Thaumasit Formation in Concretes

Project Number 199

In the last few years, the possible applications of hard coal fly ash in concrete have increased considerably as the result of a number of VGB research projects. In the meantime, hard coal fly ash is also being used in high-performance concrete and in sulphate-resistant concrete.

Very recently, sulphate damage to components with a limestone additive has occurred in Great Britain which can be attributed to the formation of thaumasite (Ca₃(CO₃ · So₄ · Si(OH)₆) · 12 H₂0. Thaumasite occurs at low temperatures as the result of the penetration of sulphate ions into the concrete. The thaumasite formation destroys the concrete's binder matrix, which results in a complete loss of strength. A thaumasite problem could arise if, according to the application guide, Portland limestone cement is used in combination with hard coal fly ash in concrete in order to improve the sulphate resistance (According to DIN 1045, the use of Portland limestone cement CEM II/A-L without fly ash is not permitted for concrete with a high sulphate resistance).

Therefore, the aim of the project is to investigate the formation of thaumasite in hard coal fly ash/Portland limestone cement concretes under worst case conditions. If the results are negative, it may be necessary to reject the use of Portland limestone cement with hard coal fly ash for sulphate resistant concretes.

The investigations will be conducted at the Institute of Mineralogy at the RWTH Aachen, Prof. Detlef Heinz, and will be completed by July 31, 2001.