Deterioration of Concrete: Application of Stable Isotopes

Abstract

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.