A new crystal
Even if the chemical processes involved in AAR have long been known, nobody had identified the physical structure of the alkali calcium silicate hydrate formed in the course of AAR. The researchers at PSI and Empa have now managed to fill this knowledge gap. They studied the substance of a Swiss bridge constructed in 1969, which has been affected heavily by AAR. Researchers from Empa cut out a material sample from the bridge and ground down a small piece of it until they were left with a wafer-thin sample that was merely 0.02 millimetres thick. The sample was then taken to the Swiss Light Source SLS and irradiated with an extremely narrow x-ray beam, fifty times thinner than a human hair. Performing so-called diffraction measurements and a complex data analysis, the PSI researchers were eventually 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. Normally, discovering an uncatalogued crystal structure means you get to name it, explains Rainer Dähn, the first author of the study. But it has to be a crystal found in nature, therefore we didnt get that honour, says the researcher with a smile. Andreas Leemann, Head of the Concrete Technology Group at Empa, had the idea for the current study. The researchers from PSI then brought their knowledge of the x-ray beam method to the table. In principle, its possible to add organic materials to the concrete that are able to reduce the build-up of tension, explains materials scientist Leemann. Our new results provide a scientific basis for these considerations and could pave the way for the development of new materials.
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