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Deterioration of Concrete by the Oxidation of Sulphide Minerals in the Aggregate



Author(s)

Josée Duchesne and Benoît Fournier

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DOI:10.17265/1934-7359/2013.08.003

Department of Geology and Geological Engineering, Université Laval, Québec G1Y 2S4, Canada

Cases of degradation of concrete associated to iron sulphides in aggregates were recently recognized in the Trois-Rivières area, Canada. The aggregate used to produce concrete was an anorthositic gabbro containing various proportions of pyrite, pyrrhotite, chalcopyrite and pentlandite. Quantitative microanalysis on sulphide minerals show that pyrrhotite contains small amount of Ni, Co, Cu and As substituting for Fe in the mineral structure. Considering element substitution, x value in the chemical formula (Fe_1-xS) was calculated to 0.099 in the pyrrhotite studied. Petrographic examination of damaged concretes showed the presence of oxidized pyrrhotite. The observation of polished samples shows, in several cases, that the pyrite is intact while the pyrrhotite presents evident signs of oxidation. In the presence of water and oxygen, pyrrhotite oxidizes to form iron oxyhydroxides and sulphuric acid. The acid then reacts with the phases of the cement paste and provokes the formation of gypsum and ettringite. These minerals were observed by SEM-EDS (scanning electron microscope/energy dispersive x-ray spectrometer) and their precipitation causes a volume increase that creates expansion and cracking of the concrete.

Sulphide minerals, oxidation, expansion, petrographic examination, concrete durability.