CMU. J. Nat. Sci. (2013) 121➔

Compressive Strength of Blended Portland Cement MortarsIncorporating Fly Ash and Silica Fume

at High Volume Replacement*

Watcharapong Wongkeo and Arnon Chaipanich*

Department of Physics and Materials Sciences, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand

*Corresponding author. E-mail: arnon@chiangmai.ac.th

ABStRACt

Fly ash has been widely used as a replacement of Portland cement at high content levels, but the compressive strength of Portland cement blended with fly ash is lower at early ages. Thus, this study compares the compressive strength of ternary blended Portland cement mortars incorporating fly ash (40-70 wt%) and silica fume (5-10 wt%) with binary blended Portland cement mortars incorporating fly ash only. Blended Portland cement mortars were designed with a constant water/binder ratio of 0.485 and the flow table tested was carried out. A fine aggregate to binder ratio of 2.75 was used. The compres-sive strength of all blended Portland cement mortars that cured in water at 23 °C was tested at 28 days. The results show that the flow of blended Portland cement mortars increased with increasing fly ash content and tended to decrease with increasing silica fume content, at the same replacement level. Moreover, the compressive strength of blended Portland cement mortars decreased with increasing fly ash content. However, compressive strength gains were obtained from ternary blended Portland cement mortars incorporating silica fume, tend-ing to increase with increasing silica fume content.

Keywords: Blended Portland cement, Fly ash, Silica fume, Compressive strength

IntRoduCtIon

Highvolumeflyash(HVFA)hasbeenwidelyinvestigated.Incommercialpractice, the dosage of fly ash (FA) is limited to 15-20%bymass of the totalcementitiousmaterial.Usually, thisamounthasabeneficialeffecton thework-ability and cost economyof concrete, but itmaynot be enough to sufficientlyimprovethedurabilitytosulfateattack,alkali-silicaexpansionandthermalcrack-ing.Forthesedurabilityproperties,HVFAwith50%ormoreflyash,bymassofthecementitiousmaterial,hasbeenstudied(Mehta,2004).TheusedofHVFAinself-compactingconcretereducesthedosageofsuperplasticizer,whichisneeded

DOI: 10.12982/CMUJNS.2013.0011

*Presentedinthe1stASEANPLUSTHREEGraduateResearchCongress(AGRC),March1-2,2012,ChiangMai,Thailand.

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toobtainasimilarslumpflowasconcretemadewithPortlandcement(PC)onlyduetothespherical-shapedparticlesofflyash.Also,theuseofflyashimprovesrheologicalpropertiesandreducescrackingofconcretedue to its lowerheatofhydration (Bouzoubaa andLachemi, 2001). However, the strengths ofHVFAconcrete are lower than that of pure Portland cement concrete, especially atearlyages,duetothedilutioneffectandverylowpozzolanicreactionofflyash (Bouzoubaa et al., 2001;Atis, 2005; Dinakar et al., 2008; Papayianni and Anastasiou,2010;Duran-Herreraetal.,2011). Previous studieshave found that theuseofflyashwith silica fume (SF)in ternary blended cement enhancesmechanical properties compared to binaryblendedcementwithflyashalone,especiallyatearlyages(Huang,1997;Poonet al., 2001;Kilic et al., 2003;Nassif et al., 2005; Erdem andKirca, 2008). Nochaiya et al., (2010) investigated ternary blends of fly ash, silica fume andPortlandcementusinganextensiverangeofmixes.Theyfoundthatincorporatingflyashwithsilica fume increased thecompressivestrengthofconcreteatearlyagesupto145%,withthehigheststrengthobtainedwith10wt%silicafume.Inaddition,usingflyashwithsilica fume in ternaryblendedcementenhances theworkabilityofbinaryblendedcementwithsilicafumealone(Longetal.,2002;SharfuddinAhmedetal.,2008;Shannag,2011).Thus,thisstudyinvestigatedthecompressivestrengthofbinaryandternaryblendedcementmortarusingflyashandsilicafumeasacementreplacementathighlevels.

MAteRIAlS And MetHodS

Portland cement type I, fly ash (obtained fromMaeMohpower plant inLampang,Thailand)andundensifiedsilicafumegrade920-U(producedbyElkem)wereused in thisstudy.ThechemicalcompositionsofPortlandcement,flyashandsilicafumepowderaregiveninTable1.XRDpatternsofPortlandcement,flyashandsilicafumeareshowninFigure1.

table 1. Chemicalcompositionsofthematerialsusedinthisresearch.Chemicalcomposition(%)

PC FA SF

SiO2 20.64 45.37 93.55

Al2O3 4.85 20.65 0.56

CaO 63.62 10.43 1.13

Fe2O3 3.17 12.31 0.17

MgO 1.14 2.13 0.75

Na2O 0.51 1.33 0.14

K2O 0.81 1.50 1.05

P2O5 0.32 0.24 0.53

TiO2 0.21 0.52 0.002

SO3 2.75 2.53 1.01

LOI 2.08 3.00 1.16

CMU. J. Nat. Sci. (2013) 123➔

Figure 1. XRDpatternsofmaterialsusedinthisresearch(a)Portlandcement(b)flyashand(c)silicafume.

TheXRDpattern indicates thatPortland cement consistsmainlyofAlite(C3S)andBelite(C2S)phases.Inaddition,theXRDpatternsofflyashandsilicafume showa broadpeak in the range of 20-30° (2θ),which is a characteristicof amorphous SiO2phases.However,flyashalsoconsistedofmainlycrystallinephasesofgehlenite(Ca2Al2SiO7),magnetite(Fe3O4),anhydrite(CaSO4),quartz(SiO2) and lime (CaO).The physical characteristics, such asmorphology andparticlesize,ofPortlandcement,flyashandsilicafumewereinvestigatedusinga scanningelectronmicroscope.ThemicrographofPortlandcement reveals an

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irregularshape,whileflyashandsilicafumearespherical(Figure2).Inaddition,riversandwithspecificgravityof2.65wasusedasafineaggregateofmortar. In this study, binary and ternary blendedPortland cementmortarswereinvestigatedusingflyashandsilicafumeasaPortlandcementreplacement.Flyashreplacedpartofthecementat40,45,50,55,60,65and70%byweight.Silicafume replacedpart of the cement at 5 and10%byweight, respectively.Afineaggregatetobinderratioof2.75andwatertobinderratioof0.485wereused.Inaddition,flowtablewasalsomeasured.Themixproportionsandmixdesignaresummarized inTable 2.Themortarmixesweremixed and cast into 50x50x50mmmouldsandcompactedusingavibratingtable.Themortarspecimenswerestoredinmouldsfor24h.Afterthespecimensweredemoulded,theywerecuredinwaterfor28daysandthenthecompressivestrengthtestedwerecarriedout.

table 2.Mixproportionsofblendedcementmortars.Mixproportions

Mixes PC(%) FA(%) SF(%)

100PC 100 - -

50FA 50 50 -

60FA 40 60 -

70FA 30 70 -

5SF 95 - 5

10SF 90 - 10

45FA5SF 50 45 5

55FA5SF 40 55 5

65FA5SF 30 65 5

40FA10SF 50 40 10

50FA10SF 40 50 10

60FA10SF 30 60 10

ReSultS

Flow test Workability property of binary and ternary blended cementmortarwastestedthroughthemeasurementofflowtable(Table3).TheflowtableofbinaryblendedPortland cementmortarwith silica fumewas lower than thePortland cementcontrolandtendedtodecreasewithincreasingsilicafumecontent.How-ever,flowtableofternaryblendedPortlandcementmortarwithflyashandsilicafumecontentincreasedcomparedtobinaryblendedPortlandcementmortarswithonlysilicafumecontent.Moreover,flowtableofternaryblendedPortlandcementmortarsincreasedwithincreasingflyashcontent.

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Figure 2.SEMmicrographs ofmaterials used in this research: (a) Portland cement,(b)flyashand(c)silicafume.

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table 3.Flowtabletestsofblendedcementmortar.Mixes Flowtable(mm)

OPC 101.5

50FA 142.8

60FA N.A.*

70FA N.A.*

5SF 66.8

10SF 47.0

45FA5SF 99.0

55FA5SF 109.3

65FA5SF 112.9

40FA10SF 65.8

50FA10SF 76.0

60FA10SF 83.8

Note:*Notavailable,cannotbemeasuredduetooverflow(>150mm).

Compressive strength Compressive strength of binary blended cementmortars at 28 days isshown in Figure 3.The compressive strength of binary blended cementmor-tarscontainingflyashwas lower than thePortlandcementcontroland tends to decreasewithincreasingflyashcontent.Whilethecompressivestrengthofbinaryblendedcementmortarswithsilica fumeat5and10wt.%washigher than thePortlandcementcontrol.

Figure 3.CompressivestrengthofPortlandcementandbinaryblendedcementmortars.

CMU. J. Nat. Sci. (2013) 127➔

Figures 4-6 show the compressive strength of ternary blendedPortland cementmortarsat50,60and70wt%cementreplacementincomparisontoPortland cementcontrolmortars,respectively.Thecompressivestrengthofternaryblendedcementmortarsarehigherthanbinaryblendedcementmortars,atthesamelevel replacement.Moreover,thecompressivestrengthofternaryblendedPortlandcement mortars increaseswith increasing silica fume content.However, compressivestrength of all ternary blended cementmortars at 50, 60 and 70wt% cementreplacementarestilllowerthanthatofthePortlandcementcontrol.

Figure 4.CompressivestrengthofPortlandcement,binaryandternaryblendedcementmortarsat50%replacement.

Figure 5.CompressivestrengthofPortlandcement,binaryand ternaryblendedcementmortarsat60%replacement.

CMU. J. Nat. Sci. (2013) ➔128

Figure 6.CompressivestrengthofPortlandcement,binaryand ternaryblendedcementmortarsat70%replacement.

dISCuSSIon

This study investigated the compressive strength of binary and ternaryblendedPortlandcementmortarsusingflyashandsilicafumeasaPortlandce-mentreplacement.Fortheworkabilitypropertyofthesamples,flowtableofbinaryblendedPortland cementmortarswith silica fumewas lower than thePortlandcementcontroland tended todecreasewith increasingsilica fumecontent.Theuse of silica fume reduces theworkabilitymortar due to its very high specificsurfacearea(Mostafaetal.,2010).However,flowtableofternaryblendedPort-landcementmortarincreasedwithincreasingflyashcontent.Thisisduetothemostlysphericalshapeoftheflyashparticles,comparedtotheirregularshapesofPortlandcement(Siddique,2004;Khatib,2008).Thus,theworkabilityofblendedPortlandcementmortarwithonly silica fume replacement canbe improvedbyalsoincorporatingflyash. ThecompressivestrengthofbinaryblendedPortlandcementmortarscontain-ingflyashwaslowerthanthePortlandcementcontrolandtendedtodecreasewith increasingflyashcontent.Thereductionofcompressivestrengthofbinaryblendedcementcontainingflyashisduetoitsslowpozzolanicreactionandthedilutioneffect (Papayianni andAnastasiou, 2010;Duran-Herrera et al., 2011;Wongkeoetal.,2012).While thecompressivestrengthofbinaryblendedcementmortarswithsilica fumeat5and10wt%washigher than thePortlandcementcontrol.Thisisduetothehighpozzolanicreactionandmicrofillereffectofsilicafume(ErdemandKirca,2008;Elahietal.,2010). ForternaryblendedPortlandcementmortars,thecompressivestrengthofternary blendedPortland cementmortarswas higher than binary blendedPort-landcementmortarsatthesamereplacementlevelsandtendedtoincreasewith

CMU. J. Nat. Sci. (2013) 129➔

increasingsilicafumecontent.Theimprovementofthecompressivestrengthofternary blendedPortland cementmortars is due to the higher pozzolanic reac-tionofsilicafumethanthatofflyashandthemicrofillereffectofsilicafume.However,compressivestrengthofall ternaryblendedcementmortarsat50,60and70wt%cementreplacementwasstilllowerthanthatofthePortlandcementcontrol.This is because themicrofiller effect and pozzolanic reaction cannotcompensateforthedilutioneffectatthetestedage(28days).

ACKnoWledgeMentS

Iwould like to thank theOffice of theHigherEducationCommission,ThailandforsupportingthisresearchthroughagrantundertheprogramStrategicScholarshipsforFrontierResearchNetworkforthePh.D.ProgramThaiDoctoraldegree.TheGraduateSchool,ChiangMaiUniversityisalsoacknowledged.

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