Expansion stress during sulphate attack on concrete - effect of additions containing aluminium

• Applicant: Dr.-Ing. Anne Heisig, since 11/2020
• Subject Area: Construction Material Sciences, Chemistry, Building Physics
• Term: from 2015 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 274235994

External sulfate attack on concrete structures in contact with sulphate-containing groundwater can lead to damage caused by the formation of ettringite and thus an expansive stress resulting in damage and high repair costs. In the first funding period, the effect of the aluminium content in Portland cement binder compositions containing ecologically and economically advantageous aluminium-rich mineral additions on the expansion stress was measured with test specimens stored in sulphate solutions. The results indicate that the aluminium content plays only a minor role compared with the pore size distribution.The investigations also showed that the reaction fronts occurring during external sulphate attack can be reliably monitored using modern spatially resolved analysis techniques. In the case of Portland cement with enhanced sulphate resistance, gypsum formation was observed to follow the formation of ettringite which produces a high degree expansion. While in-depth knowledge has been obtained on the damage mechanism of ettringite formation, the effect gypsum formation on damage development is unknown. Since gypsum formation often occurs during conventional sulphate resistance tests due to the high sulphate concentrations used, a deeper understanding of the consequences of gypsum formation, particularly with regard to sulphate-resistant Portland cements with enhanced sulphate resistance, is desirable. Surprisingly, the investigations also revealed that specimens with sulphate-resistant Portland cements do not exhibit macroscopic cracks despite a very high free expansion. Whether this phenomenon is directly related to the increased gypsum formation is the subject of this research application. It is conceivable that new micro cracks due to ettringite formation are filled with gypsum which inhibits crack growth.The proposed work programme includes the spatially resolved investigation of the expansion fronts at different low sulphate concentrations in order that the effect of gypsum precipitation can be systematically determined. In the case of Portland cements with increased sulphate resistance, this should lead to more macroscopic cracks if less gypsum precipitates. The work programme includes sulphate storage tests with hollow cylinders, with and without restraint, because the present results have shown that restraint inhibits gypsum formation. As in the first funding period, finite element modelling and thermodynamic calculations are used to aid the interpretation of the experimental results. The findings of the research project are intended to assist the design of more precise sulphate resistance test methods.

• DFG Programme: Research Grants
• Co-Investigator: Robin Edward Beddoe, Ph.D.
• Ehemaliger Antragsteller: Professor Dr.-Ing. Detlef Heinz, until 10/2020