Influence of fly ash and metakaolin in the inhibition of delayed ettringite formation in concrete – a microstructural characterisation

In the last few years it has been observed in Portugal, as well as in other countries worldwide, several cases of concrete structures affected by internal expansive reactions, particularly by delayed ettringite formation (DEF). This pathology, which is macroscopically characterized by the occurrence of cracks and fissures in concrete due to the expansion of the reaction product, is very difficult to deal with, because presently, there isn’t any efficient method to repair structures affected by DEF. Considering this, there is an urgent need to find preventive methods that may enable the inhibition of DEF in new concrete constructions. This paper presents the findings of a long-term study on the expansion rate and microstructure of heat cured concretes with different amounts of fly ash and metakaolin. For this purpose, concretes with different water-to-cement ratios and aggregate types were produced by using the same binder content. Different concrete specimens were prepared and subjected to heat curing. Length changes of specimens were measured within a period of 1000 days. The microstructures of old heat curing specimens were also investigated by SEM and EDS analysis. The expansion rates and microstructures observed were compared with the control specimens