Contribution of Thermodynamic Modeling to the Understanding of Interactions Between Hydrated Cement Pastes and Organic Acids

In numerous contexts, concrete can be exposed to chemically aggressive conditions that can damage their microstructure and reduce their lifespan. The concrete facilities from the agricultural and agro-food industries dedicated to the storage or the treatment of effluents are more particularly exposed to organic acids coming from the microbial activity naturally occurring in such media. This biodeterioration leads mainly to mineralogical transformations, such as hydrated and anhydrous phase dissolution, and to ion exchanges between acidic effluents and cement-based materials. The poorly crystalline mineralogy of hydrated cement pastes and their reactivity makes the geochemical behavior of such materials difficult to investigate and thus to predict over large periods of time and wide variety of chemical conditions. The degradation of cementitious materials in these aggressive conditions mainly leads to the leaching of calcium and the precipitation of amorphous secondary phases. The purpose of this work is (i) to assess the stability of the cement phases involved in such chemical conditions as well as to identify the alterations products, and (ii) to understand the evolution of concentration and the behavior of elements in solution such as aluminum or silicon. A thermodynamic model of cement pastes subjected to acid attacks has been developed, in order to reproduce, experimental data also presented here. Our model reproduces a major part of the behaviors shown by the experiments, i.e. a progressive decalcification of solid matrix (successive dissolution of portlandite, aluminates hydrates and C-S-H) during acid degradation and the identification of alteration zones in agreement with the experimental observations.