On the chemistry of thaumasite formation

The expansion behavior of cement mortars with 0-25% limestone content is studied in water and magnesium sulfate solution for 2.5 years at room temperature and 7 oC. The mortar with 25% limestone cured in sulfate solution at 7 oC deteriorated after 6 months and the thaumasite salt separated from its surface. The other mortars expanded without cracking because of the ettringite formation. In parallel series on pure systems, thaumasite could be prepared at room temperature. Infrared measurements on mixes composed of silica gel, calcite and ettringite indicated the formation of an intermediate phase that forms readily at room temperature and is detected by an infrared shoulder at 1012-27 cm-1. The thaumasite salt is expected to form in a further stage as a result of a combination of lime with the intermediate phase. The reaction depends on the solubility of lime which increases at low temperature and in presence of sucrose
Author: H.Y. Ghorab, M. Rizk, D. Herfort, Y.A. Osman

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