[1 ; from left: Visible structural deterioration due to ACR and ASR in New Jersey; Cracking within aggregates in reactive carbonate aggregates surrounded by high-alkali cement.]

First observed in Ontario in 1957, the alkali-carbonate reaction (ACR) is the less well understood of two expansive reactions that can occur between alkalis and certain aggregates when mixed to form concrete. The primary source of alkalis in concrete is thought to be cement; however, alkalis are also present in groundwater, seawater, and fertilizers [1]. The alkali content of cement varies widely across North America. ACR causes expansion within the aggregates due to the breakdown of dolomite [2].
Because the expansion occurs within aggregates, ACR can result in localized aggregate cracking or, if the reaction occurs on a large enough scale, cracking and deterioration of the concrete mass overall. The scale and severity of the reaction depend on (1) the alkali content of the concrete mixture, in particular the cement and (2) the chemical composition of the aggregates used.
Characteristics of ACR are often not visible except on a microscopic scale, and laboratory testing may be required to confirm ACR [1].
ACR is less common in modern construction due to careful aggregate selection [2].

(1). In eastern Canada, where ACR is known to cause significant problems, the alkali content of cement has been found to exceed 1.5% in some cement manufacturing plants; in Virginia, where ACR is less of an issue, the alkali content of most cements ranges from 0.55 to 0.70%. Changes in cement production methods are thought to have increased the amount of alkalis in cement [1].
(2) The aggregates associated with ACR are of impure dolomitic limestone [1]. However, alkali-silica reaction (ASR) can also occur within dolomitic limestone aggregates.
The proposed chemical mechanism of ACR is

CaMg(CO3)2 + 2 NaOH → CaCO3 + Na2CO3 + Mg(OH)2

Brucite (Mg(OH)2) is thought to be the direct cause of expansion in the aggregate [3].

Gitanjali Bhattacharjee