Structure of ‘concrete disease’ solved

Researchers from the Paul Scherrer Institute PSI teamed up with colleagues from the Swiss Materials Science Lab Empa to study a degenerative condition in concrete: alkali-aggregate reaction (AAR) - also called ‘concrete disease’ or ‘concrete cancer’. In the course of AAR, a material forms that takes up more space than the original concrete and gradually cracks the concrete from within as decades go by.

The researchers have now explored the exact structure of this material. They managed to demonstrate that its atoms are arranged regularly, making it a crystal. They also showed that the structure of this crystal is a sheet-silicate structure. This specific structure had never been observed before. The research results could help towards the development of more durable concrete in future.

AAR is a chemical reaction that affects outdoor concrete structures all over the world. It happens when concrete is exposed to water or moisture. According to the researchers, the basic ingredients of concrete itself cause the problem: cement contains alkali metals such as sodium and potassium. Any moisture infiltrating the concrete reacts with these alkali metals, leading to the creation of an alkaline solution.

The second main ingredient in concrete is sand and gravel, which in turn are composed of minerals, such as quartz or feldspar. Chemically speaking, these minerals are silicates. The alkaline water reacts with these silicates and forms an alkali calcium silicate hydrate. This is able to absorb more moisture, which causes it to expand and gradually crack the concrete from within. This entire process is referred to as AAR.

AAR takes place extremely slowly; the cracks are initially invisible to the naked eye. Over the course of three or four decades, however, the cracks widen and eventually jeopardise the durability of the entire structure.

“Most structures currently suffering from AAR were built between the 1960s and 1980s,” explained Erich Wieland, head of the Cement Systems Group at PSI. “The research community in Europe only became aware of the AAR problem in the 1970s.”

The researchers studied the make-up of a Swiss bridge constructed in 1969, which has been affected by AAR. Researchers from Empa cut out a material sample from the bridge and ground it down until they had a wafer-thin sample measuring 0.02mm thick. The sample was taken to the Swiss Light Source and irradiated with an X-ray beam, fifty times thinner than a human hair. Through diffraction measurements and complex data analysis, the PSI researchers were able to determine the crystal structure of the material with pinpoint precision.

They found that the alkali calcium silicate hydrate has a previously undocumented sheet-silicate crystal structure.

Andreas Leemann, head of the Concrete Technology Group at Empa, explained: “In principle, it’s possible to add organic materials to the concrete that are able to reduce the build-up of tension. Our results provide a scientific basis for these considerations and could pave the way for the development of new materials.”

Tom Austin-Morgan

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