Alkali-reactivity of a Swiss siliceous limestone caused by finely dispersed quartz

The present study was conducted to provide a better understanding of the alkali-silica reactivity (ASR) of siliceous limestones in concrete applications. A Swiss siliceous limestone, which is potentially reactive, was subjected to a series of tests. Microbar tests were first conducted to classify the limestones reactivity with respect to ASR. Two other methods were then used to characterize the amount of SiO2 present. First, an analysis of the chemical composition with petrographic and mineralogical characterizations was carried out, and second, a model reactor was used to characterize the silica reactivity. The results showed a siliceous reactive limestone. In petrography, three main types of carbonate lithology were outlined according to their SiO2 and glauconitic content. The emphasis was placed on the types of SiO2 and their sizes: a finely dispersed micro- to crypto-quartz has been highlighted by SEM and TEM, and analysed as quartz. The reactive free SiO2 content, obtained by a mineralogical calculating method, completes the petrographic results. The proportion of reactive free SiO2 could be essentially attributed to the presence of finely dispersed micro- to crypto-quartz from TEM observations which is probably very reactive towards the ASR due to its small-size. The presence of finely dispersed micro- to crypto-quartz is important to consider, due to its potential effects on the concrete mixture. The grain size of SiO2 seems to be a supplementary key parameter, in addition to crystallinity and strain, since it can promote ASR development with some siliceous aggregates.