Concrete, when exposed to high temperatures, has its properties changed, and the recovery processes depend on the intensity of these changes. This paper aimed to evaluate the influence of high temperatures on concrete properties and verify the effects generated in the material when submitted to natural and fast cooling. Three different concrete mixes were exposed to temperatures of 100 °C, 300 ºC, 450 ºC, and 600 ºC, subdivided into slow and fast cooling groups. With the increase in temperature, cracking processes and microstructural alterations intensified, generating reductions in mechanical strength and increases in absorptions. This study has identified that the elastic modulus was the property most affected by these changes. The correlations between the elastic modulus and the other properties were accompanied by increased concrete micro-cracking, water loss, and decomposition of paste phases. Also, it was found an effect of ettringite rehydration during concrete cooling.

Corrosion is one of the causes of failure in reinforced concrete structures, and forming a passive film on the steel is essential for protection. Although several studies have looked at passive film formation in concrete pore solutions, few have considered its formation in hardened concrete and the influence of silica fume (SF) in the binder composition. This study aims to evaluate the influence of the SF content on passive film formation time in concrete. Periodic measurements assessed the electrical resistivity and corrosion current density of concrete samples containing 5%, 10%, 15%, and 20% SF. The alkalinity of the mixtures and the kinetics of the pozzolanic reaction were also monitored by XRD and titration tests. The control mixtures exhibited susceptibility to corrosion, regardless of the curing age evaluated. In contrast, the partial replacement of cement with SF accelerated the formation of the passive film on the steel surface, suggesting a delayed onset of corrosion due to modifications in the physical properties of the concrete. Also, the portlandite content and pH can predict passive film formation, with SF significantly accelerating this process.

This article aims to evaluate the AAR products in the Portland cement mortars and metakaolin-based geopolymeric mortars microstructure, and the impact on their physical-mechanical properties. Portland cement and metakaolin mixed and metakaolin-based geopolymer mortars with an innocuous and deleterious mineral aggregate were produced, to characterize their alkali-aggregate reactivity, mechanical properties and microstructure. The results show that, for both geopolymer and Portland cement/metakaolin mixed mortars, the AAR reactive aggregate contributes to the mortar matrix densification and improves their mechanical strength. The microstructure of the samples also indicates that deleterious aggregates contribute to zeolite formation in both produced mortars. In conclusion, mineral phases that are susceptible to alkali-aggregate reaction can also be susceptible to alkaline activation and incorporated into metakaolin-based geopolymers matrices, improving their physical and mechanical properties.