides concrete and materials cooling.