DBPapers
DOI: 10.5593/SGEM2014/B12/S2.091

PETROGRAPHIC IDENTIFICATION OF ALKALI-REACTIVE AGGREGATES, INCLUING SILICEOUS CARBONATES

I. Fernandes, M. A. T .M. Broekmans, M. A. Ribeiro, H. Martins, I. Sims
Wednesday 1 October 2014 by Libadmin2014

References: 14th International Multidisciplinary Scientific GeoConference SGEM 2014, www.sgem.org, SGEM2014 Conference Proceedings, ISBN 978-619-7105-08-7 / ISSN 1314-2704, June 19-25, 2014, Book 1, Vol. 2, 715-722 pp

ABSTRACT
Modern day Portland-cement concrete was for decades considered to be a long lasting material, until in the 1940s it was recognised that concrete could deteriorate as the result of chemical reactions between Portland cement and some components of the aggregates. Research has been undertaken since then in order to find the best test methods for identifying the potential reactivity of aggregates before their use in new constructions. According to international guidelines, rocks are classified as ‘innocuous’ or ‘potentially reactive’ according to their mineralogical composition, texture and performance. The first method in the assessment of aggregate is optical thin-section petrography, a quick method when compared to laboratory expansion methods. Like any analytical method, petrographic assessment is subject to a number of potential errors from sample handling, analytical protocol and instrumentation, which can be minimized using rigorous procedures. However, operator error (related to petrographer experience and skill) usually exceeds analytical error. To reduce operator error, a worldwide petrographic atlas of potentially reactive aggregates was developed by an expert team of RILEM technical committee TC-219 ACS. It collates a large number of samples from all continents, from different types of rocks, revealing the microscopic differences that explain their varying performance as concrete aggregate. The present paper focuses on the main findings of this work, referring particularly to the microscopic characterisation of siliceous carbonate rocks. Supplementary analyses by SEM and EDS proved to be essential for the unequivocal identification of constituents in these very-fine grained rocks.

Keywords: Aggregates, alkali-silica reactions, texture, concrete petrography