10 the Effect of Nanosilicaxv Addition on Flowability, Strength and Transport Properties of Ultra...

7/25/2019 10 the Effect of Nanosilicaxv Addition on Flowability, Strength and Transport Properties of Ultra High Performance

1/9

MaterialsandDesign59(2014)19

ContentslistsavailableatScienceDirect

Materials

and

Design

journalhomepage:www.elsevier.com/locate/matdes

The

effect

of

nanosilica

addition

on

flowability,

strength

and

transport

properties

of

ultra

high

performance

concrete

EhsanGhafaria,*, HugoCostab, EduardoJulioa, AntonioPortugalc, LuisaDur aesc

aICIST,IST-TechnicalUniversityofLisbon,Portugal

bICIST,ISEC-PolytechnicInstituteofCoimbra,PortugalcDept.ofChemicalEngineering,UniversityofCoimbra,Portugal

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received

26

October

2013

Accepted22February2014

Keywords:

Ultrahighperformanceconcrete

Nanosilica

Pozzolanicreactivity

Microstructure

Transportproperties

a

b

s

t

r

a

c

t

The

experimental

study

herein

presented

was

conducted

aiming

to

evaluate

the

influence

of

nanosilica

(nS)

addition

on

properties

of

ultra-high

performance

concrete

(UHPC).

Thermo

gravimetric

analysis

results

indicated

that

nS

consumes

much

more

Ca(OH)2ascomparedtosilicafume,specificallyattheearlyages.

Mercury

intrusion

porosimetry

measurements

proved

that

the

addition

of

nS

particles

leads

to

reduction

of

capillary

pores.

Scanning

electron

microscope

observation

revealed

that

the

inclusion

of

nS

can

also

efficiently

improve

the

interfacial

transition

zone

between

the

aggregates

and

the

binding

paste.

The

addition

of

nS

also

resulted

in

an

enhancement

in

compressive

strength

as

well

as

in

transport

properties

of

UHPC.

The

optimum

amount

of

cement

replacement

by

nS

in

cement

paste

to

achieve

the

best

performance

was

3

wt.%.

However,

the

improper

dispersion

of

nS

was

found

as

a

deterrent

factor

to

introduce

higher

percentage

of

nS

into

the

cement

paste.

c 2014

Elsevier

Ltd.

All

rights

reserved.

1.

Introduction

Ultra-high performance concrete (UHPC) is one of the mostpromisingtypesofconcrete,whichhasbeendevelopedinthelast

decade[13].Thisinnovativehigh-techmaterialischaracterizedby

adensemicrostructure,whichpresentsbothultra-highcompressive

strengthandultra-highdurability[45].Themaincompositionof

UHPCcontainsalargeamountofcement,usuallybetween800and

1100kg/m3,whichisaroundthreetofourtimesmorethanthequan-tityofcementinnormalconcrete[5].Therefore,theblendingofce-

mentwithhighpozzolanicfinematerials,suchasnanosilica(nS),can

beasuitableoptiontoreducethehighvolumeofcementintheUHPC

proportioning.

Nevertheless,theefficiencyofUHPCisparticularlydependenton

itsdensity.Infact,byoptimizingtheparticlepacking,anultra-high

consolidationoftheconcretematrixcanbereached.Thiscanbeob-

tained

through

an

almost

perfect

grain

size

distribution,

by

incor-

poratingahomogeneousgradientoffineandcoarseparticlesinthe

mixture.Inthisscope,theuseofnSaspozzolanicadditionishighly

effective.Actually,duetoitsextremelysmallsizeparticles,nScanfill

thevoidsbetweencementandsilicafumeparticles,leadingtohigher

packinglevel(filler effect)andalsogeneratingadenserbinding

matrix,withmorecalciumsilicatehydrate(C-S-H).Consequently,a

*Correspondingauthor.Tel.:+351911831229.

E-mailaddresses:[email protected](E.Ghafari)[email protected](H.Costa)

[email protected](E.Julio)[email protected](A.Portugal)[email protected](L.Duraes).

significantimprovementonbothdurabilityandmechanicalproper-

tiesisobtained.Lietal.[6]showedthatboththecompressiveand

flexuralstrengthsofconcretecanbeenhancedbyincorporatingnS.Aconcretewithadditionofsilicafume,flyashandnSwasalsostudied

byCollepardietal.[7].Itwasconcludedthatconcretewiththisternary

combinationhasabetterperformance,intermsofbothstrengthand

durability,thanthosejustwithflyash,butsimilartothosejustwith

silicafume.Li[8]alsofoundthatanadditionofnSresultsanincrease

inbothearly-agestrengthandlong-termstrength.Alyetal.[9]re-

portedthatincorporationofahybridcombinationofcolloidalnSand

wasteglassintothecementmortarsledtoanenhancementofthe

mechanicalpropertiesincomparisonwithplainmortar.

Additionally,ithasbeenprovedthattheincorporationnSalso

improvesthedurabilitypropertiesofconcrete.HeandShi[10]stud-

iedthechloridepermeabilityandmicrostructureofPortlandcement

mortarwithdifferenttypesofnano-materials.Thisstudyconfirmed

that

an

addition

of

nS

and

nano-clay

significantly

improves

the

chlo-

ridepenetrationresistanceaswellasthegeneralionicpermeability

ofcementitiousmortar.AnexperimentalstudyperformedbyJi[11]

showedthattheadditionofnano-SiO2

tothemixtureimprovesthe

waterpermeabilityresistanceofconcrete.

Moreover,well-dispersednano-particlesactascentersofcrystal-

lizationofcementhydrates,thereforeacceleratingthehydration[12].

Qingetal.[13]statedthatthepozzolanicactivityofnano-SiO2

ismuch

higherthanthatofsilicafume.Itwasfoundthatthebondstrengthof

thepaste-to-aggregateinterface,incorporatingnS,ishigherthanthat

ofspecimenswithsilicafume.Aresearchalsoshowedthatthepoz-

zolanicactivityofflyashsignificantlyincreasesafterincorporating

0261-3069/$-seefrontmatterc 2014ElsevierLtd.Allrightsreserved.

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2 E.Ghafarietal./MaterialsandDesign59(2014)19

Table1

Chemicalcompositionandphysicalpropertiesofcementandsilicafume.

Chemicalanalysis(%) Cement Silicafume

SiO2 20.9 93.6

Al2O3 4.60 1.3

Fe2O3 3.15 0.90

CaO 62.0 0.4

MgO 2.00 1

SO2 3.60 0.4

K

2

O

99.9(%) Amorphous 15 5


4/9


Fig.3.Gaspermeabilityapparatusanddetailofatestingcell.

theWashburnequation,asinEq.(2):

D

=

4cos

P

(2)

whereDistheporediameter(nm),isthesurfacetensionofmercury(dyne/cm),isthecontactangle()betweenmercuryandsolidandP

istheappliedpressure(MPa).Thetestapparatususedforporestruc-

turemeasurementwasAutoPoreIII mercuryporosimeter.Mercury

densityis13.5335g/mLanditssurfacetensionwastakenas485

dyne/cm;theselectedcontactanglewas130.

Themaximummeasuringpressurethatcanbeappliedis200MPa

(30,000psi),whichmeansthatthesmallestporediameterthatcan

bemeasuredreachesapproximately6nm(ontheassumptionthatall

poreshavecylindricalshape).

Inordertoprovidetheinformationabouttheporesizedistribution

ofconcrete,MIPtestwasperformedonsmall-coredsamplestakenout

fromthespecimens.Thesampleswerethenwashedwithacetoneto

terminatehydration.Beforeperformingthetest,thesampleswere

alsoovendriedfor48htoremovemoistureofthepores.

Thegaspermeabilitystandsasacriticalmaterialparameter,which

characterizesthestructureandthedurabilityofconcrete.Thegas

permeabilitytestwasperformedaccordingtoRilemTC116-PCD:

Recommendations[31].Foreachmixture,threecylinderspecimens

with100mmdiameterand50mmheightwereprepared.Thecylin-derspecimensshouldbebroughttothesamemoisturecondition

beforethegaspermeabilitytestcanbeperformed.Inthisscope,the

specimenswerestoredinalaboratoryatmosphereat20 2Cand

RHof65 5%foraperiodof28days.Specimenswerethensealed

withcellophanefilmandintroducedintoplasticcontainers,which

arethenproperlysealedtoreducetheevaporationofwaterfromthe

specimen.Theschematicexperimentalsetupformeasuringthegas

permeabilityofconcreteisillustratedinFig.3.Thisapparatuscan

beusedformeasuringvaluesofthegaspermeabilityintherangeof

1019to1014m2.

ThegaspermeabilitycoefficientcanbedeterminedbyEq.(3)

whichisbasedontheHagenPoiseuillerelationforlaminarflowofa

compressiblefluidthroughaporousbodywithsmallcapillariesand

understeady-stateconditions[32].

K=2QP0L

AP2 P2a

(3)

wherekistheeffectivegaspermeabilitycoefficient(m2),Qisthe

volumeflowrateofthefluid(m3/s),Aisthecross-sectionalareaof

thespecimen(m2),Listhethicknessofthespecimeninthedirection

offlow(m),isthedynamicviscosityofthefluidattesttemperature(Ns/m2),Pistheinletpressure(absolute)(N/m2),Paistheoutlet

pressureassumedinthistesttobeequaltoatmosphericpressure

(N/m2)andP0

isthepressureatwhichthevolumeflowrateisde-

termined,assumedinthistesttobetheatmosphericpressurePa(N/m2)

Fig.4.Conductivitycurvewithtimeforpozzolanwatersuspensions.

Fig.5.Relativelossincorrectedconductivityoflimepozzolanwatersuspensions.

3.

Results

and

discussion

3.1. PozzolanicreactivityofnSandSFincementpastes

Theresultsanalysisoftheconductivityofthepozzolanwater

suspensionsareshowninFig.4.TheincorporationofbothnSandSF

intowaterresultedinanincreaseinthesuspensionsconductivity,due

toionstransferfromnSandSFintowater.ThenSwatersuspension

gavehigherconductivityvaluethanSFwatersuspensionduetoits

highercontentofwater-solubleions.

Fig.5showstherelative loss inconductivityof limewater

pozzolansuspensionwithtimeforbothSFandnSadditions.After

addingnSandSFtothissolution,thevalueofconductivitydecreased

withtime,whichimpliesthattheconcentrationofbothCa2+and

OH ionsdecreasedwithtime.Themainreasonforthisreduction

canbeattributedtotheinteractionbetweenthemandthepozzolanicmaterialsthatwereadded,whichfinallyresultedinlimefixationdue

tothepozzolanicreactivity.Theresultsindicatedthat,after30min-

utesonly,thevalueofrelativelossinconductivityreached51%and

20%byaddingnSandSF,respectively.AsitisshowninFig.5,nS

showedaconsiderablyhigherreactivitythansilicafumeatearlyages

(31%)whichcanbeattributedtorelativelyhigherspecificsurfacear-

easofnS(Table2)[33].Althoughthiseffectbecamelesssignificant

byapproachingtheendofthe240-minutetestingperiod,itwasob-

servedthatintheendthereactivityofnSwas21%higherthanSF,

whichimpliesahigherpozzolanicreactivityofnSthatSF.

TheresultsofXRDofthecementpastewithandwithoutnSat28-

daysofageareshowninFig.6.Thepozzolanicactivitiesofmaterials

werecomparedbasedonCa(OH)2consumptionwhichisobtained

bytheintensityvariationofthemaindiffractionpeaks.Asitcanbe


5/9

E.Ghafarietal./MaterialsandDesign59(2014)19 5

Fig.6.ChemicalcompoundsofA0,A1andA2at28days,obtainedfromtheXRD

technique.

Fig.7.TGAdiagramforA0,A1andA2at28days.

seen,theintensityofthecrystalfacesofCa(OH)2

ofA2samplewas

considerablylowerthanthatofthereferencesample(A0).Infact,the

intensityofcrystalfaceofA2andA1cementpastesampleis65%and

60%lower,respectivelythanthatofA0sample.Theresultsindicated

thatnSconsumed10%moreCa(OH)2

thanthepastecontainingsilica

fume.SincesomeamountofCa(OH)2inhardenedcementpasteis

inamorphousform,theamorphousconsumptionofCa(OH)2cannotbewellcharacterizedbyXRDanalysis[34].Asacomparison,itis

well-knownthatTGAanalysisgivesthemostaccurateresults[35].

TheTGAcurvesforthreedifferentcementpastesofA0,A1andA2

at28daysofageareshowninFig.7.Theweightlossvs.temperature

curveforhardenedcementpaste,betweentemperaturesof105460C,canbedividedintotwointervals.Themasslossinthefirstinterval

110380Crepresentsthedecompositionofthehydrationproducts,

andthemasslossfromtemperature380Cand460Cisassociated

withdehydroxylationoftheCa(OH)2

[36].AspresentedinFig.7,the

valuesofmasslossfortheA1andA2cementpastessamplesare

higherthanA0cementpasteinthetemperatureintervalof110380C,whichisduetotheformationofmorehydratedCSHgel.The

highestmasslossvalueof6l.18%isobservedforthepastecontaining

nS.Forthetemperatureintervalof380460C,itwasobservedthat

theincorporationofnSresultedinadecreaseinthevalueofmass

loss.Thereductionofmasslossvalueinthisintervalimpliedthe

consumptionofCa(OH)2

duetohigherpozzolanicactivityofnS.

ThenormalizedamountsofconsumedCa(OH)2inallthehardened

cementpastesasapercentageofinitiallyavailableCa(OH)2

areshown

inFig.8.Theresultsindicatethatafter90daysofcuring,theCa(OH)2

contentinA0sampleincreased,whereasinA1andA2samplethe

Ca(OH)2contentreducedwithtime.ItcanalsobeseeninFig.8that

theCa(OH)2

contentsofA2andA1samplesat7dayscuringreducedby

73%and81%respectively,comparedtoA0sample,whichisincreased

by24%.Attheendof28and90days,nSconsumed85%and89%of

theinitiallyavailableCa(OH)2whichwas6%and5%higherthanthe

consumptionamountbySF.

Fig.8.ThenormalizedCa(OH)2contentofA0,A1andA2cementpastesat7,28and90

days.

TheTGAandXRDresultsconfirmedthepozzolanicactivityevalu-

ationresultsfromtheelectricalconductivitymeasurementsoflime

pozzolansuspensions.

3.2. Freshandhardenedstateproperties

ItisusuallyexpectedthatthehigherspecificsurfaceareaofthenSimplyanincreaseinwaterdemandofconcretemixture,which

cansomewhataffecttheworkability.However,Collepardietal.[37]

studiedthepropertiesofself-compactingconcretecontainingnSand

foundoutthatthepresenceofnSmakesthemixturemorecohesive

andreducesbleedingwaterandsegregation,whichfinallyimproves

therheologicalbehaviorofconcreteinfreshstate.

ThespreadontheflowtableresultsarealsogiveninTable5.The

resultsapparentlyshowedthatthereisaconsiderabledecreasein

slumpflowvalueswhennS(wt.%)isincorporatedintothemixture.

TheadditionofnS(wt.%)intothemortarshadasignificanteffecton

thewateramountrequiredinthemixture.

Anincorporationof1%nSdidnotcauseasignificantchangeinthe

valueofminislump.However,mixtureswithnS=2wt.%andnS=3

wt.%showareductionof9mmand12mm,respectively,inthespreaddiameterincomparisonwiththereferencemixture.Ahighreduction

inspreadvalue(19mm)wasobservedbyincreasingthenScontent

from3to4wt.%.Infact,asignificantamountofwaterinthemix

wasabsorbedbynSparticles,whichfinallyresultedinasignificant

decreaseinworkability.Asaconsequence,thereisnosufficientwater

availableforlubricationallowingparticlesfreemovement[38].The

obtainedresultsinthisstudyconfirmedthefactthat,foraconstant

volumeofpowder,thereplacementofcementbyafinepowderpar-

ticlewithhighspecificsurfaceareawillincreasethewaterdemand

inordertomaintaintheworkabilityofthemixture[3940].

Theaveragevaluesofcompressivestrengthat7,28and90days

areshown,forallthespecimens,inFig.9.Theerrorbarsindicatethe

standarddeviationsontheindividualvalues.Itcanbeseenthatthe

additionofnano-SiO2significantlyincreasedtheearly-agecompres-sivestrength.

Theadditionof3(wt.%)nSresultedina24%increaseat7days,

whichis40%higherthanthatobservedwiththereferencemixture.

Thehighercompressivestrengthisduetofasterpozzolanicreactivity

ofnanoparticles,inthepresenceofCa(OH)2,makingthemicrostruc-

turedenser.Additionally,theincorporationofnSparticlescanac-

celeratethehydrationprocessofC3

Sclinkerphaseduetothelarge

andhighlyreactivesurfaceofthenanoparticles[4142].However,

theresultsshowedthattheadditionofnShadamodesteffectatthe

ageof28and90days,sothatthehighestcompressivestrengthwas

144MPaand148MPaforM3specimen,whichwasonly8%and6.5%

higherthanthereferencemixture.Thisbehaviorconfirmsthefact

thatthemostpartofthepozzolanicreactivityofnSincementpasteis

completedatearlyages[33,4344].Theseresultsareconsistentwith


6/9


Table5

Slump,waterabsorptionandMIPtestresults.

Waterabsorption(%) MIPtest

Samples Slump(mm) Totalporosity(%) Capillarypores(%)

M0(control) 193 1.212 6.35 2.65

M1 191 1.110 4.74 2.56

M2 184 0.95 4.66 2.44

M3 181 0.808 4.3 1.72

M4 174 0.856 4.8 2.33

Fig.9.Compressivestrength(MPa)ofnSparticleblendedconcretespecimens.

theresultsofTGAandconductivityanalysis.

ItcanbeobservedthatthecompressivestrengthincreaseswithnS

particlesupto3wt.%replacement.However,compressivestrength

decreasedslightlywhenthereplacementlevelreached4%wt.%.Thus,

ahigherreplacementofcementbynSdidnotleadtoanimprovement

incompressivestrength,whichcanbeduetoimproperdispersionof

nSparticlesinthemixture.InfactnSparticles,duetotheirhighsur-

faceenergy,haveapronouncedtendencytowardsagglomeration.The

dispersionofnanoparticleswithinthecementpasteisasignificant

factorgoverningtheperformanceoftheseproducts.Whennanopar-

ticlesareaddedinexcesstothemixture,thesearenotuniformlydispersedinthecementpaste,andasaconsequenceweekareas

appearinthecementmortarduetoagglomeration.Therefore,the

disagglomerationofnanoparticlesiscrucialtoachievethecomposite

materialswithimprovedproperties[12].Inaddition,theamountsof

nSinthemixturescanalsohavebeenexceededthequantityrequired

forconsumingtheCa(OH)2

andthisexcessiveamountofsilicadid

notcontributetoenhancethecompressivestrength.Theinsufficient

workabilityofM4canrepresentanextracausefordecreasingthe

compressivestrengthsincethereisnosufficientwateravailableto

contributeforlubricationwhichallowsparticlesfreemovement.

3.3. Microstructureandporestructure

Theobtainedporesizedistributionplot,coveringtheporesizerangefromaround400mdownto5nm,isshowninFig.10.Itcan

beseenthatallMIPplotsofmixturescontainingnSstandslightly

belowthereferencemixture(M0),particularlyintherange0.340

m,whichcorrespondstothecapillarypores[38].Inthiscritical

interval,M0exhibitedmorecumulativevolumeofthepores,whereas

M3specimenhadthelowestamountofporevolumeinthementioned

range.Inaddition,allthespecimenshavemuchmoreporesranging

from0.0006mto0.03m,whichcorrespondstothegelpores[45].

Theamountoftotalporevolumeandcapillaryporesisalsopre-

sentedinTable5.ResultsshowedthatporestructuresofM1,M2,

M3andM4arefinerthanthoseofthereferencemixture.Itcanbe

observedthatbyincreasingnScontent,thevolumeofcapillarypores

inspecimensalwaysdecreases,whichshowsthatthedensityofcon-

creteisincreasedandtheporestructureisrefined.Theincorporation

Fig.10.Mercuryintrusionporosimetryresults.

ofnSintothemixturesledtoadecreaseintotalporosityupto25.3%,

26.7%,32.1%and24.5%,forM1,M2,M3andM4specimens,respec-

tively.TheresultsalsoindicatedthatM3sampleshowedthelowest

valueofthecapillarypores(1.72vol.%),whichwas35%lowerthan

thecapillaryporescontentofM0mixture.Thiscanbeduetothe

discontinuityofcapillaryporesbyformationofmoreCSHgel.

Themicrostructuresofthespecimenswereanalyzedbyscanning

electronmicroscopy(SEM).Fig.11(a)and(b)showsanoverviewof

themicrostructureofM2andM0usingthesamemagnification.In

general,theanalysisofallimagesdidnotrevealanysubstantialdif-

ferencesinthemicrostructureofallspecimens.However,itwasfound

thattheamountofcapillaryporesinM0specimenswashigherthan

in

M3

specimens

(see

Fig.

11

(b)

and

(c)).

The

incorporation

of

nS

led

toafurtherdensificationinthemicrostructureandtoaconcurrent

reductionincapillaryporosityofconcretemixtures.Incomparison

withM0specimens,themicrostructureofM3specimenswasvery

denseandfewplatedshapecalciumhydroxidewasobserved.Itis

wellknownthatportlandite,amineraloxide,hasadetrimentalef-

fectonbondstrengthofaggregates-to-paste[11],whichcausesa

significantreductionindurabilityandinmechanicalproperties.Re-

sultsshowedthatnScaneffectivelyreducetheamountofportlandite,

leadingtoadensermicrostructureofinterfacialtransitionzone(ITZ)

betweenaggregateandpaste.Insufficientbondbetweenaggregate

andpastewasfoundinM0specimens(seeFig.11(c)),whileaperfect

bindingpaste-to-aggregatesadhesioninM3specimens(Fig.11(d))

wasobserved,comparedtoM0specimens(Fig.11(c)).

3.4. Transportproperties

Theresultsobtainedforwaterabsorptionarealsopresentedin

Table5.TheadditionofnSwasfoundtobeeffectiveinreducingthe

waterabsorption.Itcanbeseenthattheamountofabsorbedwater

inthesamplesdecreasedbyincreasingthenScontent.Theaddition

ofnSby1,2,3and4wt.%ledtodecreasesinwaterabsorptionin

samplesby8.5%,21%,33%and29%,respectively.

TheresultsalsoshowedthataddingnStocementpastemixesled

tomuchlowersorptivitycoefficientwhencomparedtothereference

mixture.InFig.12,thewaterabsorptionversusthesquarerootof

timeisplottedforallmixtures.Anonlinearcorrelationwasobtained

forthedatabetween1minand6h,whilealinearcorrelationwas

determinedforthedatabetween1and7days.Table6presentstwo


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Fig.11.SEMmicrographofUHPCmixturesatageof28days:(a)microstructureof

UHPC-NS;(b)microstructureofUHPC;(c)interfacialtransitionzone(ITZ)microstruc-

turebetweenaggregateandpasteinUHPCspecimen;and(d)interfacialtransition

zone(ITZ)microstructurebetweenaggregateandpasteinUHPC-NSspecimen.

Fig.12.Regressionmodelofwatersorptivityofspecimens.

differentregressionequations,determinedbyleast-squareregression

forthetimeof6hbeforeandthetimeof24hafter.Theinitialrateof

absorptioncannotbedeterminedsincethedatabetween1minand

6hdonotfollowalinearcorrelation.However,thenonlinearregres-

sionplots(Fig.12)alongwithnonlinearregressionequations(Table

6)revealedthatM3specimensabsorbedlesswaterthantheother

specimensuntil6h.Thesecondaryrateofwaterabsorptioncoeffi-

cients(kg/m2/h0.5)isdefinedastheslopeofthelinethatisthebest

fittowaterabsorptionplottedagainstthesquarerootoftime(h1/2)

usingallthepointsfrom1dayto7days.Theverylowsorptivityco-

efficientof0.0178(kg/m2/h0.5)wasobtainedfortheM3specimens,whichwas15%lowerthanthereferencemixturecoefficient(M0).

Theachievedresultsofwaterabsorptionandwatersorptivityco-efficientwereingoodagreementwithcapillaryporosityvaluesob-

tainedbytheMIPtest.Assorptionofwaterismainlythroughcapillary

pores[46],therefore,thelowerwaterabsorptionandsorptivitycoef-

ficientcanbeduetoporestructurerefinementandhigherconcrete

compactness.

Thegaspermeabilityofthreecylindricalspecimensofeachmix

wasmeasuredatthesametimebythecembureauapparatus.The

coefficientsofgaspermeabilitywereobtainedbyperformingthetest

withaninletpressureof0.5,1.5and2.5barandaregiveninFig.13.

Thegaspermeabilitycoefficientsofallthemixturesarewithinthe

sameorderofmagnitudeandtheerrorbarsindicatethestandard

deviationsontheindividualvalues.Regardlessoftheinletpressure,

resultsindicatedthatallmixturescontainingnS(M1,M2,M3andM4)

showedlowergaspermeabilitycoefficientsincomparisonwithM0

Fig.13.GasPermeabilitycoefficientofspecimens.

samples.TheadditionofnStothemixtureblendsledtoadecrease

ingaspermeabilitycoefficientupto7.5%,24.2%,31.9%and25.7%for

M1,M2,M3andM4specimens,respectively.Themaximumstandard

deviationmeasuredfortheindividualvaluesreached0.18 1019

m2.

Ithasbeenfoundthatthepenetrationofthegasmainlydepends

ontheopenporosityofthecementpaste,theaggregatesandtheproportionofthesetwocomponentsinthemixtures[47],andwitha

higherporosity,gasescanpenetratetheconcretemoreeasily[48].

Theobtainedresultsinthisstudyrevealedastrongrelationbe-

tweenporosityofconcretespecimensandthegaspermeabilitycoef-

ficient.Thisrelationhasalsobeenfoundforself-compactingconcrete

andhigh-volumeflyashconcrete[4849].

Theresultsprovedthattherefinementoftheporestructureof

concretewasakeyissuetoimprovethecharacteristicsofconcrete

samples.Infact,mostaspectsofconcretedurabilityaredirectlyre-

latedwithitsporousstructure,sincecapillaryporesareresponsible

forfluidsmigrationintheconcretematrix.Therefore,asthevalueof

capillaryporesdecreases,theresistancetoaggressiveenvironments

improvessignificantly.

4.

Conclusions

Theexperimentalstudyhereindescribedwasconductedaiming

toevaluatethepozzolanicbehaviorofnSanditseffectonthemi-

crostructureofUHPC,aswellasonitsmechanicalandtransport

properties.Thefollowingconclusionsarehighlighted:

(1)TheincorporationofnSintocementpastewithverylowwater/cementratiocanincreasetheamountofhydrationproducts

andthusitcancauseasignificantreductionintheamount

ofportlandite.ItcanbestatedthatnSwithhigherspecific

surfaceareashavehigherrateofpozzolanicreactivitythanSF,

atearlyages.TGAresultsrevealedthatnSconsumemuchmore

Ca(OH)2ascomparedtoSF.(2)TheadditionofnStothemixturesreducesthespreadonflow

table.Thewaterdemandinthemixturesalsoincreasesremark-

ablydependingonthepercentageofreplacement.Thehighest

amountofnSthatcanbeincorporatedkeepinganacceptable

rangeofslumpflowis3wt.%.

(3)CompressivestrengthofUHPCmixturesincreasedwiththein-

creaseinnScontent,especiallyatearlyages.Theoptimum

amountofcementreplacementbynS incementpasteto

achievethehighestcompressivestrengthwas3wt.%.Itwas

foundthattheproperdispersionofnSisacriticalparameter

tofacilitatetheincorporationofhigherpercentagesintothe

cementpastes.

(4)ThematrixphaseofUHPCcontainingnSissignificantlydenser

andmorehomogeneousthanthatofthereferencemixture.


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