Concrete Under Sulphate Attack: An Isotope Study On Sulphur Sources

The formation of secondary sulphate minerals such as thaumasite, ettringite and gypsum is a process causing severe damage to concrete constructions. A major key to understand the complex reactions, involving concrete deterioration is to decipher the cause of its appearance, including the sources of the involved elements. In the present study, sulphate attack on the concrete of two Austrian tunnels is investigated. The distribution of stable sulphur isotopes is successfully applied to decipher the source(s) of sulphur in the deteriorating sulphate-bearing minerals. Interestingly, delta S-34 values of sulphate in local groundwater and in the deteriorating minerals are mostly in the range from +14 to +27 parts per thousand. These delta S-34 values match the isotope patterns of regional Permian and Triassic marine evaporites. Soot relicts from steam-and diesel-driven trains found in one of the tunnels show delta S-34 values from -3 to +5 parts per thousand, and are therefore assumed to be of minor importance for sulphate attack on the concretes. In areas of pyrite-containing sedimentary rocks, the delta S-34 values of sulphate from damaged concrete range between -1 and +11 parts per thousand. The latter range reflects the impact of sulphide oxidation on local groundwater sulphate.