Thaumasite and secondary calcite in some Norwegian concretes

Abstract

Abstract Thaumasite formation (TF) and limited thaumasite form of sulfate attack (TSA) has recently been detected in several Norwegian sprayed concretes. TF and TSA is frequently associated with contemporaneous and late stage internal calcite formation by: (a) decalcification of calcium silicate hydrate (CSH): (b) decomposition of thaumasite associated with secondary liberation of SO 4 2− and occasional formation of subordinate gypsum; (c) supersaturation of fluids in voids. Popcorn calcite and other textural forms were characteristic for these reactions. Also co-precipitation of popcorn calcite + thaumasite, as well as later stage dissolution of both minerals occurred. The entire process was represented by a drop in pore fluid pH from about 13 towards 5–7. In this paper we study the TF–TSA–carbonation process in several environments: (1) three examples of 2–13 years old steel fibre reinforced sprayed concrete made with Sulfate Resisting Portland Cement (SRPC) and silica fume in contact with carbon-, calcite- and sulfide bearing Alum Shale: (2) two examples of ≈30 years old, and severely damaged, SRPC based sprayed concrete within the Alum Shale: (3) one 16 years old sprayed concrete made with Portland Cement (PC) and possibly fly ash in presence of sulfate bearing ground water and (4) one 10 years old steel fibre reinforced sprayed concrete in a sub-sea tunnel with inflow of somewhat modified seawater. This PC based concrete with silica fume had suffered localised crumbling and mush formation after less than 5 years. The critical factors for thaumasite formation are discussed together with consequences for further deterioration and timing of repair.