Pr42. Mittermayr Et Al. 2015

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Sulfate resistance of cement-reduced eco-friendly concretes

Florian Mittermayr a,,1, Moien Rezvani b,1, Andre Baldermann c, Stefan Hainer b, Peter Breitenbcher d,Joachim Juhart a, Carl-Alexander Graubner b, Tilo Proske b

a Graz University of Technology, Institute of Technology and Testing of Building Materials, Inffeldgasse 24, 8010 GRAZ, Austriab Technische Universitt Darmstadt, Fachgebiet Massivbau, Franziska-Braun-Strae 3, 64287 DARMSTADT, Germanyc Graz University of Technology, Institute of Applied Geosciences, Rechbauerstrae 12, 8010 GRAZ, Austriad OBERMEYER Planen + Beraten GmbH, Niederlassung Wiesbaden, Biebricher Allee 36, 65187 Wiesbaden, Germany

a r t i c l e i n f o

Article history:

Received 6 March 2014

Received in revised form24 September 2014

Accepted 28 September 2014Available online 13 October 2014

Keywords:

Durability

Degradation

Sulfate attack

Thaumasite

Supplementary cementitious material

Superplasticizer

a b s t r a c t

Two newly developed cement-reduced eco-friendly concretes with high limestone powder content andlow water/powder ratio were tested for sulfate resistance. Mortar samples with a paste composition ofeco- as well as conventional concretes were immersed in 30 g l1 Na2SO4 and saturated Ca(OH)2 refer-

ence solutions for 200 days at 8 C. To evaluate the reaction mechanisms of progressing sulfate attacka combined approach of mechanical, mineralogical, and microstructural methods was applied.

Gypsum and bassanite neo-formations related linearly to the expansion during sulfate exposure,except for one sample where ettringite co-precipitated. Thaumasite formation was not observed in spiteof potentially favorable conditions. This is considered to be related to the evolution of the experimental

solutions, kinetic effects, and the competing formation of CaCO3 polymorphs triggered by the usage of

superplasticizer. Both eco-friendly mixes exhibited a better sulfate resistance than their correspondingreference samples and are therefore suggested to be applicable in low sulfate-loaded environmentsaccording to DIN EN 206-1. Eco-friendly concrete based on CEM III/B performed superior against sulfate

attack and is expected to withstand even severe sulfate exposure despite a much higher water/cement

ratio than required by the standard.2014 Elsevier Ltd. All rights reserved.

1. Introduction

1.1. Principles of eco-friendly concretes

Recent cement production accounts for approximately 5% of theglobal anthropogenic CO2 emissions and thus is a major driving

force for global warming and ocean acidification [1,2]. The majorityof the produced cement is currently consumed by the concreteindustry. Hence, the short- to medium-term decrease of the cementclinker content in concrete mixtures is suggested to be essential for

the reduction of CO2emissions connected to the concrete industry.Such alternative mixtures are referred to as eco-friendly concretes.They are typically developed based on a promising concept whichincludes (i) the application of highly reactive cements, (ii) the

reduction of the water volume and (iii) the introduction of highlyefficient superplasticizers as well as non-reactive, pozzolanic andlatent hydraulic supplementary cementitious materials (SCMs)

[3,4]. Commercial Portland cement (PC) can therefore be partiallyreplaced by e.g. limestone powder, fly ash, and blast furnace slag(BFS) in the concrete mixture. Based on this approach a reduction

of more than 30% cement clinker content in comparison to conven-tional concrete can be achieved, while maintaining the requiredmechanical properties and durability of the concrete[3].

Thepositive effectof SCMs on concrete properties hasbeenstud-ied intensively in the last decades and their application in the con-creteindustry increasedexceptionally [5]. However, the availabilityof e.g. reactive fly ash and BFS is limited in many countries and the

acquisition is oftencost-intensive[6,7]. Thereforean efficient appli-cation of reactive SCMs with limited availability contributing to theconcrete performance is required. Currently, limestone powderadditives have received particular attention complying with both

ecological and economic benefits and mechanical requirements[812].

In previous studies newly developed eco-friendly concreteswithreduced cement content and low water/powder (w/p) ratio were

described [3,9,13]. Their resistance against carbonation (expositionclass XC4), slight freezethaw attack (XF1) and water penetrationdepth was tested successfully, according to DIN EN 206-1 [14]

http://dx.doi.org/10.1016/j.cemconcomp.2014.09.020

0958-9465/2014 Elsevier Ltd. All rights reserved.

Corresponding author. Tel.: +43 316 873 7159; fax: +43 316 873 7650.

E-mail address: [email protected](F. Mittermayr).1 First author.

Cement & Concrete Composites 55 (2015) 364373

Contents lists available at ScienceDirect

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and DIN 1045-2[15]. Results have shown that these eco-friendlyconcretes exhibited similar (or even better) mechanical properties

than the conventional concretes as well as an acceptable durabilityagainst freezethaw attack, water penetration and carbonation[3,9,13]. Compared to conventional concrete containing ordinaryPC, the global warming potential of the eco-concretes is reducedby more than 30% and up to 60% [3]. However, the durability of

these concretes against sulfate attack is still in question and there-fore the aim of the present study.

1.2. The influence of sulfate attack on concretes with limestone

During service life concrete elements often come into contactwith sulfate ions that are present in e.g. soil solutions or ground

water. In such sulfate-loaded environments, the formation ofettringite {3CaOAl2O33CaSO432H2O}, gypsum {CaSO42H2O} andthaumasite {CaSiO3CaCO3CaSO415H2O} is considered to causesevere damage on concrete structures [16,17]. At temperaturesP15 C ettringite (Ett) is typically formed and is known to causemicrostructural damage to the cement paste by generating expan-sive forces, particularly in small pores due to high crystallization

pressure[18,19]. It is generally accepted that aluminate phasesthat occur in the cement clinker (C3A and C4AF) support the forma-tion of Ett and hence have a strong influence on the sulfate-resis-tance of concrete[20,21]. Ett is often accompanied by gypsum(Gp) formation which results in further expansion and destruction.

Gp typically forms in high sulfate-loaded environments that arefrequently used for testing of concrete and mortar in the lab[22,23].

The sulfate resistance of concrete can be strongly improved by

using cement with a low C3AandC4AF content and/or with reactiveSCMs. For instance,Skaropoulou et al. [24] havereported that basedon lab tests mortar specimens with BFS or metakaoline showed abetter performance against sulfate attack than PC based ones inde-pendent from their limestone content. Gutteridge and Dalziel[25]

have shown that limestone additives can enhance the concretestrength by improving the hydration rate and Bonavetti et al. [10]suggested that limestone may react with alumina phases to formstable calcium monocarbonate aluminate hydrate. Furthermore,

Matschei et al.[26]have demonstrated that limestone has a signif-icant influence on the distribution of lime, alumina and sulfatephases during concrete hydration. Although the addition of lime-stone can improve the concretes performance, cements with high

amounts of calcium carbonate fillers are known to be prone tothe thaumasite form of sulfate attack (TSA). Numerous experimen-tal studies have shown that cementitious materials with limestonefillers are susceptible to TSA, especially at low temperatures

( 98 wt.%, a median diameter value (D50 volume based) of15lm and a Blaine value of 330 m2/kg. The fly ash had a similargrain size but a slightly lower Blaine value of 300 m2/kg. In allmixtures the CEN standard reference sand (EN 1961 [42]) with

a maximum grain size of 2.0 mm was used.Mixtures Ref I, Ref II and Ref III, made of the respective cement

types (I), (II) and (III), represent conventional reference mortarswith a water/cement (w/c) ratio of 0.60. This is the maximum value

according to the German standard for concrete (DIN EN 206-1/DIN 1045-2) for exposure class XA1 with a sulfate concentrationin groundwater of 200 < SO4

26 600mg l1. The cement

content of these reference mortars was in accordance with EN196-1 Methods of testing cement. Mixtures Ref II and Ref III

reflect semi-reference mortars which both have a w/p ratio of0.60 but correspondingly higher w/cratios of 1.07 and 0.76, respec-tively. The mix designs of the cement-reduced eco-mortars were

characterized by high limestone powder content and a low w/p-ratio of0.35 (Eco II and Eco III). The lowerw/p-ratio was chosento fulfill the concept of clinker-reduced concretes and to attain asufficient durability [3]. To maintain the aimed table flow of

180 mm according to DIN 1015-3[43]the use of a superplasticizerwas necessary for the Eco-mixes. The percentage of cement reduc-tion in these eco-mortars was up to 35% compared to thecorresponding concretes[3].

2.2. Casting, curing, and test procedure

Mortar specimens prisms (40 40 160 mm) and thin

prisms (10 40 160 mm) were fabricated according to EN196-1 and demolded after 48 2 h. On the basis of the GermanBuilding Authority (DIBt) testing procedure for sulfate resistance[44], the specimens were pre-cured for 14 days at 20 C in satu-

rated Ca(OH)2 solution. Afterwards, the mortar specimens weredivided into two aliquots, each of which was stored in 30 g l1 Na2SO4 and in saturated Ca(OH)2 reference solution at 8 C for200 days. Distilledwater was used for the preparation of the exper-

imental solutions. During the immersion period the volume ratio ofthe Na-sulfate and the Ca-hydroxide solutions to the samples wasfixed at 6:1. The specimens were placed on plastic spacers about

2 cm above the bottom of the tank. The test solutions wererenewed regularly every four weeks. In order to study the effectof hydration, the pre-curing time of three specimens of the Eco IIItype was extended to 28 days (Eco III-28d), followed by immersion

in Na2SO4 and Ca(OH)2 solutions at 8 C as described above. Thestrain development of the thin prisms was measured by meansof a strain gauge with an accuracy of 0.001 mm/m. Two measuringsteel points were fixed on both sides of the specimens by using a

chemically resistant epoxy resin. In order to determine changesin the expansion rate of mortar samples over time, strain measure-ments were carried out every two weeks on three samples of eachmix.

According to the SVA-guideline [44], mortars with an expansion60.5 mm/m after 91 days of reaction time indicate a high sulfate

resistance. However, the w/c-ratio of the reference mixes withregular cement contents was 0.60, instead of a required w/c-ratio

F. Mittermayr et al. / Cement & Concrete Composites 55 (2015) 364373 365
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of 0.50. This higher w/c-ratio was used to enable a direct compar-ison to the Eco-mixes. Furthermore, the temperature was set to8 C using a climate chamber, instead of 20 C, in order to trigger

TSA. The compressive strength of the mortar prisms was deter-

mined after 56 days of storage in the experimental solutions, inaccordance with EN 196-1. Visual inspection and subsequent min-

eralogical and microstructural investigations were conducted onthe mortar prisms that were immersed for 200 days.

2.3. Mineralogical and microstructural analyses

Prior to the analyses the mortar prisms were dried at 40 C.Powder X-ray diffraction (XRD) patterns were recorded for mineralidentification. Representative samples were taken and finely

ground in a ball mill for 15 min, together with 10% zincite {ZnO}as an internal standard. Randomly oriented preparations weremade using the front loading technique and subsequently X-rayedfrom 3to 902hwith a step size of 0.008and a count time of40 s

per step using a PANalytical XPert PRO diffractometer operated at40 kV and 40 mA. Phase identification was made with the PANalyt-ical XPert HighScore Plus software.

For microstructural analyses, representative sample pieces

were impregnated with epoxy resin under vacuum, followed bycutting of the samples with a diamond saw and subsequent grind-ing with diamond grinding wheels, using ethanol cooling and pol-ishing with 3 and 1 lm diamond oil suspensions. Back-scattered

electron (BSE) images were recorded on polished carbon-coatedspecimens using a Jeol JXA-8200 SuperProbe electron probe microanalyzer. For mineral identifications semi-quantitative single spotanalyses were performed using the implemented energy-disper-

sive X-ray spectroscopy (EDX) system operated at 15 kV and 10 nA.

3. Results

3.1. Visual inspection

Typical appearances of mortar samples that were stored in3 0 g l1 Na2SO4 solution for 200 days are pictured in Fig. 1 andclassified inTable 2. In order to evaluate the deterioration level,the samples were divided into four categories, reflecting no dam-

age (0), minor damage (Mi), major damage (Ma), and destroyed(D). The Ref II and Ref II mortar mixes suffered severe damagedue to intense sulfate attack and were completely decomposed,while Eco II and Ref I mixes developed a major degree of damage.

Ref III and Ref III showed only some, but rather less pervasivevisual cracks on the surface and were therefore classified into the

minor damage category. Eco III and Eco III-28d offered nomacroscopic damage, suggesting an outstanding sulfate resistance.

3.2. Mechanical properties

3.2.1. Compressive strength

The compressive strength values are presented in Fig. 2. They

were obtained after 14 days of pre-storage of the mortars in a satu-rated Ca(OH)2 solution and after the immersion period of 56 days in

Na2SO4 solution and in saturated referenceCa(OH)2 solution at 8 C.Subtraction of the compressive strength values of Ca(OH)2-immersed mortars from the sulfate-immersed ones yielded inD-strength values that are reported inTable 2.

As shown in Fig. 3 the CEM II/A-S-based reference mortar(Ref II) exhibited a compressive strength of 39 N/mm2 after14 days of pre-storage. This value is significantly higher than thatof the CEM I and CEM III-based reference mortars (Ref I and Ref III)

with 33 N/mm2. The Ref II and Ref III mixes, characterized by aconstant water content, but reduced cement content, displayedlower compressive strength values of 2730 N/mm2 compared toRef II and Ref III. This behavior reflects the increase of the w/c-ratio

from 0.60 to 1.07 and 0.60 to 0.76, respectively. The low w/p-ratioof Eco III, as a result of water reduction, resulted in an increase ofcompressive strength up to 40 N/mm2, compared to Ref III. Thisis in contrary to the behavior of Eco II and the corresponding mix

Ref II. Notably, prolonged curing of 28 days (Eco III-28d) stronglyincreased the compressive strength to 49 N/mm2 (seeFig. 3).

After 56 days of immersion in a saturated Ca(OH)2 solution asignificant increase in compressive strength was recognized for

all mortar mixes (seeFigs. 2 and 3). Notably, the Eco mixes (Eco II,Eco III and Eco III-28d) exceeded the compressive strength valuesof their corresponding reference mortars. However, after 56 daysof storage in Na2SO4solution the compressive strength of all mor-

tar mixes decreased up to 7%, except for Ref I, Ref II and Ref II. Inthe latter cases the compressive strength was even slightly

improved as indicated with black arrows inFig. 2and by positiveD-strength values inTable 2. Interestingly, these mortars also rep-

resented the samples with the highest visual damage degree(Fig. 1) and the largest D-expansion rates after sulfate exposure.The compressive strength was measured after 56 days because at

this age the strain development of some thin prisms acceleratedsignificantly also resulting in incipient macroscopic damage.

3.2.2. Strain

The strain development of thin mortar prisms obtained over a91 days period is presented in Fig. 4 and the correspondingD-expansion values are given in Table 2. The D-expansion was

determined by subtracting the strain of mortars that were storedin saturated Ca(OH)2 solution (as control specimens) from those

immersed in30 g l1

Na2SO4 solution. The mortar samplesdesignedwith CEM I 32.5 R and CEM II/A-S 52.5 N showed much higher

Table 1

Mix design for the mortar samples. Cement types are commercially available in accordance to EN197-1.

Mix ID Ref I Ref II Ref III Ref II Eco II Ref III Eco III

Water/cement 0.60 0.60 0.60 1.07 0.74 0.76 0.61

Water/powder 0.60 0.60 0.60 0.60 0.36 0.60 0.35Cement Type CEM I 32.5R CEM II/A-S 52.5 N CEM III/B 42.5 N CEM II/A-S 52.5 N CEM III/B 42.5 N

Clinkera [g] 450 405 135 262 293 102 112

Slag [g] 45 315 28 32 252 286

Total [g] 450 450 450 290 325 354 398Fly ash [g] 80 90 48 55

Limestone [g] 80 256 48 236

Total powder [g] 450 450 450 450 671 450 689

Water [g] 270 270 270 270 233 270 241

PCE-SP [g] 2.9 1.6

Sand 02 mm [g] 1350 1350 1350 1350 1350 1350 1350

a Denotes all constituents of the cement except for slag. (PCE-SP): polycarboxylatether superplasticizer.

366 F. Mittermayr et al. / Cement & Concrete Composites 55 (2015) 364373


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4. Discussion

4.1. Mechanical properties

The increase of compressive strength between 14 days of pre-storage and after 56 days of storage in Ca(OH)2 solution can beattributed to the proceeding formation of hydrate phases in the

matrix. This effect was distinctive for all mortars that containedlatent hydraulic and puzzolane additives like BFS and fly ash. After

56 days of sulfate exposure, slight improvements in compressive

strength were recognized for some mortars, i.e. Ref I, Ref II and

Ref II (see Fig. 2), which reflected the microstructural densificationof mortars due to the development of expansive products such asGp, Ett and Bas[16]. Our results revealed that specimens whichshowed increasing compressive strength values after sulfate expo-

sure exhibited also a higher expansion rate after 56 and 91 days.This behavior leads us to suggest that the initial improvementsin strength went along with negative effects of excessive expansionof the neo-formed sulfate minerals. Consequently, the compressivestrength results after Ca(OH)2exposure can be used for the charac-

terization of the mechanical properties of mortars. However, theevolution of compressive strength of specimens during 56 days ofsulfate attack did not provide reliable information about their sul-

fate resistance.Until 56 days of sulfate exposure, all mortar mixes had expan-

sion rates below the SVA-guideline limit [44] of 0.5 mm/m (seeFig. 4). After 56 days, a significant expansion was observed for Ref I,

Ref II, Ref II and Eco II mortars whereas the Ref III, Ref III, Eco III,and Eco III-28d mixes still displayed insignificant expansion until91 days of sulfate exposure. These observations clearly indicatethat both Portland (CEM I) and Portland slag composite

(CEM II/A-S 52.5 N) cement based mortars were more susceptibleto sulfate attack, compared to BFS based (CEM III/B) ones. Fromother studies it is also known that reactive hydration products suchas CH, Ett and other AFm phases can be effectively reduced when

using proper SCMs such as BFS and fly ash containing cementitiousmaterials and thus lowering the risk of the formation of secondaryexpansive phases during sulfate attack like Gp and secondary Ett[5,45]. In our samples Gp had mainly developed in the interface

transition zone (ITZ) around aggregates and in cracks parallel to

Table 2

Damage level degree from visual inspection (seeFig. 1). The D-expansion rate, D-compressive strength, D-sulfate phases (ettringite, gypsum and bassanite) and D-carbonate

phases (calcite and vaterite) were calculated by subtracting the individual parameters of Ca(OH)2immersed samples from Na-sulfate ones. Negative D-values reflect losses and

positive D-values reflect gains.

Damage DSulfate [wt.%] DCarbonate [wt.%] DExpansion [mm/m] DStrength [N/mm2]Duration [d] 200 200 200 91 56

Mix ID

Ref I Major 4.6 3.9 1.46 0.1

Ref II Destroyed 2.8 3.8 1.01 2.1Ref II Destroyed 1.7 0.1 4.78 1.5

Eco II Major 3.8 3.7 0.98 4.1

Ref III Minor 1.9 2.1 0.55 3.4

Ref III Minor 0.3 1.1 0.31 0.9

Eco III No damage 1.0 4.6 0.10 0.7

Eco III-28d No damage 0.6 0.6 0.05 0.8

0

10

20

30

40

50

60

Ref I Ref II Ref II* Eco II Ref III Ref III* Eco III Eco III28d

Mortarcompressive

strength[N/mm]

pre-storage 2 14d 20C

pre-storage 2 56d 8C

pre-storage Na2SO456d 8C

Ca(OH)

after Ca(OH)

after

Fig. 2. Compressive strength of mortars after 14 days pre-storage and after 56 days

exposure in Ca(OH)2and Na2SO4solutions, respectively. Note that Eco III-28d was

pre-stored for 28 days.

20

30

40

50

60

-Cem+

FA+Cal

-H2O+

Cal+SP

compressivestrength[N/mm

2]

CEM II/A-S

CEM III/B

-Cem+

FA+Cal

-H2O+

Cal+SP

-H2O+

Cal+SP

-Cem+

FA+Cal

-H2O+

Cal+SP

CEM II/A-S CEM III/B

-Cem+

FA+Cal

14d storage in Ca(OH)2 14 + 56d storage in Ca(OH)2

Ref I Ref II

Ref II* Eco II

Ref III Ref III*

Eco III Eco III-28d

CEM I

curing

(28+56d)

CEM I

curing

(28d)

Fig. 3. Evolution of compressive strength of mortars as a function of the mix designchange after 14 days pre-storage and after 56 days immersion in Ca(OH)2. (Cem):

cement; (FA): fly ash; (Cal): calcite i.e. limestone; (SP): polycarboxylatether

superplasticizer.

0.0

0.5

1.0

1.5

4.5

5.0

0 14 28 42 56 70 84 98

-expansion

[mm/m]

Exposure duration [d]

Ref I Ref II

Ref II* Eco II

Ref III Ref III*

Eco III Eco III-28d

max. 4.78 mm/m

SVA-guideline limit

Fig. 4. Strain development (D-expansion rate) of mortars during 91 days of

exposure at 8 in 30 g l1 Na2SO4and Ca(OH)2solutions.



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the sample surface (see Fig. 6). Commonly, the ITZ contains the

highest CH contents and a higher porosity than the rest of thecement paste[46].

The reduction of the cement content and the addition of lime-stone powder generally did not increase the expansion rates of

the mortars, despite the increase of the w/c-ratio (compare Eco IIvs. Ref II and Eco III vs. Ref III inFig. 4). Elevatedw/c-ratio resultedin weaker and potentially more porous cementitious matrixeswhich were more vulnerable for SO4

2 ion penetration followed

by the formation of expansive phases [47]. In contrary, the reducedcement clinker content combined with limestone and fly ash led to

an increased resistance against sulfate attack (e.g. Ref III

compared to Ref III). It is suggested that the low w/p-ratios resulted

in a reduced connected porosity and fly ash has effectively reduced

the CH content[48,49].

4.2. Mineralogical and hydrogeochemical aspects

Gp, Bas, and in one case Ett were found to be the mainexpansive alteration products after 200 days of sulfate exposure.These phases are considered to have caused the expansion of themortars and subsequently led to their damage. Although still

debated in the literature, this study clearly indicates that Gp iscreating expansion [22,23,50]. This is confirmed by the positive

linear correlation between the expansion of the mortars andtheir corresponding total gains of sulfate mineral phases, as

Fig. 5. XRD patterns of Ref I (top) and Eco III (bottom) mortars after immersion in 30 g l1 Na2SO4and saturated Ca(OH)2solution for 200 days, respectively. The differencediffractograms, shown in the inserted boxes, indicate mineral gains and losses related to mineral precipitation versus dissolution during sulfate exposure. The calculated D-

sulfate and D-carbonate values are reported inTable 2.

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illustrated in Fig. 7. Ref II was the only mortar where Ett was

formed. This mix had also the highest expansion of 4.78 mm/m, which was due to the more destructive effect of Ett compared

to Gp [51]. The generally good performance of CEM III/B isconsidered to be attributed to a denser microstructure and the

reducing conditions in the pore solutions due to high amounts

of BFS [48,52,53].Despite potentially favorable conditions for TSA such as high

limestone content in the Eco mixes, low temperature of 8 C andthe usage of highly sulfate-loaded experimental solutions, no

Slag

Slag

Gp

cra

ckin

g

c

r

c

k

i

n

g

Cccsecec

zQ

zQ

27575 m 42525 m

200 m00 m

375 m75 m

Eco IIIco IIINaa

2

SOO4

Ref Ief INaa

2

SOO4

Ref IIIef IIINaa

2

SOO4

Eco IIco IINaa

2

SOO4

Qzz

Qzz

Qzz

Qzz

Qzz

Qzz

Qzz

Pore Ccore Cc

Gpp

Gpp

Cal

Ccsec

Fspsp

Qzz

Gp

Ccsec crack

samplesurface

s

m

p

e

s

samplesurface

s

m

p

e

s

samplesurface

samples

urface

s

m

p

e

s

Cccsecec

Ccsec

Cccsecec

Cal

Pore Gp

Gp

Ccsec

inte

nse

cra

ckin

g

i

n

t

e

n

s

e

c

r

c

k

i

n

g

gnikcarc

esnetni

r

e

s

n

e

t

n

i

g

n

i

k

Gpp

Gp

Ref IIef IINaa

2

SOO4

375 m75 m

samples

urface

s

m

p

e

s

Eco IIco IICa(OH)a OH)

2

375 m75 m

samplesurface

s

m

p

e

s

leached

zone

l

e

c

h

e

d

z

o

n

e

Cccsecec

Qzz

Qzz

Qzz

Qzz

Cal

Cccsecec

leachedzonee chedzone

Fig. 6. Back-scattered electron images of Ref I, II, III and Eco II, III obtained after 200 days of exposure in 30 g l1 Na2SO4solution. Eco II, stored in saturated Ca(OH)2solution,

is shown for comparison. Mineral phases identified by EDX: (Gp): gypsum, (Cal): calcite, (Ccsec): secondary calcium carbonate, (Qz): quartz, (Fsp): feldspar (Slag): blast

furnace slag grains. Note that the mortar damage degree is mainly attributed to intense cracking, leaching and widely coincided with the visual degree of damage (Fig. 1).



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evidence for Tha formation was found in any of the samples. The

lack of Tha can be explained by (i) its slow reaction rates, (ii) thechemical composition and the evolution of the experimental solu-tion, which promoted the precipitation of other Ca-sulfates and Ca-carbonate minerals, and (iii) the role of polycarboxylate-ethersuperplasticizer (PCE). In the following these assumptions are

elucidated.In contrast to Tha formation, large amounts of neo-formed

CaCO3polymorphs (Cal and Vtr) were identified. These carbonatesare commonly found in association with high pH solutions (up to

13) that occur in e.g. tunnel drainages[54]. Tha is stable at highpH but also at lower pH values (down to about pH 8) but it possessslow reaction kinetics and is therefore known to occur particularlyduring the last stages of sulfate attack[27,32,55,56]. In the present

study, the 200 days of sulfate exposure were possibly not sufficient

to trigger Tha formation.Another critical point is that under real field exposure condi-

tions, pure sodium sulfate solutions are never present. In natural

surroundings, mixtures of solutions containing different aqueouscomplexes, speciations and dissolved cations and anions are con-sidered to have decisive effects on Tha formation and sulfate attackin general[55,5759]. Furthermore, cyclic events such as multiple

drying and wetting cycles induce strong enrichments in major andtrace element concentrations within the interstitial and pore solu-tions. Such processes typically enhance the formation of destruc-tive mineral phases such as Tha and thus result in the more

rapid degradation of concrete, as demonstrated by thermodynamiccalculations, lab experiments and field observations [47,48,55].

We can only speculate about the pore solution compositions

and crucial ion content of Ca2+

, SO42

, CO32

, and silica, which arerequired for Tha precipitation, but it is evident that Ca-sulfate min-erals and carbonate minerals were formed and Tha formation wassuppressed. For this reason, the absence of Tha in our samples is

expected to be also attributed to the evolution of the chemicalcomposition of the experimental solution (i.e. ion leaching, -fixa-tion and changes in pH). Additionally the formation of CaCO3poly-morphs such as Cal, aragonite and Vtr[6062]and Tha[6366]is

strongly influenced by organic substances. In our case, it is sug-gested that the PCE used in the Eco-mixtures had played an impor-tant role on controlling CaCO3versus Tha formation.

PCEs are well known to strongly attach to positively charged

cement particles but also interact with negatively charged ionssuch as SO4

2 in the pore solution, thereby affecting e.g. Ett forma-

tion during early hydration[6770]. Lothenbach et al.[53]did notfind Tha in CEM III/B-SiO2 based paste samples after 3.5 years

where PCE was used, even though Tha formation was predictedby thermodynamic modeling. Blanco-Varela et al. [64] investigated

the effect of organic admixtures during Tha synthesis. In the fourexperiments where PCEs were used, Tha was inhibited while otherorganics favored its formation.

Since in our experiments all mortar specimens were placed

together in the same containers with Na2SO4 and Ca(OH)2 solu-tions, it is assumable that organic molecules from the PCE werepresent in the solutions and pore fluids during the whole immer-sion period[53]. In these highly alkaline immersion solutions evensmall amounts of organics can affect dissolved silica speciation

[7173]. For example partial removal of aqueous silica from theexperimental solution by adsorption onto organic molecules mighthave interfered with the formation of Tha. Additionally the forma-tion of CaCO3 polymorphs has acted as a competing reaction for

Tha precipitation that lowered the Ca2+ and CO32 concentration

in the experimental solution. Consequently it seems doubtful thatdissolution of limestone filler can occur, which would serve as apotential carbonate source for Tha formation, because we had

found neo-formations of CaCO3 [28,74,75].

4.3. Exposure classification and application

The prediction of the performance of concrete in real practiceexposure based on laboratory-controlled accelerating tests on mor-tar specimens could be associated with some uncertainties. Poten-

tial variations are, for example, the higher SO42 concentrations

and/or thedefinedtemperature profileusedin conventionalacceler-ating tests, which commonly leads to the formation of mineralphases othersthan that observed in the field andconsequently non-

comparable expansion rates and deterioration mechanisms. Thus,the suitability for accurate prediction of the sulfate resistance ofconcrete based on accelerating tests is challenging and should beverified. The results presented in the previouschapters indicatethat

Gp andin particular Ett were thedriving forces for the deteriorationinthepresenceof sulfate-richsolutions, as seenin Figs.1and7.Their

formation is known to cause severe damage on concrete structures.Portland and Portland slag cement based mixtures Ref I, Ref II

and Ref II exhibited a generally low resistance against sulfateattack. Thus, the application of these mixtures in highly sulfateloaded environments is not recommended. However, the applica-

tion in low sulfate environments (XA1) with 200 < SO426

600 mg l1 is acceptable according to DIN EN 206-1/DIN 1045-2.The cement-reduced mortar Eco II had shown a better sulfate resis-tance than the reference mortar Ref II with w/c= 0.60. According to

the equivalent performance concept for concrete properties(DIN EN 206-1), a mixture like Eco II, with a w/p-ratio lower than0.35, a compressive strength of more than 40 N/mm2 and a lime-stone content lower than 35% of powder, should be acceptable

for exposure class XA1.

In spite of a higher w/c-ratio than specified by the SVA-guideline[44]Ref III, Eco III and Eco III-28d developed less than 0.5 mm/mexpansion and thus passed the test. Consequently, these concretes

are suggested to withstand moderate sulfate attack (XA2) with sul-fate concentrations from 600 < SO4

26 3000 mgl1, according to

DIN 1045-2/EN 206-1. Since the expansion rates of Eco III andEco III-28d were exceptionally low, concretes with such a paste

composition (w/p6 0.35; slagP 40% of powder; limestone6 35%of powder; fcP 45 N/mm

2) are probably suitable for applicationin concrete structures that can be exposed to severe sulfate attack(XA3), with concentrations from 3000 < SO4

26 6000mg l1.

5. Conclusion

Based on the results from cement-reduced eco-friendly mortars(Eco II and Eco III) and reference mixes the following conclusions

R = 0.90

0.0

0.5

1.0

1.5

4.5

5.0

-1.5 -0.5 0.5 1.5 2.5 3.5 4.5 5.5

-expan

sion[mm/m]

-sulfate phases [wt.%]

4.78 mm/m

ettringite + gypsum attack

gypsum

andb

assani

teattac

k

Ref I Ref II

Ref II* Eco II

Ref III Ref III*

Eco III Eco III-28d

Fig. 7. Relation between the maximum expansion rates(D-expansion rate obtainedafter 91 days) of sulfate exposure and the total gain and loss of sulfate minerals (D-

sulfate phases after 200 days) in the respective mortar mixes (seeTable 2).

http://-/?-


9/10

and recommendations for cement-reduced eco-friendly concretecan be drawn:

(i) A good correlation between expansion rate and visual dam-ages was observed for all specimens, whereas assessing thedamage levels by means of residual compressive strengthwas not beneficial.

(ii) Elevated D-sulfate phase values correlated linearly withincreasing D-expansion rates for samples that containedGp and Bas. The appearance of both Ett and Gp caused amuch higher expansion.

(iii) Despite of potentially ideal conditions such as the presenceof limestone fines in the eco-mixes, the low temperatureof 8 C and immersion of the samples in highly sulfate-bearing solutions, no Tha was detected after 200 days of

reaction time. This is believed to be related to the evolutionof the experimental solutions, the slow reaction kinetics ofTha formation, the competing effect of Cal and Vtr precipita-tion, and/or the presence of organic molecules originating

from the PCE-based superplasticizer.(iv) The cement-reduced mortars Eco II and Eco III showed less

expansion after 91 days of exposure in 30 g l1 sodium sul-

fate solution compared to their corresponding referencemortars. However, the expansion rates of BFS-based mortarmixes were significantly lower than those of the CEM I andCEM II/A-S based mortars.

(v) Eco-friendly concretes with high limestone powder contents

but low water (w/p6 0.35) and cement clinker content areappropriate for application in environments with low sulfateconcentrations (XA1), according to DIN 1045-2/EN 206-1.The application of BFS-based limestone-rich mortars with

loww/p-ratio (w/p6 0.35) in moderate sulfate-loaded envi-ronments (XA2) is possible and a sufficient durability ofthese concretes even under severe conditions (XA3) is alsoachievable, especially after a prolonged curing period.

Acknowledgement

The authors greatly acknowledge the helpful comments by JosefTritthart.

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Pr42. Mittermayr Et Al. 2015