Alkali Carbonate Reaction (ACR): Investigations on Mechanism of Dedolomitization of Dolomite in Dolomitic Limestones and Argillaceous Dolomitic Limestones

Alkali-carbonate reaction (ACR) is chemical reaction in concrete which may induce significant damage in concrete structures. Researchers have been focusing on studies of the symptoms of alkali carbonate reaction (ACR), namely expansion and loss of mechanical properties. Not enough attention has been paid to the processes of dedolomitization in dolomitic rocks. As shown in previous studies, dedolomitization in dolostones leads to the formation of a reaction rim which is similar to a nanofiltration structure, which, when thick enough, can prevent the diffusion of alkali ions and inhibit the dedolomitization process. This paper investigates the diffusion of alkali ions in dolomitic limestones and argillaceous dolomitic limestones and the process of dedolomitization of dolomite in the aggregates. Chemical, physical, and microstructural analysis of dolomitic rocks and the degree of dedolomitization were studied based on X-ray diffraction (XRD) and field emission scanning electron microscopy (SEM). It was found that the dedolomitization reaction occurred in dolomitic limestones is similar to that of dolostones where the product layer formed by the dolomitization reaction consists of an area of Mg(OH)2 crystals mixed with CaCO3 crystals. For argillaceous dolomitic limestones, some of the dolomite crystals were found to be covered by 0.5-2μm thick layer of a Mg-Al-Si phase, and the dolomite crystals wrapped by this Mg-Al-Si phase did not undergo the dedolomitization reaction, while the dolomite without such a layer shows obvious dedolomitization reaction. This suggests that the Mg-Al-Si phase prevents the dedolomitization reaction.