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INTERFACE SCIENCE 12, 411421, 2004

c 2004 Kluwer Academic Publishers. Manufactured in The Netherlands.

The Interfacial Transition Zone (ITZ) Between Cement Paste and Aggregate

in Concrete

KAREN L. SCRIVENER

Ecole Polytechnique Federale de Lausanne, Switzerland

[email protected]

ALISON K. CRUMBIELyon, France

PETER LAUGESEN

Dansk Beton Technik, Denmark

Abstract. This paper describes the so called interfacial transition zoneITZin concrete. This is the region of

the cement paste around the aggregate particles, which is perturbed by the presence of the aggregate. Its origin lies

in the packing of the cement grains against the much larger aggregate, which leads to a local increase in porosity

and predominance of smaller cement particles in this region. The ITZ is region of gradual transition and is highly

heterogeneous, nevertheless the average microstructural features may be measured by analysis of a large numbers

of backscattered electron images of polished concrete samples. Such measurements show that the higher porositypresent initially is significantly diminished by the migration of ions during hydration.

Keywords: cement, concrete, image analysis, scanning electron microscopy, porosity

Introduction, Importance of the ITZ

The interfacial transition zone (ITZ) between cement

paste and aggregate is the most important interface

in concrete. Concrete is often considered to be a two

phase composite materialcement paste plus aggre-

gates. However even in the most basic phenomena the

critical role of the ITZ is clear. This is graphically illus-

trated by comparison of the stress strain curves for ce-

mentpaste, aggregatesand concrete under compression

loading (Fig. 1). Individually cement paste and aggre-

gates both show brittle elastic behaviour, that is to say,

linear, reversible deformation up to a limit, followed by

sudden failure. In contrast, concretethe composite

materialshows significant quasi-ductile behaviour.

The load bearing capacity continues to increase be-

yond the linear elastic limit and there is a progressive

To whom all correspondence should be addressed.

decrease in load bearing capacity after the peak load.

Such behaviour, which has important practical conse-

quences, is due to the development of multiple micro-

cracking predominantly in the ITZ. This well known

behaviour leads to the common view of the ITZ as the

weak link in concrete.

TheoriginoftheITZliesinthesocalledwalleffectof packing of cement grains against the relatively flat

aggregate surface (Fig. 2). This is directly responsible

for the features of the ITZ, particularly its higher poros-

ity. Due to the way it is formed the ITZ is not a definite

zone, but a region oftransition. It effective thickness

varies with the microstructural feature being studied

and during the course of hydration. As cement is a

particulate material, the details of this transition zone

are different around each aggregate particle. The effect

of the ITZ on mechanical properties has already been

alluded to. The impact on transport properties is less

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412 Scrivener, Crumbie and Laugesen

Figure 1. Comparative stress strain curves for aggregate, paste and

concrete, the quasi-brittle behaviour of concrete illustrates the im-

portance of the ITZ on the macroscopic properties of concrete.

straightforward, but nevertheless significant. Methods

of modifying the ITZ are critical in the development

of high performance concrete. The ITZ is also critical

in determining the effective water to cement ratio of

mortars and concretes.

The aim of this article is to present the features that

give rise to the formation of the ITZ and to discuss how

Figure 2. Illustration of the wall effect. A flat solid object placed at random in an assembly of cement grains would cut through grains. As

this is impossible the packing of grains is disrupted to give a zone of higher porosity and smaller grains in the zone close to the aggregate.

this affects the properties of concrete. The Interfacial

Transition Zone has already been the subject of threetechnical committees of RILEM, whichhave published

state of the art reports [1, 2].

The Origin of the ITZPacking of Cement Grains

Against Aggregate

Cement grains range in size from less than a micron to

up to 100 microns. Aggregate particles are several or-

ders of magnitude larger. This difference in size means

that each aggregate particle is a mini wall which dis-

rupts the packing of the cement grains, resulting in the

wall effect as illustrated in Fig. 2. If a large objectwere placed in a random assembly of grains, it would

cut through many grains. As this is physically impos-

sible in the case of aggregate particles in cement paste,

the normal packing of the grains is disrupted. The re-

sult is that a zone closest to the aggregate contains pre-

dominately small grains and has a significantly higher

porosity, while larger grains are found further out.

In concrete this means that the sizeof the ITZ is com-

parable with the size of the cement grains. As packing

is a random process, each individual region of ITZ will

be differentthe ITZ is heterogeneous on the same

scale as the cement grains, therefore the average ef-fects may not be immediately apparent in images of

concrete microstructure, as for example Fig. 3. Most

diagrams of the ITZ found in the literature do not give

a correct idea of scale. As will be discussed there is no

discrete boundary between the ITZ and the bulk paste.

The changes are progressive and are most significant

in the first 1520 m closest to the aggregate (white

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Interfacial Transition Zone (ITZ) in Concrete 413

Figure 3. Backscattered electron(BSE)image of concrete,aggregateon left. Thewhite linesindicates distancesof 20and 50m frominterface.

lines marked on Fig. 3 are at approximately 20 and

50m from the interface). Therefore in good quality

concretes the ITZ is NOT apparent as distinct band of

higher porosity in, for example, thin sections impreg-

nated with florescent resin, by light microscopy.

However, even in good quality concretes, it is quite

usual to observe heterogeneities in the microstructure

on a scale of several hundred microns. Figure 4, illus-

trates sucha casewhere severalaggregatesare clustered

together. In the oval labelled 1 the microstructure hasa significantly higher porosity. In the oval labelled 2

there is a concentration of calcium hydroxide along the

lower edgeof the aggregate, butsuchconcentrationsare

not observed around all aggregates.

The challengein characterising theITZ is to measure

the average microstructure in real concretes. This can

be done by backscattered electron (BSE) image analy-

sis of polished sections of concrete [36]. Results ob-

tained from this technique (mainly from the PhD thesis

of Crumbie [6]) form the bulk of this paper. The ma-

jor advantage of this technique is that the ITZ can be

studied in context in normally prepared concretes. The

disadvantages of the technique are that it is very time

consuming and that observations are made on a two-

dimensional sections through a three dimensional mi-

crostructure. The sectioning effect means that the dis-

tance measured on 2D sections are on average greater

than the perpendicular distances in the 3D microstruc-

ture, it hasbeen estimatedthat the average lengthening

factor is around 1.2 [6]. The distances quoted here are

the uncorrected distances measured on 2D sections.In the development of the BSE/image analysis tech-

niques considerable work was done to establish the

number of fields which needed to be measured to ob-

tain representative results from a concrete. With im-

age analysis, the compromise must always be made,

between having the best resolution (highest magnifica-

tion) to observe thefeaturesof interest,whilst analysing

a large enough area to be statistically significant. Dif-

ferentmagnifications, bandwidths and number of fields

were studied in the PhD thesis of Crumbie [6]. As a re-

sult of these investigations the fractions of anhydrous

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Figure 4. BSE image of concrete illustrating typical inhomogeneities. Several aggregate grains are clustered together resulting in the formationof a porous zone (1). Along the bottom of one aggregate there is a high concentration of calcium hydroxide (2).

cement, calcium hydroxide, other hydration products

and porosity were evaluated in 30 bands of about 3 m

in width around aggregate in 100 randomly chosen im-

ages for each concrete. Taking into account the spec-

imen preparation, image acquisition and image analy-

sis, several days were needed to analyse each concrete.

Therefore, this is not a technique to be applied rou-

tinely to concretes. However, the detailed study made

by Crumbie allows the way in which the ITZ forms to

be understood.

The measured distribution of unhydrated cement in

a standard concrete at a w/c = 0.4 at different ages are

shown in Fig. 5. The earliest age at which it is practical

to prepare a polished section is 1 day; by which time,

considerable reaction of the cement has already taken

place. However the original distribution can be back-

calculated from the amount of unhydrated cement at

one day and the amount of hydration products. This

curve indicates that a deficit in anhydrous grains is only

significant in a region of about 15 m adjacent to the

aggregatesimilar to the size of the average cement

grain.

As hydration continues, the more detailed effects of

the disrupted packing on the grading of the cement

particles becomes apparent. The small grains quickly

hydrate completely, while the larger grains have a core

of unhydrated cement. At any given time the thickness

of each grain that has reacted will be the roughly the

same, so the unhydrated fraction will be greater for

larger grains. This explains the observation that, the

width of the zone with a reduced amount of anhydrous

material increases with age up to one year. At 1 year a

reduction in the amount of anhydrous cement is appar-

ent over a zone of more than 40 m.

The secondary minimum at around 35 m, was

found to occur in all the concretes measured. From

this it is deduced that as smaller grains pack close

to the interface, the region further out is depleted of

small grains, so for the large grains remaining there

is less reaction of anhydrous materials between 1 and

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Figures 5 and 6. Distribution of unhydrated cement in concrete (w/c = 0.4) at various ages. From the way in which this changes during

hydration, the effect of the aggregate on the grading of the cement grains in the ITZ can be deduced (Fig. 5). Schematic representation of grading

of cement grains in ITZ (Fig. 6).

28 days. These effects are shown schematically in

Fig. 6.

Given the relatively large zone in which the packingof the cement grains is disrupted by the aggregate par-

ticles, it is debatable if the term bulk paste really has

any meaning in concretefrom the surface area of the

aggregate and the volume of paste it can be calculated

that the average thickness of paste around aggregate

particles is only of the order of 50 m and the typical

maximum distance between aggregatesseen in sections

of concrete is only a few hundred microns. However, as

already indicated, and as will be discussed further, the

most significant differences occur in a much narrower

zone.

Effective Water to Cement Ratio of Concrete

The measurements discussed above, show that thewall effect produced by the aggregate leads to a zone

of the order of 15 m in width around each aggregate

particle, in which there are less cement grains in the

fresh state. This is equivalent to a zone of higher water

to cement ratio. In a typical concrete some 2030% of

the cement paste lies within 15 m of the aggregate.

Therefore, a higher water to cement ratio in this zone,

means that the water to cement ratio of the bulki.e.

paste more than 15 m from the aggregate must be re-

duced. For a concrete with an overall w/cratio of 0.4,

the w/cratio of this bulk paste is only around 0.35.

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Figure 7. Fluorescent light micrographs of thin sections of mortars with the same water to cement ratio but different sand contents. At lower

sand contents the lighter appearance of the paste indicates a higher water to cement ratio. The apparent water to cement ratio decreases with

increasing sand content.

The impact of aggregate content on theeffectivew/c

ratio, can be seen more dramatically in Fig. 7, from

work by Laugesen [7]. This shows a series of mortars

all with the same w/c (0.45), but with different sand

contents. The mortars are impregnated with fluorescent

resin and thin sections prepared. The individual pores

are not resolved, but the average luminosity of the hy-

drated areas is proportional to the amount of resin they

contain and so to their porosity.

It can be seen that as the sand content increase, the

paste areas become darker, indicating a lower porosity

due to a lower effective w/c ratio in the bulk of these

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paste areas. It is estimated that for a standard mortar

the overall w/c of 0.45 gives an effective w/c in thebulk paste of 0.38.

Distribution of Hydration Products in the ITZ

The packing of the anhydrous cement grains is the

origin of the ITZ, but its eventual structure is also

determined by the way in which the hydration prod-

ucts are deposited in this region. During hydration,

the microstructural development of Portland cement is

dominated by the formation of the two major hydrate

phasescalcium silicate hydrate, C-S-H1 and calcium

hydroxide, Ca(OH)2, CH1. After the first few seconds,

the concentration of silicate in solution remains very

low and consequently the C-S-H phase is mostly de-

posited directly around the cement grains. In contrast

the concentration of calcium in solution is much higher

and calcium hydroxide is mainly deposited in the open

pores. Furthermore, it has been suggested that silica

inhibits the nucleation of calcium hydroxide, which

favours the precipitation of this phase as far from the

cement grains as possible. The resulting microstructure

is shown schematically in Fig. 8.

As explained above, the packing of cement grains

leaves an initially more porous zone around the ag-gregate. This favours the deposition of more calcium

Figure 8. Schematic representation of the microstructure of

Portland cement pasteC-S-H product deposits around the ce-

ment grains, while calcium hydroxide precipitates in the pore

spaces.

Figure 9. Average distribution of calcium hydroxide in the ITZ.

hydroxide in this region as can be seen from the experi-

mental results shown in Fig. 9. As the quantity of anhy-

drous material in this region is low, most of this calcium

hydroxide must form from calcium ions coming from

the reaction of anhydrouscement outside theinterfacial

region. From the calculated initial distribution of the

anhydrous cement grains and the amount remaining at

a given time the amount of calcium hydroxide, coming

from the cement grains in each band can be calculated.

By comparison with the actual amounts, the amount

of excess or deficit, relative to the case of local depo-

sition can be calculated. These calculated amounts are

shownin Fig.10.From Figs. 9 and 10it can beseen that

the zone of increased calcium hydroxide corresponds

very closely to the zone which is deficit in anhydrous

material on mixingi.e. has excess porosity. Most of

the excess calcium hydroxide is precipitated in the first

Figure 10. Redistribution of calcium hydroxide in the ITZthe

area of excess is the % area above that which would have been

formed from the local reaction of anhydrous material. This indicates

that there hasbeen a netdiffusion of calcium andhydroxide ions into

the 10 m closest to the aggregate.

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day, with a slight increase to 28 days, but negligible

further increase to one year.Although it has been shown that the ITZ contains

excess calcium hydroxide compared to the bulk, this

amount in the ITZ is by no means a continuous layer

around all aggregates, as Fig. 3 illustrates. Even in the

3 m band closest to the aggregate the amount of cal-

cium hydroxide is only on average around 13%i.e.

about one eighth of the total volume. Higher excesses

of calcium hydroxide may be seen locally as in Fig. 4.

These heterogeneities are a normal feature of well for-

mulated concrete. In poorly formulated concrete mi-

crobleeding may occur and lead to the formation of

lenses of water beneath aggregate particles which be-

come filledwith calcium hydroxide.Such features werenot included in any of the measurements shown in this

paper.

With the BSE imaging technique it is not possible to

measure the orientation of the calcium hydroxide. Ear-

lier work from the group at Toulouse University using

X-ray diffraction and progressive polishing on model

specimens, with cement paste cast against a single mas-

sive aggregate indicated that there is a preferential ori-

entation of Calcium hydroxide with thec-axis parallel

to the aggregate surface [8].

The distribution of C-S-H and its redistribution com-

pared to formation solely from anhydrous material inthe immediate area is shown in Figs. 11 and 12. Despite

the lower mobility of silica, there is still significant re-

distribution of this product between the interfacial zone

and the bulk during the first day. However, it is appar-

ent that redistribution occurs at a more local scale

material deposited in the first 5 m is compensated by

a deficit in the next 10 (515) m. After the first day,

there is little further change over 1 year. These obser-

Figure 11. Average distribution of other hydration products, pre-

dominantly C-S-H.

Figure 12. Redistribution of C-S-H in ITZ (excess of C-S-H over

that which would be formed by the local reaction of anhydrous ma-

terial).

vation confirm the lower mobility of silica (needed to

form C-S-H) compared to calcium.

The other hydrate whose distribution in the ITZ has

been studied is ettringite. Monteiro and Mehta [9] mea-

sured an increase in ettringite in theITZ (usingin model

specimens by the Toulouse XRD technique). The small

crystals of ettringite cannot be easily resolved in BSE

images, so it has not been possible to confirm this mea-

surement in real concretes. The ions forming ettringite

are highly mobile in cement pastes, as witnessed by

the recrystallisation of ettringite into pores and voids

in mature concretes. Therefore an increased concen-

tration of this phase in the more porous ITZ would be

expected.

Porosity in the ITZ

Porosity is the volume not filled by cement grains or

hydration products and therefore is the result of all the

effects described above. The variation in distributions

with age is shown in Fig. 13. The importance of hy-

drate redistribution in modifying the excess porosity

in the ITZ are clear. On mixing the porosity adjacentto the interface is some 40% higher than that in the

bulk. After the first day this difference is reduced to

only 1020 % and the gradient is less steep. At greater

ages the porosity in the ITZ reduces by about the same

amount as it is reduced in the bulk. As there is much

less anhydrous material remaining in the ITZ, this re-

duction must be attributed to the deposition of hydrates

from the migration of ions from the reaction of cement

further from the interface.

These results of image analysis show how the over-

all amount of porosity changes in the ITZ. This overall

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Figure 13. Average porosity in ITZ at various ages.

amount will be the major factor determining the me-

chanical properties of this region. However, the trans-

port properties will depend more on the connectivity

of the porosity. It is not possible to obtain information

about connectivity from a two-dimensional polished

section of a three-dimensional structure. Nevertheless,some other work [10] gives some indication that the

connectivity of the ITZ is increased. In this study, con-

crete specimens were subject to mechanical testing.

At various load levels the samples were intruded with

Woods metal while under load and subsequently sec-

tioned and polished. In the polished sections it was

observed that the penetration of the Woods metal oc-

curred preferentially around the aggregate particles,

which suggests a higher connectivity of the porosity

in the ITZ (Fig. 14). It must be borne in mind that the

samples treated with Woods metal were dried prior

to the experiment and heated to 80C to allow the in-

trusion of the Woods metal. Although this treatmentcould have led to microcracking, linking pores which

were not connected in the original state, such cracks

were not seen.

What Happens Right at the Interface

We have seen that the ITZ arises from the packing of

the anhydrous cement grains, which produces a region

of high porosity in the first 1520 mm. However, the

other feature of the ITZ is what happens right at the in-

Figure 14. Concrete impregnated with Woods metal, which has

penetrated better through the ITZ.

terface. Early work on model systems [11] found that

a film of calcium hydroxide frequently precipitated in

this region. However, observations in real concretes

and mortars indicate that there is more often the pre-

cipitation of a thin layer of C-S-H directly at the in-

terface [12]. As discussed above during the very early

stages of hydration both calcium hydroxide and C-S-H

form through solution, so the aggregate surface may

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act as a heterogeneous nucleation site for both phases.

Siliceous rock types are essentially inert (leaving asidethe possibilities of alkali silica reaction on much longer

time scales) and there is no chemical bonding at the in-

terface. Calcareous aggregates may react very slightly

leading to the formation of calcium alumino monocar-

bonate in the ITZ.

Modification of the ITZ

Quantitative image analysis clearly shows that the ITZ

is caused by the disruption of packing the anhydrous

cement grains in this region. Thus the ITZ may be mod-

ified by changing the particle size distribution of thecementitious materials. This is now done routinely by

adding around 510% of silica fume, which consists

of particles of amorphous silica ranging in size down

to about 100 nm (0.1 m). This technology is used

widely in the production of high strength concretes.

Early work with the backscattered electron image anal-

ysis techniqueby theauthor andBentur [4]showed how

additions of silica fume densified the packing in the in-

terfacial transition zone (Fig. 15). Even at 1 day the

porosity in this region is much reduced.

Consequences of the ITZ

Despite theclear difference of theITZ from a bulk paste

it is still very difficult to precisely quantify its effect.

This is first because it is not a discrete zone but a re-

Figure 15. Distribution of Porosity in the ITZ of concretes with

and without silica fume. Adapted from [4].

gion of gradually changing microstructure. Secondly,

the preparation of model specimens in which the prop-erties of the ITZ can be measured separately tend to

produce different ITZs from those in real concrete.

As described in the introduction the impact of the

ITZ on mechanical properties is not in doubt. Never-

theless, evaluation of the mechanical properties of the

ITZ from those of the overall concrete is an inverse

problem. Monteiro has reviewed the possibilities for

mechanical modelling [13] and recently published an

analysis with Hashin [14] based on the generalised self

consistent scheme in which the elastic modulus of the

ITZ was estimated to be about 50% lower then the

bulk paste.

Theimpact of theITZ on Transport properties, whichdetermine durability, is more complex and has been

well reviewed by Marchand and Delagrave [15]. Al-

thoughthe higherporosity andprobablehigher connec-

tivity of this porosity suggest that transport of species

should be faster in the ITZ, this effect is counteracted

by several other effects:

The overall volume of the cement paste (the perme-

able component of concrete) is reduced.

The presence of aggregates increases the tortuousity

of the path for transport.

The porosity of the bulk paste is reduced comparedto a pure paste with the same overall w/cratio.

Experimental studies (such as [16, 17]) in which the

aggregate content has been varied keeping the water

to cement ratio constant indicate that rates of trans-

port are decreased as aggregate content increases, de-

spite the increase in the amount of ITZ. This indicates

that the effects of decreasing paste volume and tortu-

ousity outweigh any effect of increased transport in the

ITZ.

Summary

The ITZ is a zone oftransitionnot radically differ-

ent to the rest of the cement paste and its effective

width depends on the microstructural feature be-

ing considered and the degree of reaction.

Quantitative characterisationof the interfacial transi-

tion zone (ITZ) between aggregate and cement paste

in concrete, confirms that it arises due to the packing

of cement grains against the larger aggregate par-

ticles. This initial packing leads to a more porous

zone some 15 to 20 m in width. The deposition of

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hydration products, especially calcium hydroxide

tends to fill this zone, but even in mature pastes theITZ still has a significantly higher porosity.

Because of the packing effect extra water is incor-

porated in the ITZ and the effective water to cement

ratio of the bulk paste in reduced by around 0.05.

The most effective way to modify the ITZ is to add

a proportion of fine particles, such as silica fume,

which can pack closer to the aggregate surface.

The impact of the ITZ on mechanical properties is of

considerable significance, leading to increased duc-

tility amongst other effects.

Any impact of the ITZ on transport properties is

counteracted by other factors which generally lead

to a reduction of transport rates with increasing ag-gregate content if other variables are held constant.

Note

1. These abbreviations use cement chemist notation, C = CaO;

S = SiO2; A = Al2O3; F = Fe2O3; H = H2O; the nomencla-

ture C-S-H indicates the variable stoichiometry of this phase.

References

1. J.C. Maso(ed.), Interfacial Transition zonein Concrete, RILEM

report 11 (E&FN Spon, London, 1996).

2. M.G. Alexander, G. Arliguie, G. Ballivy, A. Bentur, and J.

Marchand, Engineering and Transport Properties of the Inter-

facial Transition Zone in Cementitious Composities, Report 20

(RILEM Publications S.A.R.L., 1999).

3. K.L. Scrivener and E.M. Gartner, in Bonding in Cementitious

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6. A.K. Crumbie, PhD Thesis, University of London, 1994.

7. P. Laugesen, presentation 4th Euroseminar on Microscopy Ap-

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8. J. Grandet and J-P. Ollivier, inProceeding of 7th International

Congress of the Chemistry of Cement(Editions Septima, Paris,

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9. P.J.M. Monteiro and P.K. Mehta, Cement and Concrete Research

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10. K.L. Scrivener and K.M. Nemati, Cement and Concrete Re-

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11. B.D. Barnes,S. Diamond,and W.L. Dolch, Cement andConcrete

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12. K.L Scrivener and P.L. Pratt, in Proc. 8th Int. Cong. on the

Chemistry of Cements, Rio de Janeiro (1986), vol. III, p. 466.

13. P.J.M. Montiero, in ref. 1, p. 64.

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15. J. Marchand and A. Delagrave, in ref. 2, p. 157.

16. A. Delagrave, J. Marchand, and M. Pigeon, Advanced Cement

Based Materials7, 60 (1998).

17. N.R. Buenfeld and E. Okundi, Magazine of Concrete Research

50, 339 (1998).

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