Influence of delayed ettringite formation on the durability of mortars: the importance of thermal control

Introduction. This study is part of a broader investigation into the phenomenon of delayed ettringite formation (DEF) in concrete and its consequences on the durability of concrete–mortar systems. The objective of the research is to analyze the influence of the initial thermal conditions of fresh concrete on the development of internal sulfate reaction (ISR), the mechanical properties of concrete, and the behavior of mortar applied to the surface after hardening. Materials and Methods. Three concrete mixtures, designated BP1, BP2, and BP3, were produced and subjected to DEF testing on cubic specimens, including continuous temperature monitoring. The temperature of fresh concrete was intentionally varied in order to evaluate its influence. The experimental program was carried out in Africa, specifically in Benin. Compressive and tensile strength tests were performed. After verifying that the concrete was suitable, mortar was applied to the concrete surface. The behavior of the mortar was then monitored over a period of 90 days through visual observation of the mortar surface. It was particularly interesting to observe the evolution of the mortar surface over time. Results and Discussion. The results has shown that for concretes BP1 and BP2, when the initial temperature was below 35 °C, the thermal behavior during the DEF test was stable, and the maximum temperature did not exceed 80 °C. Under such conditions, appropriate mechanical properties and a uniform coating of mortar were obtained without any visible cracking or surface damage. Nevertheless, for concrete BP3 with initial temperature higher than 35 °C, the thermal behavior became unstable, but the maximum temperature remained lower than the critical one. It was observed that this instability negatively affected the mechanical properties and reduced the durability performance of the material. Conclusion. The control of the initial thermal conditions of fresh concrete is a key parameter for limiting the development of DEF, preserving the mechanical properties of concrete, and ensuring the durability of applied mortars. In particular, the initial temperature of fresh concrete should not exceed 35 °C, since higher temperatures may promote the development of internal sulfate reaction (ISR) within the concrete itself.