The occurrence of thaumasite in modern construction – a review

Abstract

This paper provides a review of the reported cases of deterioration in modern construction, where the formation of the sulfate-bearing mineral thaumasite has been identified. The formation of thaumasite can cause varying degrees of deterioration within a wide variety of building systems including lime–gypsum plasters, exposed sulfate-bearing brickwork, concrete foundations and floor slabs exposed to sulfate-bearing ground, concrete roads and sub-bases, sewer pipes, tunnel linings and old blastfurnace slags used as landfill. In Spring 1998, the thaumasite form of sulfate attack was found in the foundation concrete of 10 motorway bridges in Gloucestershire, western England. As a result, the UK Government convened the Thaumasite Expert Group (TEG), who were commissioned to prepare a Report `The thaumasite form of sulfate attack: Risks, diagnosis, remedial works and guidance on new construction' [Thaumasite Expert Group, The thaumasite form of sulfate attack: Risks, diagnosis, remedial works and guidance on new construction, DETR, London, 1999]. This paper was one of the references used in the preparation of the TEG Report, which was published in January 1999. Deterioration as a result of the formation of thaumasite has become recognised as a separate form of sulfate attack, which has the potential to affect a wide variety of components and a range of building materials. The full extent of the problem is still unknown. There are two distinct ways in which thaumasite can precipitate as a reaction product, especially within concretes and mortars. These have been termed the thaumasite form of sulfate attack (TSA) and thaumasite formation (TF) by the Thaumasite Expert Group. TSA refers to cases where there is significant damage to the matrix of a concrete or mortar as a consequence of replacement of cement hydrates by thaumasite. TF, on the other hand, refers to incidences where thaumasite can be found in pre-existing voids and cracks without necessarily causing deterioration of the host concrete or mortar. In relation to the current paper, it was not always easy to identify retrospectively whether the occurrence of thaumasite in the reported field cases was associated with TSA or TF. Distinctions were therefore only made when it was possible to do so. The references reviewed in this paper span a time period of over thirty years and some of these, especially the more recent UK cases, prompted the TEG to provide new interim recommendations in order to minimise the effect of TSA in new construction. However, in many of the remaining cases reported in the following paper, deterioration could have been avoided simply by following good practice of the period, either by the use of more suitable materials for given service conditions or by better workmanship on site.