C, O and S isotopic signatures in concrete which have suffered thaumasite formation and limited thaumasite form of sulfate attack

Abstract

Abstract Thaumasite formation and limited thaumasite sulfate attack has recently been discovered in sprayed concretes in contact with pyrrhotite-, pyrite- and calcite bearing Alum Shale in Oslo. In concretes, several types of calcite occur, including internal Popcorn calcite formed by replacement of both thaumasite and calcium silicate hydrate. In an attempt to throw further light on the origin of carbonates and sulfates involved, we have used the laser ablation probe to characterise these secondary minerals with respect to stable isotopes (C, O and S). Mitigation as well as repair may in several cases depend much on correct characterisation and location of the fluids provenance, and stable isotopic characterisation may be an appropriate tool to do so. The preliminary results of this study indicate a complex open system with influence of fluids from several sources. There is a general difference in signatures between ordinary surface carbonation and internal carbonation associated with thaumasite. Calcite deposits within the Alum Shale/concrete contact zone show highly variable isotopic signatures reflecting a composite origin, probably significantly influenced by atmospheric CO 2 . The sulfur isotopes in thaumasite appear to be much lighter than the Alum Shale constituents. This might possibly be explained by a contribution from atmospheric SO 2 , or alternatively by sulfide oxidation in Alum Shale assisted by bacterial activity. A certain internal contribution from gypsum in cement clinker cannot be excluded.