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
cracking in concrete structures worldwide. Volcanic rocks have been
found to be potentially alkali-reactive in a number of countries such
as Argentina, Iceland, Japan, New Zealand and Turkey.
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
(Characterization of Potential Reactivity of the Volcanic Aggregates
from the Azores Archipelago: Implications on the Durability of
Concrete Structures) that comprises mainly the petrographic
assessment of the aggregates. In Santa Maria Island aggregates were
collected from two important quarries and also four concrete cores
were drilled in the local airport. The petrographic examination and
the geochemical analyses of the rock samples showed a significant
alteration of these rocks. The petrographic examination is accepted
internationally as the first step in the assessment of aggregates. The
concrete petrography of the airport pavement showed that the cement
paste has almost completely been replaced by an alkali-silica gel,
which was confirmed by scanning electron microscopy.
Keywords: Volcanic aggregates, Alkali-silica reaction, Petrography,
Azores islands.
Resumo: As reações álcalis-sílica têm causado, a nível mundial, a
deterioração de um grande número de estruturas de betão. As rochas
vulcânicas utilizadas como agregado têm sido consideradas
potencialmente reativas em vários países como a Argentina, Islândia,
Nova Zelândia e Turquia.
Os agregados açorianos empregues na produção de betão são
principalmente basaltos e mais raramente traquitos. A caracterização
da reatividade destes agregados tem sido implementada através do
projeto de investigação ReAVA (Caracterização da reatividade
potencial de agregados vulcânicos do Arquipélago dos Açores:
implicações na durabilidade do betão) que compreende
principalmente o exame petrográfico dos agregados. A análise
petrográfica é aceite internacionalmente como o primeiro método de
estudo a desenvolver para a avaliação da reatividade potencial aos
álcalis. Na ilha de Santa Maria foram recolhidos agregados de duas
importantes pedreiras e foi realizada uma campanha de amostragem
no aeroporto local. O exame petrográfico e a análise química
evidenciam uma alteração significativa das rochas amostradas. A
petrografia do betão proveniente do pavimento do aeroporto mostra
que a pasta de cimento junto a grandes fissuras no pavimento foi
substituída por um gel sílico-alcalino, tal como foi confirmado
através do microscópio eletrónico.
Palavras-chave: Agregados vulcânicos, Reações álcalis-sílica,
Petrografia, Ilhas dos Açores.
1
Universidade dos Açores, Departamento de Geociências, Apartado 1422,
9501-801 Ponta Delgada, Açores, Portugal.
2
Departamento de Geociências, Ambiente e Ordenamento do Território,
Faculdade de Ciências da Universidade do Porto e Centro de Geologia da
Universidade do Porto, Porto, Portugal.
*
Corresponding author / Autor correspondente: [email protected]
1. Introduction
The Azores archipelago is the most occidental region of
Portugal and consists of nine volcanic islands located in
the North Atlantic Ocean. The archipelago magmatism
belongs to the alkaline series with compositional
characteristics that range from basalts to trachytes.
Alkali-silica reaction (ASR) is a deterioration
phenomenon that occurs between alkaline (Na+ and K+)
and hydroxyl (OH-) ions in the cement and certain reactive
forms of silica in the aggregates. The reactivity of volcanic
rocks is usually associated with the presence of volcanic
glass, altered minerals and SiO2 content of the rock
(Korkanç & Tuğrul, 2005). The deterioration of concrete
by ASR is well known in several countries such as Japan,
Iceland and Turkey. In Portugal, the only information
about the alkali performance of volcanic rocks is related to
the study of the pavement of Santa Maria Island airport in
the Azores archipelago (Braga Reis et al., 1996).
The ReAVA research project is a R&D project under a
business context with contributions from private and
public partners. Aggregates from a total of thirteen places
(quarries, excavation and crushing plant) were collected in
all the islands to produce thin sections and to perform
expansion tests in laboratory. Corvo Island was the only
exception due to the inexistence of local aggregate
production. One of the main goals of this project is the
petrographic assessment of both aggregates, for the
classification towards potential reactivity, and concrete, for
the identification of cracks and ASR products.
Santa Maria Island is the oldest island of the Azores
archipelago with 8.12 Ma (Abdel-Monem et al., 1975). It
is formed by volcanic rocks of basic composition and
sedimentary rocks (e.g. calcareous fossiliferous
sediments). In order to identify the potential reactivity of
Santa Maria aggregates a program was carried out to
produce thin sections from: (1) rock samples of two
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S. Medeiros et al. / Comunicações Geológicas (2014) 101, Especial III, 1133-1136
quarries of the island and (2) concrete core samples of
Santa Maria airport pavement.
2. Materials and methods
2.1. Quarries
The rock samples (SMA-SM1 and SMA-SM2) were
collected in two quarries of the island. One of the quarries is
the main source of aggregates in Santa Maria Island. This
quarry is part of a submarine cone with a small amount of
subaerial pyroclasts on the top (Serralheiro, 2003). The
quarry shows submarine lava flows with fractured and
altered zones and intercalated levels of reddish
hialoclastites. The other quarry was exploited for the airport
construction in the mid of 1940's and is no longer in activity.
The rocks show some signs of alteration which is revealed
by the spheroidal weathering of the basalt in several areas of
the quarry.
Four concrete cores were drilled in Santa Maria Airport
pavement aiming at characterizing the aggregates included
in the manufacture of the concrete and to identify possible
products of alkali-silica reactions.
2.2. Aggregate and Concrete Petrography
The petrographic examination was performed under a
polarizing microscope Nikon Eclipse E 400 POL, with
automatic camera AXION cam MRC. The microphotographs
were obtained by Axion Vision 3.1. software. In order to
determine the composition of the reaction products, a
scanning electron microscope (SEM) FEI QUANTA 400
FEG ESEM/EDAX PAGASUS X4M equipped with energy
dispersive spectrometer (EDS) was used.
3. Results
The two samples (SMA-SM1 and SMA-SM2) were
classified according to their mineralogical composition and
textural features. The rock bulk chemical analysis was
used to plot the data on a TAS diagram in order to obtain
the correct designation of the rock. The geochemical
analyses of these basanites show a relatively high value of
loss on ignition (LOI), suggestive of an alteration that
formed iddingsite and analcite (in SMA-SM1).
Both basanites are composed of olivine with thick
iddingsitized
rims,
clinopyroxene,
plagioclase
phenocrysts and opaques (ilmenite and magnetite) with a
matrix formed by the same minerals but showing a lower
amount in olivine. In SMA-SM1 sample carbonate
minerals were identified filling some of the cracks of the
rock. The scanning electron microscope study (SEM) and
the analysis by energy dispersive spectrometry (EDS)
also detected anhedral intergranular analcite in several
areas (Medeiros et al., 2012). In SMA-SM2 iddingsite
minerals seem to fill some of the cracks present on the
rock. Volcanic glass was not found in any of these rocks.
The concrete cores from the Santa Maria Airport
pavement were drilled in the places where the concrete
was cracked, as identified during site inspection. The cores
sampled close to the longer longitudinal cracks show a
white deposit covering the crack walls. White rims are also
found lining the coarse aggregate particles (Fig. 1). The
concrete petrography confirmed that the aggregates are
composed of crushed basalt. The sand fraction contains
mainly mineral grains of olivine, pyroxene and plagioclase
minerals. However, several sand particles are composed
mainly of rock fragments in which volcanic glass is the
dominant component. The glass is light-brown, yellowish
or dark-brown, always isotropic. The observation by
SEM/EDS showed that the glass is mainly composed of Si,
Al and much lower Ca, Mg, Na and K (Fig. 2a, b, c). The
volcanic glass is, in some locations, strongly altered to
clay minerals which form yellow rims. In the concrete
cores of the deeply cracked concrete several microcracks
were observed crossing both the aggregates particles and
the cement paste.
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.
The reactivity of Santa Maria volcanic aggregates
The main characteristic of the strongest cracked
concrete core is the texture shown by the cement paste.
Under optical microscope, the cement paste exhibits
abundant cracks similar to the retraction cracks in clay
soils (Fig. 2d, e, f). It is densely cracked following a
pattern named map cracking, which is typical of alkalisilica reactions. The images obtained by SEM reveal this
cracks which occur throughout the cement paste. EDS
analysis of the cement paste show a composition
substantially different from the characteristic composition
of the cement paste in which Si and Ca occur in similar
contents. From the EDS spectrum it can be conclude that
the cement paste has been heavily leached and is now
1135
composed mainly of Si with much lower contents of Al,
Ca, Na and residual Mg (Fig. 2h). Some of the
aggregate particles are rimmed by a whitish fibrous
material which shows first order polarizing colours to
dark grey, usually with a radial habit. The smaller
particles of the aggregate are totally replaced by this
product (Fig. 2e, f). The fibrous crystals develop
dominantly in the direction perpendicular to the
aggregate particles’ limits. Cracks crossing the
aggregate particles are also partly filled with these
radial acicular crystals. The compositions obtained by
SEM/EDS suggest that the crystals correspond to a
mineral of zeolitic composition (Si-Al-Ca) (Fig. 2g).
Fig. 2. Concrete petrography: a) volcanic glass (N//); b) SEM image of glass and crystallites; c) EDS spectrum of the volcanic glass; d) cracked cement
paste (N//); e) fibrous material (N//); f) SEM image of the same field; g) EDS spectrum of the fibrous material (Z1); h) EDS spectrum of the cement paste
(Z2).
Fig. 2. Análise petrográfica do betão: a) vidro vulcânico (N//); b) imagem SEM do vidro com cristalitos; c) EDS do vidro vulcânico; d) pasta de cimento
fissurada (N//); e) produto fibroso (N//); f) imagem SEM da mesma área; g) EDS do produto fibroso (Z1); h) EDS da pasta de cimento (Z2).
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4. Discussion and conclusions
Acknowledgements
Santa Maria Island started to emerge during Miocene but
the Pliocene age is well reflected in the aggregates studied.
The two rocks studied are basanites with a low silica
content (SiO2 < 50% in the bulk rock chemical analysis).
Olivine phenocrysts are altered to iddingsite. The presence
of analcite (zeolite) in the cavities or seams of these rocks
usually occurs as secondary minerals. The absence of
volcanic glass could be justify by its alteration, because
glass tend to devitrify with time and produce alteration
products (e.g. montmorillonite). The alteration of these
rocks is also confirmed by the geochemical analyses, with
a LOI of 3.25%.
Concerning ARS it is important to refer that andesitic
glass is considered not deleterious. However, volcanic
glass becomes deleterious with increasing of SiO2 content
(> 62-65%) in the course of the crystallization of magma,
e.g. dacitic glass and rhyolitic glass (Katayama et al.,
1989). The analyses performed by SEM/EDS, although
qualitative testify the high content in Si (dominant) for the
light-brown volcanic glass in most of the sand fraction
particles.
According to literature (Braga Reis et al., 1996) the
previous study of the pavement of Santa Maria Island
airport revealed that the deterioration of the concrete was
due to a complex process, which involved expansive
reactions associated with altered volcanic aggregates such
montmorillonite and zeolites. In this study the occurrence
of fibrous minerals of zeolitic composition and habit was
confirmed. Similar results were found by Batic et al.
(1994) in the study of ASR with basalt rocks in Argentina.
The composition of cement paste is quite different from
the cement paste in the preserved concrete. The
composition obtained for the paste is very similar to the
alkali-silica gel identified in ASR affected concrete by
other authors (e.g. Thaulow et al., 1996; Diamond, 2000),
which confirms the occurrence of ASR reactions in this
pavement.
The analyses by SEM/EDS were performed at CEMUP
which equipment was funded by the projects
REEQ/1062/CTM/2005 and REDE/1512/RME/2005 of
Portuguese Foundation for Science and Technology
(FCT). The authors also acknowledge the financial support
of the FRC under the project M5.2.2/I/05/2011.
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