Quantitative assessment of alkali-reactive aggregate mineral content through XRD using polished sections as a supplementary tool to RILEM AAR-1 (petrographic method) Nélia Castro a, ⁎, Bjørn E. Sorensen a , Maarten A.T.M. Broekmans b a Norwegian University of Science and Technology (NTNU), Department of Geology and Mineral Resources Engineering, Sem Sælands vei 1, N-7491 Trondheim, Norway b Geological Survey of Norway, Department of Industrial Minerals and Metals, PO Box 6315 Sluppen, N-7491 Trondheim, Norway abstract article info Article history: Received 22 June 2012 Accepted 9 August 2012 Keywords: Alkali-aggregate reaction (C) Aggregate (D) X-ray diffraction (B) Petrography (B) Opaline silica The mineral content of 5 aggregate samples from 4 different countries, including reactive and non-reactive aggregate types, was assessed quantitatively by X-ray diffraction (XRD) using polished sections. Additionally, electron probe microanalyzer (EPMA) mapping and cathodoluminescence (CL) were used to characterize the opal-CT identified in one of the aggregate samples. Critical review of results from polished sections against traditionally powdered specimen has demonstrated that for fine-grained rocks without preferred orientation the assessment of mineral content by XRD using polished sections may represent an advantage over tradi- tional powder specimens. Comparison of data on mineral content and silica speciation with expansion data from PARTNER project confirmed that the presence of opal-CT plays an important role in the reactivity of one of the studied aggregates. Used as a complementary tool to RILEM AAR-1, the methodology suggested in this paper has the potential to improve the strength of the petrographic method. © 2012 Elsevier Ltd. All rights reserved. 1. Introduction Alkali-aggregate reactions (AAR) cause severe damage in concrete structures worldwide. The most widespread type of AAR is the alkali– silica reaction (ASR), in which alkali–reactive silica sensu lato in the aggregate forms a hygroscopic and hydraulic gel with alkali inherited from the cement paste. The alkali gel expands upon hydration and cracks up the surrounding concrete, thereby reducing structure service-life and increasing cost for society. The incubation time need- ed before ASR damage starts ranges from a few months to several de- cades, much depending on aggregate type, binder type, and exposure climate. There are two generalized classes of siliceous aggregates known to be potentially reactive with alkalis in concrete [1]: the normally reactive aggregates (those that react in a time scale of 5 to 20 years) and the slowly reactive aggregates (those that react in a time scale greater than 15–20 years). Normally reactive aggregates are characterized by the presence of very fine grained quartz and disorder forms of silica (e.g. opal, chalcedony). Slowly reactive aggregates are typically crystal- line quartz-bearing rock types (e.g. mylonite, granite, gneiss, quartzite, greywacke, phyllite, and argillite). Though the exact mechanism of AAR is a matter of dispute, there is general consensus that AAR at some point involves silica dissolution. Silica dissolves at extreme pH values in strongly acidic or strongly alkaline conditions, and less around neutral pH. For this reason it is designated as an amphoteric material. Extreme values of pH are not very often found in geological ambient. However, the pore solution of concrete is an extreme alkaline environment (pH > 13), which will be a driving factor on the dissolution of silica minerals within the aggregates. A summary of a number of fundamental consider- ations related to the solubility of silica minerals under geological ver- sus ASR conditions can be found in [2–4]. One of the key issues pointed out by [3,4] is that different silica species have different dis- solution rates. Thus, the predominant silica species largely governs the alkali-reactivity potential of the aggregate, a disordered silica structure being more reactive than cristobalite, which in turn is more reactive than ‘orderly quartz’ [5]. Test methods to assess the ASR-potential of aggregate for concrete have been under development for several decades. To meet the needs of the building and construction industry these test methods are re- quired to provide an accurate and precise result in the shortest time possible using the least resources possible. The petrographic method is used as a “first step” to assess the potential alkali-reactivity of aggregates for concrete followed by accelerated laboratory tests to confirm the results obtained. Mortar or concrete prisms are exposed to severe conditions of temperature and alkalinity to provoke expan- sion within days, weeks or years, depending on the method. Different European test methods were evaluated for their suitability for use with the wide variety of aggregates found across Europe in the EU-funded PARTNER project. The overall experience from the PARTNER project is that in most cases the RILEM tests could successfully identify the Cement and Concrete Research 42 (2012) 1428–1437 ⁎ Corresponding author. Tel.: +47 73 59 48 41; fax: +47 73 59 48 14. E-mail address: nelia.castro@ntnu.no (N. Castro). 0008-8846/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.cemconres.2012.08.004 Contents lists available at SciVerse ScienceDirect Cement and Concrete Research journal homepage: http://ees.elsevier.com/CEMCON/default.asp