Microstructural Consequences of Alkali-Carbonate Reaction

During alkali-carbonate reaction (ACR), dolomite present in the aggregate particles reacts with the hydroxide ions present in the concrete pore solution. The mechanism of expansion of ACR has generated controversial discussion in the scientific community. A detailed microstructural investigation was performed to characterize the changes in a concrete containing ACR-susceptible Kingston carbonate rock as aggregates. Four types of reaction products are present in the aggregates resulting from the dissolution of dolomite, illite and quartz. However, no alkali-silica reaction (ASR) products were observed. The cement paste adjacent to reactive aggregates is altered with the formation of a patchy layer of calcite embedded in de-calcified C-S-H with a high alkali content. A decrease of pH together with this alteration was indicated by the absence of S-containing phases within the reaction rim.