Microstructural modifications of the expansive hydrates formed under different curing and restraining conditions in expansive concretes

Trabajo presentado al 15th International Congress on the Chemistry of Cement (ICCC), celebrado en Praga (República Checa) del 16 al 20 de septiembre de 2019.
The application of expansive concretes to construction projects is gradually becoming popular for both new construction and refurbishment. Expansive concretes are mainly made by using expansive agents with different chemical compositions that result in the increase of certain hydrates contents within the concrete matrix. The most usual agents promote the formation of ettringite (type-K) or portlandite (type-G). Many parameters influence the efficacy of the expansive agents and the performance of the corresponding expansive concretes such as curing, restraining conditions or concrete composition. These parameters affect the content, chemical composition and morphology of the expansive hydrates, and the resulting expansion is closely related to these modifications in the hydrates characteristics. Therefore, different concrete mixes were designed in this work, most of them selfcompacting concretes, in order to evaluate these effects. Two expansive agents, two supplementary cementitious materials, two expansion conditions and two curing conditions were considered. The microstructure of the concretes was evaluated by BSEM and XRD. The results obtained indicate that restraining conditions influence the morphology and the chemical composition of the expansive hydrates formed. An eminently amorphous ettringite type was identified as responsible for the expansion when using type-K agent. Regarding the concretes with type-G agent, denser portlandite plates better integrated in the cement paste matrix were formed in the concretes cured in more restrained conditions. Curing conditions and concrete composition also influence the reactivity of the expansive agents.
Authors gratefully acknowledge the Spanish Ministry of Economy and Competitiveness for the financial support given to this research in BIA2015-64363-R project.