This paper presents research carried out on carbonate aggregate – on three dolomite aggregates, one dolomitic limestone and one limestone – reference. Their physical and mechanical properties, structure and chemical composition were determined, and then made from them a samples of mortar and alkali reactivity tests were performed according to the AAR-2 method (developed by RILEM), keeping the reference samples in water. On the samples after the alkali–reactivity test and on the reference samples the comparative studies were performed, including compressive strength, porosity, phase composition by X-ray diffraction and thermal analysis. Based on the results of tests it can be said that with more advanced dedolomitization reaction, the porosity of mortar and a decrease in compressive strength after testing is less.