Thaumasite Field Trial at Shipston on Stour: Three Year Chemical and Mineralogical Assessment of Buried Concrete

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Title: Thaumasite Field Trial at Shipston on Stour: Three Year Chemical and Mineralogical Assessment of Buried Concrete

Author(s): N.J. Crammond, R.G. Sibbick, and G. Collett

Publication: Special Publication

Volume: 234

Issue:

Appears on pages(s): 539-560

Keywords: field trial; microscopy; sulfate thaumasite attack

Date: 3/22/2006

Abstract:
Researchers at the Building Research Establishment in the UK have recently excavated buried concrete specimens from a field trial site in central England. The specimens had been exposed to sulfate-bearing groundwater for a period of three years and as a consequence, many had undergone the thaumasite form of sulfate attack (TSA). Conventional sulfate attack, leading to the formation of ettringite and/or gypsum did not play a role in the deterioration process. TSA requires a source of carbonate ions, which is usually supplied through the use of a carbonate aggregate but, in this study, it has occurred in portland cement concretes containing an all-in siliceous aggregate. Evidence therefore points to carbonate ions dissolved in the groundwater as being an additional source. A selection of the concretes were examined using X-ray diffraction, chemical analysis and microscopy studies and this paper discusses the results obtained. A variety of binder/aggregate combinations were investigated and a clear distinction could be made after three years between TSA-susceptible and TSA-resistant mixtures. An approximate ‘depth of attack’ measurement has been determined for each of the susceptible mixtures. It was noticed however, that several of the unattacked concretes showed high total sulfate values (in excess of 6% by weight of binder) in their outer surfaces. This was a surprising result, the further investigation of which is reported in this paper.