When bridges, dam walls, and other structures made of concrete are streaked with dark cracks after a few decades, the culprit is a chemical reaction termed the “alkali-aggregate reaction” (AAR). Also called the “concrete disease” or even “concrete cancer,” it is a reaction between substances contained in the material and moisture seeping in from outside. AAR damages concrete structures all over the world and makes complex renovations or reconstructions necessary. Researchers from the Paul Scherrer Institute (PSI), the largest research center for natural and engineering sciences in Switzerland, and the Swiss Materials Science Lab Empa have now solved the structure of the material produced in the course of AAR at the atomic level – and have thereby discovered a previously unknown crystalline arrangement of the atoms. The results were published online on October 14, 2015 in Cement and Concrete Research, the top journal in the field of building material research. The article is titled “Application of Micro X-Ray Diffraction to Investigate the Reaction Products Formed by the Alkali-Silica Reaction in Concrete Structures.” Researchers from the PSI teamed up with colleagues from Empa to study a degenerative sign of aging in concrete: the so-called alkali-aggregate reaction (AAR). In the course of AAR, a material forms that takes up more space than the original concrete and thus gradually cracks the concrete from within as the decades go by. The researchers have now explored the exact structure of this material. They managed to demonstrate that its atoms are arranged extremely regularly, making it a crystal. They also showed that the structure of this crystal is a so-called “sheet-silicate structure.” This specific structure had never been observed before.
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