Deterioration of cementitious materials by organic acids in agricultural effluents: experiments and modelling
Résumé
Combining experiments with modelling, this paper investigates how Portland cement pastes are attacked when exposed to three aggressive solutions made of acetic acid, oxalic acid or a mixture: 1/3 oxalic - 2/3 acetic acids, the mixture being more representative of real effluents such as sugar cane molasses. Oxalic acid is known to have a protective effect toward OPC matrices whereas acetic acid has a strong leaching effect. The aim was to better characterize the degradation mechanisms by discriminating between acidic attack and complexation effects, determining the cement phase evolution, and assessing leaching sensitivity versus the solubility and molar volume of calcium salts. Under acidic pH, oxalate anions are stronger complexing agents of aluminium and calcium cations than are acetate anions. Calcium oxalate salt precipitates whatever the pH, while there is no precipitation of the highly soluble calcium acetate salts. Both oxalic acid alone and the acid mixture were protective, showing the predominant role of oxalic acid. Experiments and modelling led to similar results in terms of degradation rate and depth.