DBPapers
DOI: 10.5593/SGEM2014/B12/S2.092

PETROGRAPHY AND CATHODOLUMINESCENCE MICROSCOPY IN THE STUDY OF ALKALI REACTIVE GRANITIC ROCKS IN CONCRETE

I.Fernandes, H. Couto, A. Santos
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, 723-730 pp

ABSTRACT
Alkali–silica reaction (ASR), a deleterious chemical reaction in concrete that involves certain mineral phases in the aggregates and the alkalis from the cement paste, is in the origin of the deterioration of many large structures such as dams, bridges and pavements, some of which have already been demolished. Many of these structures contain fast reactive aggregates such as greywacke, sandstone, chert/flint, siliceous limestone and quartzite. ASR has been identified with slow reactive deformed rocks showing manifestations of deformation such as undulatory extinction, strain lamellae and sub-graining and also with granitic rocks with light deformation manifestations. In order to understand the features in granitic rocks that can be related with the occurrence of ASRs, concrete samples from expansion laboratory tests were selected. Alkali-silica gel was identified in the thin sections mainly in rims partially surrounding the coarse and fine aggregate particles, mainly related to crystals of quartz. Cathodoluminescence (CL) microscopy was applied in an attempt to explain the occurrence and location of alkali-silica gel and to find the possible correlation between the presence of ASRs and the crystalline defects of the quartz grains in the rock. The quartz grains showed heterogeneous luminescence, although always very weak, in reddish-brown. The study performed evidenced there are open spaces between the quartz grains, close to the places where gel was found. In fact, the contacts between the different grains are opened and highlighted by a lighter brown material. Cracks, not visible under polarizing microscope, were observed, showing a lighter luminescence colour. The study showed that CL microscopy is an important tool to complement the optical microscopy observation used in concrete petrography as it contributes to the identification of the features in the aggregates that can be in the origin of ASR. This technique can therefore constitute a promising useful input for the future identification of the compositional and textural features in new aggregates and can allow the classification of their potential alkali reactivity before the use in concrete.

Keywords: Alkali-silica reaction, deformed quartz, cathodoluminescence.