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Sulphate resistance of cements under the microscope.

In the context of a pre-normative research, funded by the Belgian Federal Public Service “Economic Affairs”, possible factors controlling the sulphate induced deposits of secondary minerals in cement based materials, such as ettringite, thaumasite and gypsum, were investigated. The examined influencing factors are temperature, cement type and type of sulphate solution. For this purpose, mortar test samples of different compositions were prepared and stored under different conditions. After having been exposed to a sulphate solution, several destructive and non-destructive tests were carried out. This article focuses on the microscopic analysis and the flexural and compressive tests as complementary information. The petrographic analysis reveals interesting results concerning the secondary mineral deposits within mortar bars and the related mechanical strength. The temperature of the sulphate solution has an influence on the type of secondary minerals formed and the degradation of the mortar related to these minerals. Low temperatures (5 °C) enhance the formation of ettringite/thaumasite accompanied with an intense leaching of the hardened cement paste. Also could be determined that no thaumasite was formed at temperatures higher than 15 °C. However, these higher temperatures lead to gypsum deposits which cause expansion and spalling with leaching of the cement paste.


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