Degradation of cement-based materials by various organic acids in agro-industrial waste-waters

The paper analyses the mechanisms of the degradation of cementitious materials by three organic acids: oxalic, citric and tartaric acids, and evaluates their relative aggressiveness compared to that of acetic acid. These acids have different chemical characteristics in terms of poly-acidity and chemical and physical properties of their calcium salts. The aim was to highlight the links between these properties and the aggressiveness of each acid toward concrete. Degradation mechanisms and kinetics were investigated (using EPMA, XRD and SEM) on cement paste specimens immersed in acid solutions for one year.The results showed that oxalic acid was not aggressive to the matrix. Citric acid caused the highest kinetics and severe degradations in the paste. Tartaric acid attack developed in two stages: no visible degradation was observed in the first weeks and then damage occurred, the kinetics remaining moderate. Acetic acid had intermediate aggressiveness.It was confirmed that the solubility of the calcium salts was a major parameter of aggressiveness. It also appeared that, even if salts precipitated, they were not necessarily protective, the protection being correlated with the salt's molar volume. Moreover, poly-acidity seemed, in some cases, to increase the aggressiveness of the acid.