Weathering of iron sulfides and concrete alteration: thermodynamic model and observation in dams from Central Pyrenees, Spain

Abstract

The concrete of the Graus and Tabescan dams present significant durability problems. The cement paste is altered to expansive phases such as ettringite and gypsum, following fractures and aggregate-paste interfaces. The alteration is initially attributed to the acidic solution produced by the weathering of the pyrrhotite contained in the aggregate fragments. A chemical model, based on ion association and thermodynamic equilibrium, permits the calculation of the mass transfer between the solids and the pore solution, and the prediction of the progress of the acidic attack. The results of the calculations have been compared with the alteration features observed in concrete of these dams. Despite the simplifications, this model is able to predict the observed alteration stages, and confirms the hypothesis of acidic-sulfatic alteration of the cement.