Materials Characterization

Structural properties of quartz and their potential role for ASR

Maarten A.T.M. Broekmans*

Geological Survey of Norway, Department of Mineral Resources, N-7491 Trondheim, Norway

Received 30 August 2004; accepted 31 August 2004

Abstract

Quartz is the predominant silica polymorph in alkali-reactive aggregate materials. Being a major constituent of the Earth’s

crust as well, quartz is stable under a broad range of conditions including pressure, temperature, fluids present, postdeformation,

etc. The conditions under which a particular quartz was formed are reflected in its qualities, including the crystal structural

details and its (trace) element composition. However, current testing methods to determine the alkali–silica reaction expansion

potential of aggregate materials do not address these qualities directly. To gain an idea which qualities affect the dissolution

behavior of quartz under ASR conditions, it might be useful to first consider the geological conditions, and apply these to

concrete.

D 2004 Elsevier Inc. All rights reserved.

Keywords: Quartz dissolution; Crystal structure; Crystal chemistry; ASR

1. Introduction

Three constituents are essential to the alkali-silica

reaction (ASR): alkalies and silica as reagents and

moisture (both as a reagent and a transport medium).

The alkalies are supplied by the cement paste or,

under certain circumstances, can be derived from the

aggregate material, and the water is mostly provided

by dexposure conditionsT, i.e., meteoric or otherwise.

Considered in their pure state, these constituents

behave consistently when reacting to form a silica

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

doi:10.1016/j.matchar.2004.08.010

* Tel.: +47 7390 4152; fax: +47 7392 1620.

E-mail address: maarten.broekmans@ngu.no

(M.A.T.M. Broekmans).

gel, regardless of their origin. The mechanism of silica

dissolution is not so much controlled by the alkali

species Na and K, but rather by water molecules and

dissolved OH-ions breaking silica bonds, which then

later recombine with alkali. Of course, differences in

concrete composition and local microenvironment

sensu lato do affect the alkali-reaction (i.e., silica

dissolution) conditions.

Already back in 1955, Ralph Iler published the first

edition of bThe colloid chemistry of silica and

silicatesQ [1], a monograph that, over the years, has

become famous, if not monolithic. It provides an

excellent introduction to the chemistry of silica

dissolution. However, since the publication of the first

edition and the subsequent expanded and updated

53 (2004) 129–140

M.A.T.M. Broekmans / Materials Characterization 53 (2004) 129–140130

second edition in 1974, much more data on quartz

and silica have become available. The 1994 compiled

review volume edited by Heaney et al. [2] covers the

behavior of silica under elevated geological con-

ditions as well as its physical properties, and capita

selecta from its application as an industrial material.

The book bStructure and imperfections in amorphous

and crystalline silicaQ [3] contains a valuable se-

lection of papers dealing with just that, from an

industrial point of view. Silica dissolution under

ambient weathering conditions is extensively treated

in Ref. [4].

Aggregate materials as used in concrete are of

diverse geological origin, and consequently, so is

the quartz they contain, having formed under

diverse conditions, essentially different for each

rock type with respect to their specific combination

of pressure, temperature and fluid history. To begin

with, it may be convenient to lump a mineral

formed under broadly variable conditions under one

single general denominator. Using combined min-

eralogical and geochemical techniques, however, it

is possible to identify different properties and

qualities of quartz from different geological envi-

ronments, potentially affecting its alkali-reactivity

potential.

Many publications have been dedicated to the

relationship between the nature and reactivity of

aggregate [e.g., Refs. [5–7], and references therein].

However, the different properties and qualities of

quartz sensu stricto with respect to its ASR-potential

seem to have received only little attention from a more

fundamental point of view. It is therefore useful to

review the mineralogical and geochemical properties

and qualities that possibly affect the dissolution of

quartz.

2. Silica dissolution controls

2.1. General

Parameters controlling quartz dissolution have

been well studied by many researchers. Recent

reviews are given in Refs. [4,8], and new contribu-

tions are frequently published in international jour-

nals. The key point in the alkali-silica reaction is the

hydrous dissolution of silica in the presence of

dissolved Na and K at high pH (N13), as is common

in OPC concrete.

The properties and qualities of the silica and its

environment may affect its solubility under the given

conditions. These will be discussed below in separate

paragraphs, with special emphasis on ambient pres-

sure (P)–temperature (T) conditions.

2.2. Equilibrium dissolution

The dissolution reaction of silica can be repre-

sented by the equation:

ð1Þ

However, the true nature of the silica in dissolution

is not simply H4SiO4 (aq), of which notation is

merely used for convenience, but rather depends on

solution properties such as pH, time, etc. [1,9]. Due

to dimerization and polymerization of the dissolved

silica upon ageing, the ratio SiO2:H2O trends

towards less than 1:2. However, if H-bonded waters

are counted in, the balance goes the other way.

Reaction Eq. (1) is an equilibrium reaction with a

constant K written as:

K ¼ aH4SiO4

aSiO2d a2H2O

ð2Þ

The principal shape of this equilibrium equation is

independent of the dissolving silica polymorph, even

when its solubility differs. A general rule of thumb is:

the lower the activity (or alternatively DGf8) of the

silica, the lower the solubility. This means that,

though the shape of Eq. (2) for quartz is the same

for all silica polymorphs, the respective a-values are

different [4]. It also means that a plotted graph of such

equations for each polymorph in essence will have the

same shape, but will be shifted along the vertical axis.

From thermodynamic data as experimentally con-

firmed by laboratory experiments, it appears that

quartz is the least soluble polymorph, glassy silica the

most soluble, whilst other are intermediate [10]. Apart

from the differences attributed to a different bulk

crystal structure, a higher lattice integrity with fewer

defects also affects silica solubility. Thus, the con-

centration of the dissolved species strongly depends

M.A.T.M. Broekmans / Materials Characterization 53 (2004) 129–140 131

on the whole nature of the solid silica with which the

solution is in equilibrium.

2.3. Effects of solution pH

Silica is an amphoteric material, which means that it

dissolves at extreme pH values in strongly acidic or

strongly alkaline conditions, and less around neutral

pH. At high pH in a basic environment, dissolved silica

as H4SiO4 behaves like a weak acid. From pH 9 and

upwards, silica dissolves according to the equation:

SiO2 þ 2H2O W H3SiO�4 þ Hþ ð3Þ

The predominant species in solution is no longer

H4SiO4 (aq), but the deprotonized variant of that. At

higher pH values, stepwise deprotonization runs to

final completion according to equation:

H3SiO�4 W SiO4�

4 þ 3Hþ at pHN12; ð4Þ

with equilibrium constant for each increment similar to

the model of Eq. (2). Eqs. (1), (3), and (4), in fact,

represent a scheme of subsequent reactions with

incrementally increasing deprotonization ((3) and (4))

that develop when pure silica dissolves in water at

pHN12.

If the final concentration of dissolved silica is high

enough, then it will polymerize to form H6Si4O72�

complexes, merely an effect of solution ageing and

not of pH. However, the speciation of freshly

dissolved silica does depend on pH.

3. Silica crystal structural features

3.1. Natural silica polymorphs

Some 12% of the volume of the entire earth’s crust

is made of the dpureT oxide SiO2, representing a

weight of ~3.2�1021 kg. In total, some nine silica

polymorphs are known to exist, each of the compo-

sition SiO2 but each with its own unique crystal

structure different from all others. Best known are a-

quartz, h-quartz, tridymite, cristobalite on the high-T

side, and coesite and stishovite on the high-P side.

Very recently, a completely new silica phase that

appears to be stable at extreme pressures beyond

stishovite stability was identified [11].

Lechatelierite is an amorphous silica glass from

fused sand found in fulgurites, and keatite is

crystalline but has as yet only been synthesized in

the laboratory [13]. The polymorph moganite has

officially been acknowledged by the Commission on

New Minerals and Mineral Names of the International

Mineralogical Association (CNMMN/IMA), but is

nevertheless still a matter of dispute among scientists.

A monograph on quartz varieties and their natural

appearances (and including some other silica poly-

morphs) was written by Rykart [12].

Most of the silica in the earth’s crust occurs as the

polymorph a-quartz, due to its large stability region in

the P,T-field [13]. Other polymorphs are in principle

thermodynamically unstable in the quartz P,T-region,

but may nevertheless persist due to the very sluggish

reconstructive phase transformations to quartz. An

extensive review on the crystal structures and other

properties of micro- and noncrystalline silica poly-

morphs can be found in Ref. [14].

3.2. Crystal structure of a-quartz

The crystal structure of quartz can be represented

as a 3D framework of [SiO4]-tetrahedra. Each

tetrahedron is built of four isometric triangles with

608 corners, with one O-atom at all four apices and the

Si-atom placed in its center of gravity. Adjacent

tetrahedra share their apical oxygen, reducing the O-

content of the bulk material to SiO2.

The [SiO4]-tetrahedra are arranged in sixfold rings,

where the top apex of tetrahedron 6 shares its oxygen

with the bottom apex of tetrahedron 1, forming a helix

that can be either left- or right-handed. Thus, quartz is

a chiral material with l (levus; left-handed) and d

(dexter; right-handed) enantiomorphs. At room tem-

perature and pressure conditions, the symmetry of

these six-fold rings is reduced from hexagonal to (di-)

trigonal as three of the six angles are wider than the

three other angles in the same ring. This arrangement

is called a-quartz and is the most stable polymorph

under ambient conditions.

3.3. Phase transformations of silica

Despite a-quartz being the most ubiquitous silica

polymorph, other polymorphs might be important in

specific environments and bear importance for con-

M.A.T.M. Broekmans / Materials Characterization 53 (2004) 129–140132

crete materials science. It is therefore interesting to

take a closer look at these, beginning with the P,T-

diagram for silica.

At room temperature, increasing pressure will

transform a-quartz to coesite above 1.9 GPa (19

kbar) and to stishovite above 7.6 GPa (76 kbar). Both

polymorphs immediately reverse to a-quartz when

pressure is released, so neither of these polymorphs

exists metastably at ambient conditions. The com-

pressive yield strength of highest performance con-

cretes produced and tested under laboratory

conditions is on the order of a few hundred MPa

and of modern bulk concrete in practice under 100

MPa. Together with a safety strength margin of 50%

the concrete that modern structures are designed with,

and in older days double that value, mechanical

stresses will be considerably lower than 100 MPa in

daily practice. Thus, the coesite and stishovite silica

polymorphs can be safely neglected in an ambient

concrete environment.

At ambient pressure, a-quartz will transform to h-quartz above 573 8C, changing its crystal structure

from di-trigonal to hexagonal by shifting the tetrahe-

dra to more symmetric places in the sixfold ring by an

intricate set of rotations. This also results in a stepwise

volume change of around 10% at the transformation

temperature: expansive upon heating, shrinking upon

cooling. This type of transformation is called dis-

placive as it merely comprises tilting of tetrahedra

affecting bond angles. Therefore, transformation is

rapid and reversible. A commensurate intermediate

phase is known to exist at temperatures near the a–htransition, but its treatment here is beyond the scope

of this paper [13].

Further heating will result in the h-quartz trans-

forming into tridymite above 867 8C and into

cristobalite above 1470 8C. Finally, cristobalite melts

at 1713 8C. Although formally a melt, molten silica is

extremely viscous and does not even need a container

as it will not flow. The melt’s viscosity can be

attributed to its extreme polymerization, as it still

represents a 3D silica network.

Upon cooling, the melt crystallizes as cristobalite,

which then transforms to tridymite below 1470 8C, incontrast to the heating trajectory. Further cooling does

not result in a transformation to h-quartz below 867

8C, but instead immediately to a-quartz at 573 8C,provided the cooling is done slow enough to allow the

lattice reconstructions to take place. If quenched,

silica glass, cristobalite, and/or tridymite will persist

down to ambient temperatures.

Neither the crystal structure of tridymite nor that of

cristobalite can be constructed from the h-quartzstructure by rotation of the [SiO4]-tetrahedra. Instead,

O–Si–O bonds have to be broken and mended

requiring a relatively high activation energy, making

this type of phase transformation reconstructive and

hence sluggish, so much so even that some phases are

skipped. Adding a fluxing agent such as, e.g., sodium

tungstate significantly speeds up reconstructive phase

transformations in silica [15].

Remarkably, 3D-network polymorphs tridymite

and cristobalite also crystallize under authigenic

(near-ambient) conditions in amorphous opal-A,

transforming it to opal-CT. Similarly, devitrification

of silica glass also leads to the formation of

cristobalite/tridymite intergrowths (eventually dstuffedderivativesT) like the dsnowflakesT in natural obsidian

[16], or spots in Medieval stained glass [17].

Intermediate crystallization into metastable cristoba-

lite/tridymite before recrystallization to a stable silica

phase (mostly a-quartz) is preferred due to the close

resemblance of their crystal structures to that of glassy

silica, requiring a smaller activation energy for the

(re-) construction of the crystal structure [18].

A number of SiO2 species are of particular

interest with respect to the alkali–silica reaction,

including opal, chert, and chalcedony [5,7]. Apply-

ing the appropriate nomenclature to fine grained

silica varieties is a complicated matter, as are their

respective crystal structures [14]. It is proposed that

only moganite is a polymorph sensu stricto [19], and

that agate, chert, flint, chalcedony, etc., are instead

rather to be regarded as rock names [20]. The crystal

structure of moganite can be described as being

derived from a-quartz by lamellar Brazil twinning,

polysynthetic at unit cell scale [13]. The moganite

structure has been applied to the domain structure of

chalcedony as a three dimensional construct of

quartz and moganite domains of variable size and

degree of (mis-) orientation. As per today, moganite

has not yet been mentioned as participant in a

deleterious alkali-silica reaction [e.g., Refs. [21–24]],

whereas it has been demonstrated to be a common

constituent in selected chert and other fine-grained

silica varieties from all over the globe [25]. Porous

M.A.T.M. Broekmans / Materials Characterization 53 (2004) 129–140 133

chert is listed as a severe ASR hazard in the

Netherlands, Belgium, Germany, and Denmark,

among other countries.

The silica polymorphs tridymite, cristobalite, and

moganite are thermodynamically less stable than

quartz to various degrees, and hence more prone to

dissolution and/or alkali reactivity than quartz. If the

solubility rate of quartz in pH-neutral water at 25 8C is

set at unity, then cristobalite dissolves 2.05 times as

fast, tridymite 2.71 times [8], and moganite at pH 3.5

even 7.4 times [26]. The respective volume of quartz

and moganite and the degree of misorientation

between domains are thought to affect the dissolution

of this type of silica, and may also explain the

variations in reference values for the solubility of

chalcedony [18].

Although the speciation of dissolved silica depends

on pH (and ageing), the concentration of the dissolved

species also depends on the nature of the solid silica

with which the solution is in equilibrium. All this

implies that, although the template of equilibrium Eq.

(2) will not change, the effective concentration of

dissolved silica will, depending on the actual poly-

morph and its degree of crystallinity.

3.4. Effect of the quality of the crystal lattice

The exact definition of a grain boundary differs

with the boundary type. In general terms, a grain

boundary is defined as a btwo-dimensional lattice

defect that introduces a misorientation with no long-

range stress fieldQ. In this respect, a high-angle

misorientation is, in fact, a plane between two

individual grains with differently oriented lattices, in

contrast to a low-angle misorientation that is to be

regarded as a defect within one single crystal [27].

Thus, a grain boundary is comparatively low in stress.

Within a certain volume embedding a structure

imperfection in quartz, the crystal structure is distorted

and deviates from the proper structure, more easily

accommodating foreign ions than an undistorted

structure. Consequently, aggressive agents have an

easier job breaking the stressed bonds in and near

imperfections than elsewhere, whence the solubility of

quartz also reflects its structural perfection.

The above provides the basis for the supposedly

increased alkali-solubility of deformed quartz as

compared to defect-free quartz [see, e.g., Ref. [6]].

Experimental mortar bar expansion testing on heavily

deformed rocks (Norwegian mylonites) confirmed

these are violently alkali-reactive indeed [66]. Thus,

there seems to be more to the alkali-reactivity of

deformed quartz in mylonite than merely undulous

extinction [e.g., Ref. [6]] and a strongly oriented grain

fabric.

Twin planes also represent a discontinuity in a

crystal structure, albeit for a totally different reason. In

quartz, Dauphine twins are most common followed by

Brazil twins [28,29]. Less-common twin modes are

summarized in Refs. [30,31]. The structures of both

twin individuals are differently oriented by a crystallo-

graphical symmetry operation, such as, e.g., a

rotation, a mirror or a glide plane, or a combination.

The difference in orientation results in a distortion of

the interface’s crystal structure. For instance, in

Dauphine-twinned quartz, the twin interface acquires

a distorted h-quartz structure with stressed interatomic

bonds, as a kind of average of the two di-trigonal

individuals. Other twinning modes may have different

interface structures, with different amounts of bond-

stress [13]. The thermodynamic stability of the silica

at a twin interface is reduced relative to bulk material.

Hence, twin interfaces are in principle more prone to

attack in aggressive environments, including high pH.

In natural quartz, Dauphine-twinning occurs at all

sizes down to the transmission electron microscopy

scale [e.g., Ref. [32]]. Dauphine twinning can also be

mechanically induced by applying oriented stress. As

Dauphine twinning is optically inactive, it remains

invisible in petrographic microscopy. Thus, its macro-

scopic identification in quartz oscillator plates and

thin sections is traditionally done using etching

techniques, giving good but mainly qualitative results

[28,31]. Etching techniques have long been known

but have a major disadvantage in that they often use

health-hazardous chemicals [e.g., Ref. [33]]. In con-

trast, electron backscatter diffraction mapping is a far

safer alternative that provides quantitative data on

twinning and domain building as well as (mis-)

orientation at the same time.

High-temperature experiments in the transmission

electron microscope have demonstrated that Dauphine

twins disappear above the a-h transition temperature

at 573 8C, conforming to expectation from crystal-

symmetry points of view. However, when the sample

is cooled through the transition temperature, the

M.A.T.M. Broekmans / Materials Characterization 53 (2004) 129–140134

original Dauphine twin pattern reemerged. Repeated

thermal cycling resulted in only minor changes [34].

Early high-temperature experiments on quartz oscil-

lator plates back in the 1940’s gave very similar

results on the macroscopic scale [28]. Amazingly, it

was also found that even sustained heating for 14 days

to temperatures up to 1000 8C could not eliminate

initial/natural Dauphine twinning. In an additional

experiment, synthetic Dauphine twins were induced

mechanically on quartz plates already containing

natural twins. The natural Dauphine twins were as

persistent as previously observed. However, the

synthetic Dauphine twins did disappear, even after

short heating to only 600 8C, providing a great

contrast to their natural counterparts.

From similar experimental results on anorthite, it

has been speculated that this memory-effect might be

attributed to foreign ions present at the location of the

twin interfaces. The deviations from the structure at

the twin interface may accommodate foreign species

that effectively pin the twin boundary and cause it to

reemerge at its original position [35]. Dry heating only

slowly moves these species away from their initial

positions, explaining why the original twin pattern is

so persistent and with only minimal change. On a

much larger size scale, interspersed small mica grains

have been proven to be effective in pinning grain

boundaries in quartzite [27].

The deformation inflicted on the quartz lattice by

polishing may result in the formation of a surface

layer with an amorphous, noncrystalline structure,

until a depth of a few hundred Angstrom, called the

Beilby layer after its discoverer [36]. Beilby layers

have been identified in a variety of materials,

including oxides (i.e., gem materials), and were

originally thought to be caused by polish-induced

flow of the top surface. The original theory of Beilby

layer formation was dismissed in the 1980s [37].

Amorphous Beilby layers also form when material is

being crushed. Analysis by A-differential thermal

analysis (DTA) of quartz ground in a ball mill for

20 h clearly showed that the material was not

completely amorphous and still had a relic a–h phase

transition near 573 8C, although peak broadening

already occurred after 1 h. However, in similar

experiments with a regular DTA (i.e., non-A), evenextended milling for 400 h could not remove

completely the transition peak [38].

Beilby layers are known to affect the surface

reactivity of silica with respect to its toxicity in

respiratory issues; bulk silica is less harmful. Beilby

layers on silica powder can be removed by acid

etching or simply by ageing in a wet environment

[39]. Therefore, the surface of freshly crushed

aggregate could be expected to be more reactive in

a similar way.

3.5. Effect of foreign species in the solid silica

Despite being known as one of the purest mineral

species in nature, quartz usually contains a small

amount of foreign species, including hydrous species.

The helical crystal structure of quartz proper is very

rigid and does not easily accommodate ions of

deviating size or charge, limiting replacement to

minor amounts at specific locations. The most

common substituents for tetravalent Si4+ are trivalent

Al3+ and Fe3+, leaving uncompensated charges in the

structure. To maintain electrical balance, small mono-

valent cations such as H+, Li+, Na+, and/or K+ enter

the quartz structure in interstitial spaces, not at

original Si-locations [40]. Alternatively, two Si4+

can be substituted by one Al3+ plus one P5+, some-

times dubbed the berlinite replacement. Berlinite is a-

AlPO4 which is iso-structural with quartz. Substitu-

tions with other elements (Ti4+, Mg2+, etc.) have also

been reported [31,41–43].

As the Al3+ and/or Fe3+ substituents are bonded in

the quartz structure, they are rather immobile under

ambient conditions. In contrast, interstitial charge

compensating ions are rather mobile and can be

exchanged for other species by electrochemical treat-

ment [44], in particular, through the helical channels

in the quartz structure that run parallel to the c-axis.

This effect was described for the first time almost 120

years ago, in the late 1880s, by the brothers Jacques

and Pierre Curie and contemporaneous researchers

[Ref. [45], and references therein].

Alternatively, silicon in quartz can be substituted

by four H+ for one Si4+ [40]. All four H+-s are each

bonded to their own O2�, thus in fact making up four

hydroxyls filling an Si4+ vacancy. This entire arrange-

ment is called a silanol group. In larger structural

vacant volumes, additional water molecules may

become H-bonded to the silanol groups, forming

nano-inclusions of essentially bonded water mole-

M.A.T.M. Broekmans / Materials Characterization 53 (2004) 129–140 135

cules. Due to the bonding, the freezing temperature of

the water is considerably reduced, by tens of degrees

centigrade. In natural quartz, silanol groups are

common at dislocations, in previous cracks healed

under elevated conditions as demonstrated experi-

mentally by Bakker [46], and in neogenic quartz in

compacted sandstone as demonstrated from geolog-

ical practice [47]. Silanol groups may become an

active color center after activation, e.g., by the

electron beam in cathodoluminescence. They make

quartz luminesce in the far red, with intensity

increasing after prolonged beam exposure.

3.6. Effect of silica grain size

Grain size is in itself not a fundamental material

property. Nevertheless, the grain size of quartz does

affect its solubility, because of the increasing surface

area with diminishing grain size. If more area is

available for dissolution, then the material will

dissolve in a shorter time. In addition, the small

(quartz) grains need to be accessible for the solvent,

requiring porosity and permeability of the aggregate

grain to facilitate (chemical) communication. For

instance, very fine grained silica will react delete-

riously in porous chert, whereas it will not do so in

nonporous chert, also for porous and dense parts

within one single chert grain. In addition, the

logarithmic plot of NBRI-expansion vs. mean quartz

grain size in Norwegian mylonites is suggestive of an

davailable surface areaT-related relationship [48].

Quartz solubility increases significantly at grain

sizes below ~0.1 Am due to an inflection point

occurring in the particle’s free Gibbs energy DGf

[4]. Quartz particles with a convex curvature (i.e.,

spheres) have a measurably higher solubility. As a

result, the finest quartz particles sub ~0.1 Am will

dissolve, whereas larger particles will grow. This

process of overall grain coarsening has long been

known as Oswald ripening. In contrast, quartz with a

concave curvature experience decreased solubility.

Consequently, voids in fine-grained porous chert will

fill with newly precipitated quartz, becoming more

equidimensional and clotting initial permeability.

Related to the above discussion on grain size and

enhanced solubility is the definition of grain bounda-

ries. A grain boundary can be defined as a certain

misorientation angle between two separate volumes in

the same crystal structure, essentially a planar array of

dislocations. Two volumes separated by a low-angle

dislocation array are considered as one crystal struc-

tural entity; if separated by a high-angle boundary

instead, they consist of two contiguous individuals.

Arbitrarily, the distinction between low- and high-

angle is put at 108–158. Alternatively, a nonplanar

Dauphine twin interface and planar arrangements like

a Brazil twin interface or deformation lamellae can

also be regarded as a grain boundary [27].

3.7. Catalytic effects of coexisting minerals

In the last few decades, more and more articles

have appeared describing the catalytic action of

certain minerals upon dissolution reactions/rates.

Biotite has been documented to promote the formation

of sillimanite at elevated conditions [49], and of

hydrogarnet at lower grade conditions [50]. Phyllosi-

licates, clay minerals, and zeolites have proven

catalytic properties on the most diverse chemical

processes [51,52], and it has even been postulated that

clay minerals could be closely involved with the

origin of life [53].

Several minerals are known to affect the pH of

pore solutions. In 1983, Boles and Johnson [54]

published results from an elegantly simple experiment

demonstrating that fine-grained muscovite suspended

in water affected the final pH of the suspension

differently than biotite did. Depending on mineral

composition and detailed crystal structure (in the

experiment mentioned above: dioctahedral vs. triocta-

hedral mica), phyllosilicates may either spawn H+ into

the solution or absorb it from, in both cases altering

the pH. In addition, alkalies rather easily leach from

the sides of phyllosilicate flakes into the pore water by

incongruent dissolution.

The effect of phyllosilicates on quartz solubility

has previously been described for muscovite in

sandstone. Upon compaction of sand into sandstone

under diagenetic conditions, single flakes of musco-

vite appear to penetrate deeply into quartz grains

sideways, without kinking or even bending. The

quartz grains, in particular, are of detrital origin and

not neogenic precipitates, as confirmed by cathodo-

luminescence. A simple calculation using published

E-module data for mica precludes mechanical intru-

sion into quartz, simply due to lack of strength of the

M.A.T.M. Broekmans / Materials Characterization 53 (2004) 129–140136

mica. Thus, the only way to penetrate into quartz is

to dissolve away the quartz at the side of the mica

flake [55].

Adjacent quartz grains in the same sandstone

material often have a serrated interface, whereas,

thermodynamically, a smooth boundary would be

more advantageous. Element maps of K, Al, and Si at

high magnification correlate directly with a back-

scatter image of the same area suggesting the presence

of clay minerals (e.g., kaolinite, smectite) at the

interface, but the mineral grains themselves proved

beyond resolution of the scanning electron micro-

scope used [55]. Such (serrated) grain boundaries are

described as having denhanced visibilityT under a lightoptical microscope. Independently, a similar observa-

tion on grain boundaries had been made in alkali-

reactive sand-/siltstone in Dutch concrete [56,57].

Both penetration of mica into detrital quartz and the

formation of serrated grain boundaries are attributed

to the catalytic action of mica and/or clay phyllosili-

cates [58].

4. Comparison with quartz in the concrete

environment

4.1. Geological vs. ambient conditions

Conditions in the earth’s crust range from ambient

up to roughly 1.5 GPa and 1200 8C, whereas ambient

conditions in concrete structures may range up to ~30

MPa at temperatures ranging from around �20 to +30

8C in a temperate climate, although extreme values

may be substantially different. Under ambient con-

ditions, some minerals initially formed under higher-

grade geological conditions deeper in the earth’s crust

will dissolve and reprecipitate in another assemblage.

Under ambient conditions, dissolved species are

transported by liquid water, resulting in the local

depletion of particular chemical species and enrich-

ment elsewhere. Transport of locally dissolved matter

necessarily also affects porosity and permeability of

the rock.

4.2. Silica gel composition

The true chemical composition of silica gel from

laboratory, natural, or concrete systems cannot be

represented by H4SiO4, although such is often done

for brevity. As explained above, the solubility of

quartz (and silica in general) is influenced by other

species in solution, each with a specific affinity for the

quartz, dissolved or solid. The more outspoken the

affinity of a given species, the more firmly that

species attaches itself to the quartz [4].

Real-life compositions of alkali–silica gel typically

contain significant amounts of Na+, K+, and Ca2+,

whereas microscopic observations on the mineralogy

of the alkali–silica reaction and its products suggests

that Al3+, SO42�, CO3

2�, and possibly also Mg2+ and

Fe2+/3+ play an important role [59]. The alkali–silica

gel reaction product may, in an advanced stage of

deleterious ASR, be extruded along cracks, even

outside the concrete. Element species with a high gel-

affinity will be transported along with the extruded

gel, whereas other species reside with the residual

material and behave immobile [60].

4.3. Silica particle size, aggregate porosity, and fluid

access

Although grain size in itself is not a fundamental

material property of quartz, it does affect its solubility,

especially in combination with porosity and perme-

ability of the aggregate grain. Thus, aggregate grains

containing quartz with a small initial grain size and/or

coarse-grained quartz that suffered extensive grain

size reduction and/or subgraining due to geological

deformation (e.g., load compaction, mylonitization)

will be more prone to develop a deleterious ASR due

to an increase in available surface, provided that the

increased surface is sufficiently accessible for the pore

solution. In addition, quartz solubility is higher for

grain sizes below ~0.1 Am for a fundamentally

different reason.

Nota bene: referring to optics theory, the resolving

power of a modern light optical microscope is limited

to around ~1 Am, under optimum conditions with very

thin (submicron) samples, and using oil immersion. In

20- to 30-Am thin petrographic sections, studied in air

with a cover glass mounted (standard thickness: 0.17

mm=170 Am), optical resolution is typically reduced

to 3–5 Am [78]. Per geological definition, siltstones

have a grain size ranging from 63 Am down to 2 Am.

Cherts have an average observed grain size of 8–10

Am [18]. Thus, in a typical petrographic thin section

M.A.T.M. Broekmans / Materials Characterization 53 (2004) 129–140 137

of chert or fine-grained siltstone, several quartz grains

will be stacked on top of each other, blurring view and

reducing resolution. Grains that eventually would

enjoy enhanced solubility due to their small enough

grain size are completely beyond optical resolution.

The discussion of grain size and subgraining is

obviously related to the phenomenon of undulous (or

dwavyT) extinction. Extinction angles may vary con-

siderably within a single quartz grain, up to 108–158or more between its extremities, and obviously also

depending on its size [6]. According to the (arbitrary)

criterion of 108–158 described above, an undulous

grain must contain several low-angle boundaries

separating domains with a slightly different orienta-

tion, effectively reducing overall grain size. Fluid

access is facilitated along the angular misfit between

adjacent domains separated by a planar dislocation

array. There, fluids may have an easy job attacking the

dislocation-rich subgrain walls (compare Refs.

[40,46,61]).

Not all twin modes are optically active and/or may

be easily overlooked, especially in fine grained

material like chert or siltstone. The interfaces between

twin individuals in a-quartz do not have a proper

quartz structure but a deformed one, sometimes

actually closer to h-quartz. Foreign species tend to

favor the deformed interface structure, inducing dtwinmemoryT by pinning twin boundaries [28,35].

Chemical impurities and foreign ions, including

water and silanol groups, tend to associate with

dislocations, vacancies and other structural irregular-

ities in quartz, providing easy access for attack by

chemical and/or physical forces [62]. The effect of

lattice defects and grain boundaries has recently been

addressed in general terms by Wigum [48]. He

recognizes the impact of the problem in proper

characterization of aggregate materials and he devel-

oped a point-counting method for application to

mylonites. Whether, and if, and to what extent the

method works for other rock types has not been

verified.

4.4. Identification and characterization of silica

According to ASR literature and field experience,

quartz is by far the most occurring silica polymorph in

confirmed alkali-reactive aggregate materials. In most

studies, the reactive species is mostly only identified

as quartz (eventually in chert, siltstone, or other rock

types), but only rarely characterized in detail by

mineralogical or geochemical methods in terms of

crystal structure, the presence of domain building and

twinning, and/or its chemical composition [e.g., Refs.

[63,64]]. Whereas identification of quartz by petrog-

raphy is straightforward and mostly occurs by face-

factor, quartz qualities affecting its solubility and

susceptibility for deleterious ASR have to be assessed

with more intricate analytical methods, especially in

fine-grained materials. That includes the presence of

moganite that appears to be more prominent in fine

grained silica varieties [13,25].

Quartz containing Norwegian felsic mylonites

have been reported to be violently alkali-reactive

[65,66]. Although a correlation of total quartz area

versus accelerated expansion testing was demonstra-

ted [48], the fundamental reason for the increased

quartz solubility remained unaddressed. Several

crystal structural and compositional options exist that

can increase quartz solubility, which, in principle,

also could affect quartz crystallinity, including dis-

location density. However, the Normin2000 project

[67] puts blastomylonites that have enjoyed

dequilibrium recrystallizationT after their deformation

in the same alkali-reactive category. The recrystalli-

zation changes the strongly oriented fabric into a

more equiangular fabric, also reducing the dislocation

density. As a result, the quartz has smooth extinction

as opposed to undulous in a non-recrystallized

mylonite, suggesting a decrease in reactivity potential

[compare Ref. [6]]. Recently, two regular Norwegian

mylonites (i.e., non-recrystallized) were found to

behave inertly in repeated accelerated mortar bar

experiments [68], despite their true mylonite texture

and fabric.

A similar thing occurs with sand-/siltstones that,

according to the current Norwegian regulations,

classify as potentially alkali-reactive, without any

further specification. However, recent long-term field

experience shows that some sandstones apparently do

not react deleteriously, despite their classification. In

contrast, Dutch sand-/siltstone is regarded as nonalkali

reactive according to the latest regulations (unless

containing chert fragments, chalcedony or opal), but

field experience has actually confirmed that some

grains do react [69]. Geologically speaking, sand-

stones display a broad variation in composition,

M.A.T.M. Broekmans / Materials Characterization 53 (2004) 129–140138

structure, texture, and other properties; however, that

is not reflected in the Dutch and/or Norwegian ASR

classification.

Apparently, classifying a rock as potentially alkali-

reactive by simply applying nomenclature is not quite

good enough as innocuous rocks may be included as

well, obviously an undesirable situation. Therefore,

proper characterization of silica species in potentially

and actually alkali-reactive aggregate must also

include these aspects.

4.5. Catalytic actions of coexisting minerals

The presence of some nonsilica minerals has been

tied to the occurrence of ASR in several cases, e.g.,

phlogopite [70], dawsonite [59,71], and basalt rock

[72,73]. The possible active contribution of mica to

deleterious ASR in Dutch sand-/siltstone has been

suggested, describing bell-shaped areas of increased

porosity at the edges of detrital muscovite grains

[56,57]. In thin sections, the grain boundaries in such

sand-/siltstones appear optically enhanced, matching

descriptions in sedimentology literature on catalytic

action of detrital mica [55,58].

Catalytic or other (inter-) action of minerals has

not too often been demonstrated in natural rock

systems, and not at all for ASR or other causes of

concrete deterioration. Such interaction can for

instance consist of incongruent dissolution preferently

releasing alkalies [e.g., Refs. [74,75]], or a truly

catalytic action affecting pore water pH [Ref. [54];

also see Ref. [76]]. Recently, it was observed that

portlandite occurred intimately intergrown with and

within frayed biotite flakes in Dutch concrete [77],

strongly suggesting a pH-dependent relationship

present in concrete, at least during the precipitation

of portlandite [see also Ref. [50]].

5. Conclusions

There are a great number of qualities and properties

about quartz as a material that may affect its undesired

dissolution under ASR conditions. Some of them are

quite obvious and relatively easy to assess, but most

of quartzTs intricacies are more difficult to character-

ize as they require specific instrumentation. Detailed

and thorough analysis of alkali-reactive silicas from

different origins with a range of techniques and

methods will be needed to understand why some

silica is alkali-reactive and why others are not, what

their differences are, in order to be able to develop

reliable (and preferably quick) testing procedures.

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