Alkali-silica reactions in granite-based aggregates: The role of biotite and pyrite

Abstract

Concrete structures built in granite-based aggregate develop and proliferate contour-like cracks as they aged. Such cracks are often associated to alkali-aggregate reactions (AAR). Though, in most cases, the alkali-silica reaction (ASR) involving these aggregates has shown to be non-destructive to concretes, their contents of iron-contained minerals, including biotite and pyrite predict novel chemical route for AAR. This study aims to investigate the alkali reactivity of granite-based aggregates, and the rule of biotite and pyrite as potential contributors to AAR. Aggregate samples were collected from three different deposits in tropical Africa, denoted: Deposit I, Deposit II and Deposit III. Mineralogical and microstructural characteristics of the aggregates, by XRD, optical microscopy and ESEM-EDX, indicated micaceous, felspathic, garnet and sulphide mineral groups. XRF shows quite significant levels of FeO + Fe2O3, ranging from 6.38 wt% to 9.86 wt%. Optical microscopy and ESEM analysis of NaOH-leached residues showed quite elevated fractions of Fe Sulphur was quantified in the leached residues of most corroded aggregate samples from deposit III, pointing to the dissociation of pyrite. The absence of S and the presence of ample amounts of Fe in the less degraded aggregates leached residues of deposits I and II points to the release of Fe by other iron containing minerals, including biotite. Findings of this study is a reference point for the development of control and mitigation of AAR involving pyrite and biotite – rich aggregates.