Alterações na microestrutura, causadas pela exposição às altas temperaturas, de argamassas com potencial formação de etringita tardia

Abstract

The delayed ettringite formation (DEF) is a pathological manifestation that can occur in concretes and mortars when cement with high sulfate content in the production process and thermal cure in temperatures high up to 70 ºC. The DEF causes cracks which can shorten the lifespan and aggravate the damage suffered in atypical situations such as a fire. This work aimed to evaluate the microstructural changes due to high-temperature exposure in mortars with potential delayed ettringite formation. For this, a reference mixture was compared with a mixture in which the SO3/Al2O3 molar ratio was changed to 1.1 to favor the occurrence of DEF. After the production of the specimens, they were subjected to an initial wet cure for 12 hours at 23 or 80 °C, followed by a wet cure at room temperature for 96 days to simulate deterioration and subsequent recrystallization from ettringite. Subsequently, the CPs were kept for 56 days in a controlled environment at 23 ± 2 ºC and 50 ± 5 % humidity to stabilize the internal humidity and avoid possible detachment during heating due to moisture evaporation. After the moisture conditioning period, the CPs were heated at a rate of 1 °C/min to levels of 200, 400, 600, and 900 °C. Using X-ray diffraction and scanning electron microscopy techniques, it was verified that mortars with a SO3/ Al2O3 molar ratio equal to 1.1 had a higher incidence of ettringite with type II morphology, which has an expansive character. In addition, the curing process resulted in the decomposition of the ettringite. Regarding heating, for the specimens submitted to 200 ºC, it was observed that there was no increase in the intensity of portlandite peaks in the thermally cured samples. As for the change in the SO3/Al2O3 molar ratio, its influence was evident in the samples heated to levels of 600 and 900 ºC in which the presence of anhydrite was observed, which can make it difficult to recover the material in a post-fire scenario. Therefore, it is concluded that the mortar with more significant potential for the occurrence of DEF can also present greater damage when subjected to high temperatures, such as in the event of a fire.