Maarten Broekmans has studied the alkali-silica reaction in two Dutch concretes by means of optical microscopy, geochemical analyses on bulk material and after selective digestion in acid, and with element mapping in polished thin sections. He furthermore characterized the nature of the silica/quartz in the Norwegian mylonites studied previously by Wigum in his PhD-thesis (1995), in the Ohio cherts previously studied by Kneller (1967), and some Dutch cherts by assessing their crystallinity indices following the XRD-method of Murata&Norman (1976). The thesis suggests a simple and straight-forward system to rate concrete damage by assessing and classifying the crack fabric in impregnated full cores under fluorescent illumination. This method does not distinguish between different causes for the observed cracking, but it is quick and does not require expensive instrumentation nor special training. However, identifying the true cause of the cracking is of course essential and can be done as a second stage by thin section petrography. Typically, chert is the main alkali-reactive constituent in Dutch concrete. However, petrographic observations on sand- and siltstone grains in the aggregate indicate that these generate ASR-gel as well, which was not generally accepted in the Netherlands until before recently. The necessary alkali may very well be provided by interstitial clay minerals, and eventually by detrital muscovite on a very local scale with very limited reach. In geological-sedimentary literature, the catalytic effect of sheet silicate minerals (clays, micas) upon the dissolution of quartz is well documented. There is a strong resemblance of the observations made in the ASR-concrete to the features described from sedimentary-diagenetic sandstone compaction, and a similar process is suggested to occur in ASR. This is partly supported by element maps on Na, K, Ca, Si, Fe and S on intact and ASR-cracked chert, and intact and ASR-cracked sandstone. There are marked differences in their alkali-household. The ...