Alkali-aggregate reaction-identifying reactive silicates in complex aggregates by ESEM observation of dissolution features

摘要

A petrographic examination can only be used to qualify a concrete aggregate in regard to its potential for AAR when the reactivity of the corresponding lithologies and minerals are known. In this study, two complex Swiss aggregates with polished surfaces were immersed in an alkaline solution in order to identify reactive minerals. Afterwards, dissolution and precipitation phenomena were investigated by environmental scanning electron microscopy. In general, quartz is the most reactive mineral in both aggregates. Feldspar, biotite and chlorite exhibit dissolution phenomena, and liberate alkalis and calcium. Various morphological types of gels are formed as precipitates on the polished surface: thin calcium silicate hydrate structures exhibiting card house morphology; small agglomerates (2-20 um) with a smooth surface; and large agglomerates (20-500 urn) characteristic of common ASR reaction products. These gels mainly consist of Si and Ca with variable amounts of Na, Mg, Fe and Al.