ABSTRACT
XRD pattern of sound concretes and shotcretes display poorly crystalline CSH-phases, portlandite (Ca(OH)2) as well as silicate and carbonate aggregates. Mineralogical investigations (XRD, EPMA, SEM) clearly show that severe concrete deterioration is caused by sulphate attack. Complete disintegrated materials consist mainly of thaumasite, calcite and silicate aggregates. Gypsum was found in small quantities and ettringite could not be identified unambiguously due to the structural similarity and the overwhelming presents of thaumasite. Furthermore significantly reduction and even total lacking of dolomite aggregates were detected by comparing sound and deteriorated material. Surprisingly ground water and drainage water analyses generally showed only low to moderately elevated SO42− content with values ranging from about 3 to 500 mg/l. Consequently arising the question: Is dissolved SO42− of ground water the only source or are we facing other S sources coming from e.g. internal from concrete/shotcrete, oxidation of pyrite, atmospheric contribution, or organic matter such as soot. The latter source was the most likely candidate for a more than 100 years old railroad tunnel which was not properly cleaned before shotcrete was applied about 50 years ago. In spite of rather low SO42− content in the ground water S-isotope measurements clearly indicate that SO4 in thaumasite is stemming from infiltrating ground water implicating the dissolution of local occurring gypsum and anhydrite rocks. δ34S values of thaumasite, ground water SO42− and host rock are mostly within the range of +14 to +27‰ (Mittermayr et al., 2012a). In a few locations significantly lower δ34S signals were detected in thaumasite resulting from pyrite oxidation. Interestingly internal sulphate attack or contributions from soot or the Earth’s atmosphere can be ruled out by the given δ34S ranges which opens up a new subject: How does ground water with about 500 mg/l of sulphate or even less may cause intensive sulphate attack? Therefore we analyzed the chemical and isotopic composition of interstitial solutions extracted from heavily damaged concrete samples by using a hydraulic press. Proportions of extracted solutions correspond to about 5 up to 20 wt.% of the solid material and extreme accumulation of Na+ and SO42− is observed compared to the locally occurring ground water. Considering the dramatic increase of univalent cations compared to the local ground water and the rather conservative behavior of e.g. K+ and Rb+ suggests evaporation of water to be responsible for extreme SO42− concentrations of up to 30000 mg/l (Mittermayr et al., 2011). Proof is gained from analyses of δ2H and δ18O values of H2O molecules which display a strong enrichment
of the heavy stable isotopes. The respective kind of isotopic evolution clearly indicates evaporation of H2O from the local ground water. A further curiosity with a high need to be verified was the incongruent dissolution of dolomite aggregates. Previously we have suggested that dolomite aggregates are preferentially dissolving incongruently governed by high Ca2+/Mg2+ ratio in the interacting aqueous solution. Calcite becomes more stable versus dolomite. As long as the pH remains above 10.5 the continuous removal of dissolved Mg2+ from the solution is leading to brucite (Mg(OH)2) formation. Surprisingly in all cases where sulphate attack in combination with dolomite aggregates was found, a partial dissolution of the latter and precipitation of secondary calcite and brucite was observed. Thus we assumed carbonate released by dolomite dissolution to have contributed to thaumasite formation. Various studies suggest that the carbonate can be stemming from the absorption of atmospheric CO2, carbonate aggregates or DIC of ground water. To investigate the carbon source in thaumasite we applied stable carbon isotopes. In contrary to our assumption that carbonate is originating from dolomite aggregates, we found the carbon in thaumasite to be related to the uptake of ground water DIC. δ13C values from thaumasite and DIC are mostly in the same range from −11 to −5‰ (Mittermayr et al., 2012b). Most carbonates used as aggregates are usually much heavier in isotopic values and are plotting at 0 ± 2‰. In contrary Dietzel (1995) has pointed out that the formation of calcium carbonate related to the absorption of atmospheric CO2 is leading to a strong depletion of 13C versus 12C. The latter reaction occurs at high pH and is governed by a kinetic isotope fractionation leading to δ13C values of −25 ± 3‰.
