How to identify carbonate rock reactions in concrete

This paper summarizes the modern petrographic techniques used to diagnose carbonate rock reactions in concrete. Concrete microbar specimens of the prototype RILEM AAR-5 test, provided by the Austrian Cement Research Institute, and typical Canadian concrete that had undergone alkali–carbonate reaction (ACR) were examined. Scanning electron microscopy, element mapping and quantitative analysis using electron-probe microanalyzer with energy-dispersive spectrometry (EPMA/EDS: around × 2000, <0.1 nA) were made of polished thin sections after completing polarizing microscopy. Dedolomitization produced a myrmekitic texture, composed of spotted brucite (<3 μm) and calcite within the reaction rim, along with a carbonate halo of calcite in the surrounding cement paste. However, no evidence was detected that dedolomitization had produced the expansion cracks in the cement paste, while the classical definition of alkali–carbonate reaction postulates their development. It was found that the alkali–silica reaction (ASR) due to cryptocrystalline quartz hidden in the matrix, always associated with dedolomitization in all the carbonate aggregates tested, was responsible for the expansion of both the laboratory and field concretes, even with the Canadian dolomitic limestone from Kingston, the reference material for alkali–carbonate reaction. It is suggested that the term alkali–carbonate reaction is misleading.