Multi-scale study of delayed ettringite formation (DEF) in concrete

Internal swelling reactions (ISR) are important degradation mechanisms for cement based applications. ISR can potentially cause swelling, which leads to cracking, thus affecting the performance of the material either in the short or long term. The study of these mechanisms is important in civil and nuclear sectors. This PhD will focus on a particular ISR, delayed ettringite formation (DEF), because this degradation mechanism is relevant for assessing the lifetime of nuclear power plants (NPPs) and assessing the conformity of nuclear waste cementitious immobilization matrix. Despite research efforts in the last decades, still major knowledge gap exists between the diagnosis and prognosis of DEF mechanisms, and finally the quantification of macroscopic expansion. This research aims at better understanding the pathology of DEF in order to be able to estimate the level of expansion at different environmental and material conditions, to estimate the mechanical behavior of the material under DEF and to evaluate best measures to avoid such reactions. The proposed methodology utilizes multi-scale numerical modelling complemented by experimental analytical techniques. Multiscale modelling involves two major modelling approaches, pore-scale modelling where the spatial resolution is at the micrometer scale and meso-scale modelling at a larger spatial resolution. Pore scale model is based on lattice Boltzmann numerical method coupled with geochemical solver and gives the understanding on the process interplay between chemical reactions, ion transport and pore geometry evolution. With a meso-scale model we will capture heterogeneities of concrete (aggregates and cement paste) and is intended to provide information of stress evolution and crack formation. Experimental part will identify changes in C-S-H, AFm and monosulfates as well as pore structure. This data will be used as input for numerical modelling.