Versão online: http://www.lneg.pt/iedt/unidades/16/paginas/26/30/185 Comunicações Geológicas (2014) 101, Especial III, 1133-1136 IX CNG/2º CoGePLiP, Porto 2014 ISSN: 0873-948X; e-ISSN: 1647-581X Alkali-silica reactions with volcanic aggregates in Santa Maria Island, Azores Agregados vulcânicos e reacções álcalis-sílica na ilha de Santa Maria, Açores S. Medeiros1*, I. Fernandes2, J. C. Nunes1 Artigo Curto Short Article © 2014 LNEG – Laboratório Nacional de Geologia e Energia IP Abstract: Alkali-silica reactions have caused serious expansion and 2Departamento de Geociências, Ambiente e Ordenamento do Território, cracking in concrete structures worldwide. Volcanic rocks have been Faculdade de Ciências da Universidade do Porto e Centro de Geologia da found to be potentially alkali-reactive in a number of countries such Universidade do Porto, Porto, Portugal. * as Argentina, Iceland, Japan, New Zealand and Turkey. Corresponding author / Autor correspondente: [email protected] The Azorean rocks used as aggregates are mainly basalts and some trachytes. The characterization of the reactivity of these aggregates has been implemented through the research project ReAVA 1. Introduction (Characterization of Potential Reactivity of the Volcanic Aggregates The Azores archipelago is the most occidental region of from the Azores Archipelago: Implications on the Durability of Concrete Structures) that comprises mainly the petrographic Portugal and consists of nine volcanic islands located in assessment of the aggregates. In Santa Maria Island aggregates were the North Atlantic Ocean. The archipelago magmatism collected from two important quarries and also four concrete cores belongs to the alkaline series with compositional were drilled in the local airport. The petrographic examination and characteristics that range from basalts to trachytes. the geochemical analyses of the rock samples showed a significant Alkali-silica reaction (ASR) is a deterioration alteration of these rocks. The petrographic examination is accepted phenomenon that occurs between alkaline (Na+ and K+) internationally as the first step in the assessment of aggregates. The - concrete petrography of the airport pavement showed that the cement and hydroxyl (OH ) ions in the cement and certain reactive paste has almost completely been replaced by an alkali-silica gel, forms of silica in the aggregates. The reactivity of volcanic which was confirmed by scanning electron microscopy. rocks is usually associated with the presence of volcanic Keywords: Volcanic aggregates, Alkali-silica reaction, Petrography, glass, altered minerals and SiO2 content of the rock Azores islands. (Korkanç & Tuğrul, 2005). The deterioration of concrete Resumo: As reações álcalis-sílica têm causado, a nível mundial, a by ASR is well known in several countries such as Japan, deterioração de um grande número de estruturas de betão. As rochas Iceland and Turkey. In Portugal, the only information vulcânicas utilizadas como agregado têm sido consideradas about the alkali performance of volcanic rocks is related to potencialmente reativas em vários países como a Argentina, Islândia, the study of the pavement of Santa Maria Island airport in Nova Zelândia e Turquia. the Azores archipelago (Braga Reis et al., 1996). Os agregados açorianos empregues na produção de betão são The ReAVA research project is a R&D project under a principalmente basaltos e mais raramente traquitos. A caracterização da reatividade destes agregados tem sido implementada através do business context with contributions from private and projeto de investigação ReAVA (Caracterização da reatividade public partners. Aggregates from a total of thirteen places potencial de agregados vulcânicos do Arquipélago dos Açores: (quarries, excavation and crushing plant) were collected in implicações na durabilidade do betão) que compreende all the islands to produce thin sections and to perform principalmente o exame petrográfico dos agregados. A análise expansion tests in laboratory. Corvo Island was the only petrográfica é aceite internacionalmente como o primeiro método de estudo a desenvolver para a avaliação da reatividade potencial aos exception due to the inexistence of local aggregate álcalis. Na ilha de Santa Maria foram recolhidos agregados de duas production. One of the main goals of this project is the importantes pedreiras e foi realizada uma campanha de amostragem petrographic assessment of both aggregates, for the no aeroporto local. O exame petrográfico e a análise química classification towards potential reactivity, and concrete, for evidenciam uma alteração significativa das rochas amostradas. A the identification of cracks and ASR products. petrografia do betão proveniente do pavimento do aeroporto mostra Santa Maria Island is the oldest island of the Azores que a pasta de cimento junto a grandes fissuras no pavimento foi substituída por um gel sílico-alcalino, tal como foi confirmado archipelago with 8.12 Ma (Abdel-Monem et al., 1975). It através do microscópio eletrónico. is formed by volcanic rocks of basic composition and Palavras-chave: Agregados vulcânicos, Reações álcalis-sílica, sedimentary rocks (e.g. calcareous fossiliferous Petrografia, Ilhas dos Açores. sediments). In order to identify the potential reactivity of Santa Maria aggregates a program was carried out to 1Universidade dos Açores, Departamento de Geociências, Apartado 1422, produce thin sections from: (1) rock samples of two 9501-801 Ponta Delgada, Açores, Portugal. 1134 S. Medeiros et al. / Comunicações Geológicas (2014) 101, Especial III, 1133-1136 quarries of the island and (2) concrete core samples of used to plot the data on a TAS diagram in order to obtain Santa Maria airport pavement. the correct designation of the rock. The geochemical analyses of these basanites show a relatively high value of 2. Materials and methods loss on ignition (LOI), suggestive of an alteration that formed iddingsite and analcite (in SMA-SM1). 2.1. Quarries Both basanites are composed of olivine with thick The rock samples (SMA-SM1 and SMA-SM2) were iddingsitized rims, clinopyroxene, plagioclase collected in two quarries of the island. One of the quarries is phenocrysts and opaques (ilmenite and magnetite) with a the main source of aggregates in Santa Maria Island. This matrix formed by the same minerals but showing a lower quarry is part of a submarine cone with a small amount of amount in olivine. In SMA-SM1 sample carbonate subaerial pyroclasts on the top (Serralheiro, 2003). The minerals were identified filling some of the cracks of the quarry shows submarine lava flows with fractured and rock. The scanning electron microscope study (SEM) and altered zones and intercalated levels of reddish the analysis by energy dispersive spectrometry (EDS) hialoclastites. The other quarry was exploited for the airport also detected anhedral intergranular analcite in several construction in the mid of 1940's and is no longer in activity. areas (Medeiros et al., 2012). In SMA-SM2 iddingsite The rocks show some signs of alteration which is revealed minerals seem to fill some of the cracks present on the by the spheroidal weathering of the basalt in several areas of rock. Volcanic glass was not found in any of these rocks. the quarry. The concrete cores from the Santa Maria Airport Four concrete cores were drilled in Santa Maria Airport pavement were drilled in the places where the concrete pavement aiming at characterizing the aggregates included was cracked, as identified during site inspection. The cores in the manufacture of the concrete and to identify possible sampled close to the longer longitudinal cracks show a products of alkali-silica reactions. white deposit covering the crack walls. White rims are also found lining the coarse aggregate particles (Fig. 1). The 2.2. Aggregate and Concrete Petrography concrete petrography confirmed that the aggregates are The petrographic examination was performed under a composed of crushed basalt. The sand fraction contains polarizing microscope Nikon Eclipse E 400 POL, with mainly mineral grains of olivine, pyroxene and plagioclase automatic camera AXION cam MRC. The microphotographs minerals. However, several sand particles are composed were obtained by Axion Vision 3.1. software. In order to mainly of rock fragments in which volcanic glass is the determine the composition of the reaction products, a dominant component. The glass is light-brown, yellowish scanning electron microscope (SEM) FEI QUANTA 400 or dark-brown, always isotropic. The observation by FEG ESEM/EDAX PAGASUS X4M equipped with energy SEM/EDS showed that the glass is mainly composed of Si, dispersive spectrometer (EDS) was used. Al and much lower Ca, Mg, Na and K (Fig. 2a, b, c). The volcanic glass is, in some locations, strongly altered to 3. Results clay minerals which form yellow rims. In the concrete cores of the deeply cracked concrete several microcracks The two samples (SMA-SM1 and SMA-SM2) were were observed crossing both the aggregates particles and classified according to their mineralogical composition and the cement paste. textural features. The rock bulk chemical analysis was Fig. 1. Concrete core showing the longitudinal crack observed during the site inspection. White rims are identified in the interfaces between the aggregate particles and the cement paste. Fig. 1. Carote de betão mostrando a fissura longitudinal observada na inspeção local. Identificam-se orlas de um produto de reação nas interfaces entre as partículas de agregado e a pasta de cimento.