Published Online First on April 05, 2013 © 2013 The Geological Society of London

Since alkali–silica reaction (ASR) in concrete was identified for the first time in the 1940s, many papers and books have been published to try to understand the phenomena involved. However, the reliable identification of the potential reactivity of an aggregate seems to remain a great challenge. The correct identification of potentially alkali–silica reactive rock types is important for the prevention of ASR in new concrete. Deficient knowledge of aggregates is respon-sible for part of the large amount of money spent every year on reha-bilitation of damaged structures (Fernandes et al. 2008). The methods that have been developed over the years for the identification of aggregate constituents and characterization of the alkali-reactivity potential are commonly cascaded in two subsequent stages: (1) vis-ual or optical identification and quantification by petrographic assessment, variously using both raw unprepared material and thin-sections; (2) assessing the performance of the aggregates under accelerated laboratory conditions by testing expansion potential. Although a number of standards for assessment of concrete aggregate by petrography are available (e.g. BS 7943:1999 (BS