Origin and mobility of alkalies in two Dutch ASR-concretes. II: Microscale element distribution around sandstone and chert. Implications for the mechanism of ASR

River gravel used as aggregate for concrete in the Netherlands contains several potentially deleterious components with respect to alkali-silica reaction (ASR), viz. porous chert, chalcedony, and impure sandstones (greywackes. mica- and sericite-rich sandstones, siltstones, arkoses etc.). Whereas cherts and chalcedonies are virtually free from alkalies prior to their incorporation in concrete, impure sandstones are not. Current regulations therefore impose a cumulative limit in terms of Na2O-equivalents on concrete. i.e. the sum of bulk Na20-equivalents of individual Components (cement, aggregate. Mier. additives, etc.). However, for understanding the fundamental mechanism of alkali-silica reaction itself, knowledge of the internal relationships within concrete, i.e. exchange between different types of aggregate (chert, impure sandstone), cement paste and fluid phase, is essential.