Dietmar Klammer

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, δ34S values of sulphate in local groundwater and in the deteriorating minerals are mostly in the range from+14 to+27 ‰. These δ34S 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 δ34S values from−3 to+5 ‰, and are therefore assumed to be of minor importance for sulphate attack on the concretes. In areas of pyrite-containing sedimentary rocks, the δ34S values of sulphate from damaged concrete range between−1 and+11 ‰. The latter range reflects the impact of sulphide oxidation on local groundwater sulphate.

MASS CHANGE DURING EXTREME ACID-SULPHATE HYDROTHERMAL ALTERATION OF A TERTIARY LATITE, STYRIA, AUSTRIA

Abstract The Tertiary latitic rock from Gossendorf, in the Gleichenberg Volcanic Area, Styria, Austria, has in places been completely altered to various associations of the secondary minerals opal-C/-CT, alunite, kaolinite, and montmorillonite. The associations occur in a zonal arrangement and correspond to the characteristics of the silicic, advanced argillic and argillic alteration terminology. All elements investigated were mobilized during the hydrothermal acid-sulphate alteration process, except Ti, which was selected as the immobile monitor element to calculate mass changes. Drastic differences in gains and losses of certain elements which refer to the different alteration conditions were found to exist not only between different alteration zones but also within such zones. The gains depend on the newly formed minerals, the losses, however, on the increase of the porosity in the altered rocks. For one altered rock characterizing theoretically the alteration processes in Gossendorf, mass grains were calculated for SO 3 , H 2 O, V and Sr, and mass losses for SiO 2 , Al 2 O 3 , Fe 2 O 3 , MnO, Mg0, CaO, Na 2 O, K 2 O, P 2 O 5 , Cr, Co, Ni, Cu, Zn, Rb, Y, Zr, Nb and Ba with respect to an unaltered latite. The net mass balance calculation, i.e. the difference of elemental gains and losses, shows a net mass loss of about 14.60 g. The mass exchange was accompanied by a moderate average decrease in volume of 6.6%. The chemical species added to the rock during alteration, SO 3 , V 2 O 3 , and SrO, probably originated from a volcanic source. H 2 O could have been of exogenous and/or endogenous origin. The results of the mass change calculations of major, minor and trace elements suggest that the secondary mineral associations and their zonal arrangement at Gossendorf have been formed by a strongly acidic hydrothermal solution rich in SO 4 2− , which underwent chemical variations by reaction with the latite. The calculations indicate that for the formation of the altered rocks and their zonal distribution the elements of the latitic precursor rock, a volcanic source for S, V, and Sr and probably exogenic water were necessarily involved. Based on these results a general chemical characteristic for alteration assemblages in epithermal acid-sulphate districts is presented. Al 2 O 3 , Fe 2 O 3 , Mg0, CaO, SO 3 and H 2 O% by weight proves to be suitable to characterize silicic, advanced argillic and argillic alteration.

Deterioration of Concrete: Application of Stable Isotopes

Groundwater interaction causing concrete damage and drainage clogging is of great economic interest due to reduced service life and additional costs for remediation and maintenance. Although numerous studies are available, detailed understanding of the reactions leading to concrete deterioration is still lacking. By introducing multiproxy approaches for the water–cement aggregate system, new fundamental insights are gained. This includes trace element and isotope signatures supplementary to the main elemental compositions, mineralogy, and microstructure. Our results are obtained from samples that were taken from Austrian tunnels. Possible sources for CO3 2– in thaumasite or newly formed calcite are atmospheric CO2, aggregates, or carbonate from solutions. Results acquired by stable carbon isotopes (δ13CVPDB) indicate that dissolved inorganic carbon of infiltrating groundwater is the main carbonate source for thaumasite formation, as values are in the range of –7‰. Contrarily, calcite sinters that are formed by CO2 absorption show much more depleted values from –25‰ to –40‰, and carbonates from marine limestone aggregates are close to 0 ± 2‰. The sulfate source for thaumasite, namely, secondary ettringite and gypsum, can be deciphered by the δ34SVCDT values. The δ34SVCDT values of thaumasite, sulfate from interacting groundwater, and local host rock were analyzed. For a case study, the δ34SVCDT values of thaumasite and ground water were close to 20‰. Therefore sulfate in thaumasite is related to infiltrating groundwater and sulfate from oxidation of local sulfides, organic matter, or atmospheric influence can be ruled out. Moreover, interstitial solutions of deteriorated concrete were separated by a hydraulic press. Extracted solutions contain up to 65 g/L total dissolved solids (TDS) and are extremely enriched in Na+ and SO4 2–. Concentrations reach values of up to 17 and 30 g/L, respectively. The analyzed 2H/H and 18O/16O values of the squeezed interstitial solution display a strong enrichment of the heavy isotopes versus the local infiltrating solutions. As this trend is in accordance with respective enrichment of conservative trace elements, elevated TDS can be quantitatively related to the isotope fractionation during evaporation of the interstitial solution.