The enigma of the ‘so-called’ alkali–carbonate reaction

The so-called alkali–carbonate reaction (ACR) consists of two distinct reactions: the non-expansive dedolomitisation reaction and deleteriously expansive alkali–silica reaction of cryptocrystalline quartz hidden in the dolomitic limestone aggregate. In the past, dedolomitisation was confused with ACR and regarded as associated with the expansion mechanism, whereas alkali–silica gel was not observed using conventional optical microscopy. This led to a long-standing controversy, but the enigma has been resolved by careful petrographic examination of typical ACR concretes made with the type aggregate from the Pittsburg quarry, Ontario, including an experimental sidewalk in Kingston. This paper introduces the historical background of this issue and the essence of recent studies from the petrographic viewpoint by the authors who actually examined the typical ACR in Ontario.