The microscopical characterisation of thaumasite

Abstract BRE’s Microscopy unit have been extensively involved in the identification of new and unusual occurrences of the thaumasite form of sulfate attack (TSA) and thaumasite formation (TF). Using both optical and scanning electron microscopy a substantial database of information describing the location, form, composition and characteristics of TSA and TF in a series of different examples both within the UK and abroad has been developed. This paper describes the form taken by the TSA/TF within these various localities, and compares it with the characteristic taken by naturally occurring thaumasite. Earlier work on the M5 concrete bridges in Gloucestershire, UK defined a four-stage degradation process, resulting in the formation of at least three microscopically distinct forms of thaumasite. This paper takes these basic stages and assesses their presence and makes some revisions to the characteristics of the TSA reaction, and its evolution within lower quality cement-derived materials, such as mortars and masonry. Finally the observed strong association of TSA and TF, with the “Popcorn” calcite form of deposition within more depleted areas of affected cement paste from degraded concrete-based material is also discussed along with a proposed mechanism of deterioration.