Electrochemical reactions between iron sulfide minerals and their implications for concrete durability

Electrochemical reactions between iron sulfide minerals and their implications for concrete durability

Ebrahimi, Nafiseh (National Research Council Canada, Canada)
Ghaziaskar, Amin (National Research Council Canada, Canada)
Makar, Jon M. (National Research Council Canada, Canada)

2021-04-29

It is well known that iron sulphides such as pyrite (FeS2) and pyrrhotite ((Fe1-xS, 0-x-0.125) are unstable in presence of oxygen and moisture, oxidizing to produce sulfuric acid and iron sulphate. Further reactions in concrete cause the formation of expansive reaction products such as ettringite and thaumasite, potentially leading to structural failure. However, the possibility of accelerated oxidation when pyrite and pyrrhotite come to contact with each other or with a steel rebar has not yet been investigated. This type of accelerated oxidation is more commonly known as corrosion when it happens in metals such as steel reinforcements. Since both pyrite and pyrrhotite are semiconductors and therefore sources of electrons, they can, however, promote this type of electrochemical reaction. Using electrochemical techniques, the electrochemical properties of pyrite, pyrrhotite, and steel and their open-circuit potentials were measured at different pH values. A zero resistance ammeter was also used to investigate the possibility of an accelerated oxidation reaction when any two of pyrite, pyrrhotite or steel are in electrical contact. The results show that pyrrhotite oxidation is accelerated in the presence of pyrite and both minerals could lead to an increased corrosion rate for steel rebar. These results are particularly significant as the Maskimo aggregate that was responsible for over $400 million dollars in damage to concrete in the Trois Rivières region of Quebec, Canada contains significant quantities of both pyrite and pyrrhotite. While pyrrhotite alone will cause damage to concrete through expansive reaction products, the combination of the two minerals may have contributed to both the extent of the damage and the small amount of pyrrhotite that was necessary to cause the damage. In addition to the results of the electrochemical studies, on-going work to investigate the electrochemical behaviour of the two minerals in concrete will also be outlined.

concrete
Iron sulphide
pyrrhotite
pyrite
electrochemical reaction

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