Carbonate aggregates will react with alkalies from the cement paste in concretes under certain conditions. Several types of reaction are possible, and one involving dolomitic aggregates which contain intergranular clays can lead to disruptive expansion in the concrete. This reaction of dolomite with cement alkalies is essentially one of dedolomitization, a complex reaction rim containing three recognizable zones develops at the interface between the cement paste and aggregate. The details of the reaction processes and of ionic migration with time have been followed using electron probe microanalysis. The effect of the presence of additional alkalies and of sodium chloride on the reaction processes and speed are also discussed on the basis of microanalytical data.