microscopic analysis of alkali–aggregate reaction products in a 50-year-old concrete

Materials Characterization

Microscopic analysis of alkali–aggregate reaction

products in a 50-year-old concrete

Isabel Fernandesa,*, Fernando Noronhaa, Madalena Telesb

aDepartamento de Geologia, Faculdade de Ciencias da Universidade do Porto, Praca Gomes Teixeira, 4099-002 Porto, PortugalbDepartamento de Engenharia Civil, Faculdade de Engenharia da Universidade do Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal

Received 10 February 2004; received in revised form 23 July 2004; accepted 2 August 2004

Abstract

Fifty-year-old concrete from a large dam was examined in the scope of an investigation program concerning the properties of

granite as aggregate material for concrete. Site inspection, which was developed in order to detect possible signs of deterioration

of the concrete, revealed the existence of efflorescence and exudations.

Scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) analyses were attempted to identify the

composition of these materials and their morphology. From the analyses, it was concluded that some of the exudations were

composed by alkali–silica gel. In these samples, an interesting behavior was observed in different moments after a 3-month

interval. It was noticed that the initially noncrystalline alkali–silica gel transformed into sodium-rich needles and tablets after a

few months kept in a desiccator in the laboratory. Therefore, it was concluded that the materials identified corresponded to

different stages of evolution of an alkali–aggregate reaction product.

D 2004 Elsevier Inc. All rights reserved.

Keywords: Exudations; Alkali–aggregate reaction; Alkali–silica gel; Calcium carbonate

1. Introduction

The Cabril dam is a thin concrete arch dam built in

the 1950s in the Zezere River, Central Portugal (Fig. 1).

The dam is 136 m high and it has four horizontal

internal galleries, as shown in Fig. 2, and a drainage

gallery in the abutments, close to the foundation.

1044-5803/$ - see front matter D 2004 Elsevier Inc. All rights reserved.

doi:10.1016/j.matchar.2004.08.005

* Corresponding author. Tel.: +351 223 401 471; fax: +351 222

056 456.

E-mail addresses: [email protected] (I. Fernandes)8

[email protected] (F. Noronha)8 [email protected] (M. Teles).

Cracking was observed shortly after the first

reservoir impounding at the upper levels of the

downstream face. These cracks were mainly horizon-

tal and were due to structural causes related to the

high rigidity of the crest and its geometry which was

modified to support a road not included in the original

design [1]. During the first decades of the service life

of the dam, displacements were recorded by instru-

mental monitoring installed in the galleries and by

geodetic methods. At the beginning of the 1980s, a

full grouting treatment was performed in the concrete

body as well as in the foundation rock mass.

53 (2004) 295–306

Fig. 1. The Cabril dam (Central Portugal, viewing to NE).

I. Fernandes et al. / Materials Characterization 53 (2004) 295–306296

Previous studies referred the existence of manifes-

tations of alkali–aggregate reactions in the concrete.

According to Ref. [1], the inspection of the galleries

inside the dam showed the existence of exudations

similar to alkali–silica gel and [2] stated that the

deterioration of concrete was due to the following: (1)

slow alkali–aggregate reactions; (2) reactions between

sulfate and alumina from the alteration products of the

aggregates; and (3) reactions between sulfate and

hydrated calcium silicates from the cement paste.

The present work is part of an investigation

program in which it is intended to characterize granite

as aggregate material for concrete. This study refers to

the importance of a complementary microscopic

Fig. 2. The Cabril dam. Cross-section s

examination using scanning electron microscopy

(SEM) and energy dispersive spectrometry analyses

(EDS) in the identification and characterization of

secondary products originated from reactions within

the concrete, especially those occurring between the

particles of the aggregate and the cement paste.

1.1. Geological background of the aggregate material

The coarse and fine particles of the aggregate are

dominantly of granitic composition and were obtained

from the crushing of the granites from a quarry that

was opened for the construction of the dam and

abandoned after conclusion. The granites are Hercy-

hows horizontal internal galleries.

I. Fernandes et al. / Materials Characterization 53 (2004) 295–306 297

nian in age and are intrusive in a series of schists and

greywackes late Precambrian to Cambrian in age. The

batholith is heterogeneous in structure and near the

contact with the host rock the granite can exhibit an

oriented texture.

The petrographic analysis of the material from the

quarry led to the identification of two types of granite

mainly differing in texture [3]. The petrographic

examination did not reveal preferred orientation of

minerals, although there was a slight deformation of

the rocks.

Quartz grains have low undulatory extinction

angles of about 118 and deformation is also present

in muscovite crystals, with bent cleavage planes, and

in feldspars which show undulatory extinction and

bending of twin planes.

2. Materials and methods

Site investigation was developed inside the four

horizontal galleries and all along the drainage

gallery of the dam. The galleries were examined

in the last 3 years in the scope of the present

investigation program in order to recognize signs of

deterioration of the concrete: inspection occurred in

May 2001, in July 2002, in November 2002, and in

July 2003. In the access galleries to the upper level

gallery (293.65 m), in both abutments, exudation

samples were collected. These access galleries are

closer to the rock mass foundation and in some

places show damp walls with areas of efflorescence.

In the upper level gallery, there were locally small

exudations and also a cavity resembling a pop-out

with 60 mm diameter, but the walls were dry and

sound.

In the galleries at the levels 274.50, 255.50, and

239.00 m, the walls were also dry and there were no

exudations in all the extension of the galleries.

Stalactites were observed and sampled close to the

drain holes.

At the top of the drainage gallery, in the right

abutment, spalling of the concrete was visible in a

limited area of about 0.50 m length. A small piece of

concrete was broken by hand and kept in an airtight

container for examination in the laboratory. The

drainage gallery showed a damp surface in just two

places with efflorescence.

Samples from described efflorescence and exuda-

tions were extracted and kept in airtight containers in

order to preserve them for examination and for

analyses in laboratory. In order to study the concrete

itself, in the sites where efflorescence and/or exuda-

tions were extracted, places for drilling cores were

selected.

Cores with 90 mm diameter and 600–900 mm

length were drilled for petrographic examination of

the concrete. They were extracted with a diamond

drilling machine Milwaukee model 4094-5 chilled by

water circulation, with 230 W of power and a rotation

velocity of 450 cycle/min. Due to equipment’s

characteristics (sampler of 300 mm length), the cores

could not be extracted as a continuous length and

were fragmented in two or three pieces of about 300

mm each. Following the instructions stated in the

report [4], the pieces were labeled with a reference

number and the sample’s orientation indicated with a

waterproof ink. To avoid desiccation, a cling-film was

wrapped around the samples and they were sealed in

polythene bags. The cores showed a compact concrete

with no damp patches. They were taken immediately

to the laboratory.

When in the laboratory, the pieces were reassembled

to reconstruct the cores to be photographed. Places for

thin sections production were selected along the core

samples. Slices of the concrete were cut (25�45�10

mm) and glued with araldite to glasses to produce thin

sections. The samples were impregnated with resin by

heating at Tb70 8C until drying, if necessary several

times, and no vacuum was utilized. The thin sections

were produced totally by manual processes from the

progressive grinding to the final polishing.

In order to examine the voids and the bonds

between the cement paste and the aggregate particles,

small samples of concrete were also prepared for

observation. The samples were sawn from the broken

tops of the fragmented pieces resulting from drilling.

They were glued with araldite, with the broken

surface up, to metallic cylinders 6 mm thick and

sealed in airtight containers. The sample of the

spalling concrete was also prepared.

The samples from exudations were taken from the

containers, glued to metallic cylinders, and put back in

airtight containers.

Within two weeks the samples were taken to the

scanning electron microscope laboratory where they


were submitted to vacuum and sputtered by gold by

JEOL JFC 1100 equipment, just before they were

examined. The examination occurred under a scan-

ning electron microscope model JEOL JSM-6301F

equipped with a NORAN-VOYAGER energy disper-

sive spectrometer. The accelerating voltage used was

15 kV with a working distance of 15 mm. The

collection time for the microanalyses was 60 s with a

dead time of approximately 30% to obtain semi-

quantitative standardless analyses.

The examination by scanning electron microscope

permitted the characterization of the morphology of

the predominant products. The energy-dispersive

spectrometer analyses gave qualitative information

about their compositions.

3. Results

From the inspection of the galleries, it was

concluded that there are no signs of map cracking in

the walls and ceiling of the galleries or sweat patches

in the concrete surfaces.

The exudations, which predominate in the ceiling

of both access galleries, are white or yellowish,

most of them are solid and hard whereas others may

show transparent fluid droplets and are viscous in

appearance, as shown in Fig. 3. There were also a

few exudations in the ceiling of the upper level

gallery, mainly in the blocks closer to the left

abutment, similar to those observed in the access

galleries.

Fig. 3. Different types of exudations from the top-level gallery. In pane

centimetres.

The EDS spectra of the exudations were grouped

according to the predominant elements. In Fig. 4,

typical spectra of the exudations from the access

galleries in both abutments and from the upper level

gallery are shown. In the composition of most of the

samples, silicon is present and there are a great

number of samples containing alkalis sodium and

potassium, which was assumed to correspond to

alkali–silica gel. However, in these semiquantitative

analyses, no calcium was identified.

The gel shows different views and textures,

although the general chemical composition remains

constant. It is often amorphous, with a smooth surface

showing diagnostic characteristic shrinkage cracks. In

some samples a spongy texture was observed.

In the samples that show transparent droplets, gel

exhibited a curious behavior when examined under

SEM. Although the samples were coated by Au, when

under vacuum the solid surface broke down and the

viscous material created bubbles. The bottom spec-

trum in Fig. 4 corresponds to this noncrystalline

fraction, where the Au coating is lacking, showing its

evolution during the examination by SEM. On the top

of this viscous gel, Na-rich platy crystals were

identified, corresponding to the initial solid surface.

Some of these samples were again coated by Au

and observed by SEM after a 3-month interval. There

was a considerable evolution in the viscous alkali–

silica gel observed, both under SEM and through the

EDS analyses.

In fact, 3 months later, in some samples the gel

showed that the fluid material produced more cracks

l a, translucent droplets are yet present in the exudation. Scale in

Fig. 4. Spectra of alkali–silica gel. Different views of the alkali–silica gel. Scale bars are 2, 3, and 1 Am, from top to bottom.



in the solid surface and maintained the appearance,

which was noticed in the first observation. The

samples had abundant needles of Na-rich crystals

growing over and through the gel surface, as shown in

Fig. 5, and the amorphous gel behind the needles had

a lower Na content than previously measured. As a

consequence, it was assumed that a different combi-

nation of the elements occurred although the samples

were kept in a dry, clean environment.

The Na-rich crystals have two predominant habits.

In some samples they are prismatic and dense,

growing through the alkali–silica gel or forming

rosette-like agglomerates, sometimes with a random

orientation. When they are over the amorphous gel,

they show long thin acicular crystals growing parallel

Fig. 5. (a and b) Evolution of the alkali–silica gel after 3 months in the la

observed as bubbles grew from behind the solid surface. Scale bar is 6 Amsilica gel. Scale bars are 2 Am (left) and 3 Am (right).

to the gel surface, as shown in Fig. 6, or needles

growing in clusters at right angles to the surface of the

gel, similar to those referred for Norwegian concrete

reaction products [5].

In lower galleries, dense deposits of calcium

carbonate form long white stalactites resulting from

lime leaching. This product is totally crystalline,

exhibiting different habits with similar composition,

as shown in Fig. 7, and is not caused by alkali–silica

reactions.

The aggregates were also examined in order to

evaluate their potential reactivity in a strong basic

environment. Fresh granites were taken from the

quarry close to the dam site. According to Ref. [6],

these rocks can be considered as potentially reactive

boratory. Na-rich crystals formed over the viscous gel. Cracks were

. (c and d) Different aspects of Na-rich crystals growing over alkali–

Fig. 6. Sodium-rich needles growing parallel and perpendicular to the alkali–silica gel. Scale bars are 2 Am (a) and 20 Am (b).


as they show signs of deformation and contain

minerals like feldspars and muscovite that can

contribute with alkalis to the reaction.

The macroscopic examination of concrete cores

revealed a sound concrete with no visible cracks or

deterioration. The petrographic examination of the

concrete shows that some aggregate particles are

different from the granites of the quarry. There are a

few fragments of metamorphic rocks and of a rock of

granitic composition showing stronger deformation

than the granites from the quarry, expressed in mica

and in quartz grains, and also containing microcrystal-

line quartz. This rock might belong to the outer layers

Fig. 7. Spectrum and view of deposits of calcium carbo

of the batholith, close to the contact with the schist

and greywacke formation. It was probably exploited

from the quarry but it is no longer observable in

outcrop. The characteristics of this stressed granite

suggest potential alkali reactivity. However, no alkali–

silica gel was identified in thin sections and no

microcracking was observed in relation with the

aggregate particles, as shown in Fig. 8.

Samples of the concrete from the cores were also

selected for examination by SEM wherever there were

white deposits around aggregates or in voids. In these

samples, it was possible to identify aspects of the

attack to the quartz grains by strong alkali fluids, as

nate related to concrete drains. Scale bar is 3 Am.

Fig. 8. Aspects of the concrete in thin section. The fine aggregate particles show angular and elongated shapes resultant from the crushing of the

granite. Interfaces between aggregate particles and the cement paste are closed. Circular voids at the right top and bottom in panel b are filled by

ettringite. Plane-polarized light.


shown in Fig. 9. Surfaces with spongy appearance

were observed resulting from the corrosion of quartz

caused by alkali dissolution due to fluid circulation.

In the concrete samples, no alkali–silica gel was

identified, although there were Na-rich crystals

showing habits similar to those of the previously

referred exudations.

Ettringite was identified filling totally or partially

the voids in the interior of concrete and in the contacts

between aggregates and the cement paste, close to the

exposed walls, as shown in Fig. 10. Alkali–silica gel

was only identified by EDS in the aggregate-paste

bonds coexisting with ettringite.

Fig. 9. Aspects of the corrosion of quartz surface due to the actio

In the examination of the samples of concrete other

products were identified, although they were not

directly related to the alkali–silica reactions. Fig. 11

shows some of the materials observed, such as tabular

and prismatic crystals of Na-K-S, corresponding

probably to aphthithalite ((K2Na)2SO4) and portlan-

dite (Ca(OH)2).

Some efflorescence was also observed. This is

frequent in the access galleries to the top level gallery

in the left abutment, where there are fluffy white

products covering a part of the sidewall, as shown in

Fig. 12, and in the right abutment. A few fluffy white

patches of efflorescence can be also observed in the

n of alkaline fluid. Scale bars are 2 Am (a) and 0.5 Am (b).

Fig. 10. Spectrum and image of ettringite crystals over a quartz grain of the aggregate in the sample of the spalling concrete from the wall of the

drainage gallery, and ettringite crystals partially filling the voids in concrete from the cores. Scale bar is 3 Am (b), 20 Am (c), and 20 Am (d).


drainage gallery. They consist of sodium-rich crystals

probably corresponding to trona.

4. Discussion

There are a number of different factors that cause

the deterioration of concrete structures. They are

usually classified as human causes, natural actions,

and accidents. Human factors can be related to the

design, the construction, or the maintenance during

the service life of the structure. Natural factors can be

classified as physical, chemical, or biologic, and

accidents can also be natural or have human causes.

Concerning natural chemical actions, the alkali–

aggregate reaction consists in the reaction between the

hydroxyl ions and sodium and potassium ions in the

pore fluid within the concrete [7] and susceptible

components of the aggregate. These reactions take

place when three conditions are fulfilled at the same

time: (1) reactive aggregates in appropriate amount,

(2) available alkalis, and (3) water [5,8–10]. Factors

concerning the properties of the concrete such as

porosity, alkali content of the cement paste, grain size

of the aggregates, conditions of exposure to environ-

ment, and temperature can affect the reaction.

The type of reaction depends on the composition

of the aggregates and there are three categories of

Fig. 11. Views of other materials identified by SEM: (a) aphthitalite and (b) portlandite. Scale bars are 2 Am (a) and 20 Am (b).


alkali–aggregate reactions: (1) the reaction with

silica, which is the most frequent and well known;

(2) the alkali reaction with complex silicates; and (3)

the alkali reaction with dolomite and dolomitic

carbonates. Alkali–silica reaction, the most wide-

spread, results in a swelling gel sometimes being

deleterious to the structure. This product may have a

variable composition and initially shows a resinous

transparent aspect, becoming white and hard due to

the contact with the air.

The reaction with silicates is considered by some

workers similar to the reaction with silica though

slower [10]. A range of rock types and a number of

minerals are considered potentially alkali reactive.

The most commonly referred materials are opal,

chalcedony, cristobalite, tridymite, microcrystalline,

or glassy quartz and strained quartz.

Fig. 12. Efflorescence in the sidewall of the access gallery to the

upper level gallery, in the left abutment. Scale bar is 15 cm.

Alkali–aggregate reactions are typically slow

reactions. The first visible signs of alkali–aggregate

reaction can occur after 5, 10, or even 20 years after

the construction. The reaction ceases when the

reactive aggregates, the hydroxyl, or the alkalis

sodium and potassium ions are exhausted.

Exudations are generally considered as one of the

manifestations of alkali–aggregate reactions. Exuda-

tions of alkali–silica gel, as a result of the migration

of the gel to the surface of concrete, have been the

primary focus of investigations on the chemical

composition of the gel [11]. However, they are not

common and when present, indicate that there has

been sufficient moisture to carry the gel through to

the surface [10].

According to Ref. [11], the alkali–silica gel found

in the cracks far from the reaction site shows a high

content of CaO of approximately 20%, while gel close

to the reaction site and gel taken from the surface of

the concrete show a low content of CaO.

According to Ref. [12], the potential alkali

reactivity of granites is related to the percentage and

strain effects in quartz, particularly quartz grains with

moderate to strong undulatory extinction. Textural

characteristics such as undulatory extinction and

intense fracturing, which result from the mechanical

strain induced in quartz grains during deformation

processes, are related to the chemical reactivity with

cement alkalis.

However, recent studies [13] state that the grain size

reduction of quartz enhances reactivity by increasing


the total area of quartz boundaries available for

reaction, and a new quantitative petrographic method

was developed for the evaluation of the total grain

boundary area of quartz.

Concerning the coexistence of ettringite and

alkali–silica gel, Ref. [14] notes the frequent

observation of the occurrence of ettringite and

isotropic gel as indicative of a possible connection

between sulfate attack and alkali–aggregate reaction.

However, Ref. [15] discuss the formation of alkali–

silica gel and of ettringite and the authors concluded

that there is no evidence of any mutual interference

of both phenomena. However, the authors refer that

alkali–silica gel and ettringite can recrystallize in air

voids, microcracks, and cement-paste interfaces. In

the concrete from the Cabril dam, ettringite was

identified in voids and cracks and the alkali–silica

gel was mainly identified in exudations. In rare

cases, EDS pointed out the coexistence of ettringite

and alkali–silica gel. These facts suggest that there is

no evidence of any correlation between the two

phenomena.

5. Conclusions

The walls and ceiling of the internal galleries of the

Cabril dam show a sound concrete, which was

confirmed by the observation of the concrete cores.

There are no signs of cracking or sweat patches in the

superficial concrete. The only manifestation of the

existence of alkali–silica reaction consists of a number

of exudations and efflorescence, as referred in the

literature [1,2], which occur mainly in the access

galleries to the upper level horizontal gallery (293.65

m). These galleries also show damp walls due to the

proximity to the foundation rock mass. In the 293.65-

m level gallery, just a few exudations were collected

from the ceiling.

Most of the exudations were in a solid state, but

some showed a transparent viscous appearance.

Examination under SEM showed that they are mainly

composed of alkali–silica gel. In their semiquantita-

tive analysis obtained by EDS, no calcium was found,

in agreement with the literature about exudations.

The transparent gel maintained a viscous behavior

when examined by SEM, creating bubbles from inside

the solid surface as the vacuum was applied. When

kept in a desiccator, the product evolutes into Na-rich

crystals within a few month’s interval.

From the petrographic examination of thin polished

sections, the presence of alkali–silica gel in voids or

cracks within the concrete was not, however, identified.

The analysis by EDS pointed to the possible coex-

istence of alkali–silica gel and ettringite acicular

crystals in some voids but it could not be concluded

that there was a relation between their origins.

The other horizontal galleries show deposits of

calcium carbonate close to the drain holes and the

drainage gallery presents some efflorescence of

sodium-rich crystals.

Therefore, it was concluded that there were alkali–

silica reactions taking place locally, consisting of a

superficial phenomenon. No gel was identified in the

interior of the concrete, analyzed in thin sections by

optical and scanning electron microscope. This super-

ficial phenomenon seems not to be deleterious to the

concrete at this stage of the reaction process, as no other

manifestations of deterioration were observed on the

exposed surfaces of the concrete or in its interior.

Acknowledgements

We are particularly indebted to EDP-CPPE, Direc-

cao de Producao Hidraulica for allowing the access to

the Cabril Dam and for the facilities during the

fieldwork.

The authors are grateful to reviewers for the

constructive and interesting comments.

References

[1] Silva HS. Estudo do envelhecimento das barragens de betao e

de alvenaria—alteracao fısico-quımica dos materiais. Relatorio

de Tese. Lisbon, Portugal7 LNEC; 1992.

[2] Reis MOB, Silva HS, Santos Silva A. Ocorrencia de reaccoes

alcalis-inerte em Portugal. Estudos de Casos. Presented at

Betao Estrutural 1996-68 Encontro Nacional sobre Estruturas

Pre-esforcadas, Lisbon, Portugal, p. 2.37.

[3] Fernandes I, Noronha F, Guedon-Dubied S, Teles M. Petro-

graphic analysis of hardened concrete with granitic aggregates.

Proceedings of the 9th Euroseminar on Microscopy Applied to

Building Materials, Trondheim, Norway; 2003.

[4] BCA. The Diagnosis of Alkali–Silica Reaction. Report of a

Working Party. Slough, U.K.7 British Cement Association;

1992.


[5] Jensen V. Alkali aggregate reaction in Southern Norway. PhD

thesis. Trondheim, Norway: Norwegian University of Science

and Technology (1993).

[6] E 415-1993-Especificacao. Inertes para argamassas e betoes-

Determinacao da reactividade potencial com os alcalis.

Analise petrografica, Lisbon, Portugal (1993).

[7] Berra M, Bertacchi P. Alkali–aggregate reaction in concrete

dams. Water Power Dam Constr 1991;43/4:12–6.

[8] Hobbs DW. Alkali–Silica Reaction in Concrete. London7

Thomas Telford; 1988.

[9] Sims I, Brown B. Concrete aggregates. In: Hewlett PC, editor.

Lea’s Chemistry of Cement and Concrete, 4th edition.

London7 Arnold Publisher; 1998. p. 903–89.

[10] St John DA, Poole AB, Sims I. Concrete Petrography—A

Handbook of Investigative Techniques. London7 Arnold

Publisher; 1998.

[11] Knudsen T, Thaulow N. Quantitative microanalyses of alkali–

silica gel in concrete. Cem Concr Res 1975;5:443–54.

[12] Gogte BS. An evaluation of some common Indian rocks with

special reference to alkali–aggregate reactions. Eng Geol

1973;7:135–53.

[13] Wigum BJ, Hagelia P, Haugen M, Broekmans MATM. Alkali

aggregate reactivity of Norwegian aggregates assessed by

quantitative petrography. Proceedings of the 11th International

Conference on Alkali–Aggregate Reaction, Quebec, Canada,

p. 533–42.

[14] Pettifer K, Nixon PJ. Alkali metal sulphate—a factor common

to both alkali aggregate and sulphate attack on concrete. Cem

Concr Res 1980;10:173–81.

[15] Johansen V, Thaulow N, Skalny J. Simultaneous presence of

alkali–silica gel and ettringite in concrete. Adv Cem Res

1993;5:23–9.

microscopic analysis of alkali–aggregate reaction products in a 50-year-old concrete

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