Physio-Microstructural Properties of Aerated Cement Slurry for Lightweight Structures

15 Pages·2017·27.79 MB·English

materials
Article
Physio-Microstructural Properties of Aerated Cement
Slurry for Lightweight Structures
AreejT.Almalkawi1,* ID,TalalSalem1,SameerHamadna2,A.G.N.D.Darsanasiri2,
ParvizSoroushian2,AnagiBalchandra2andGhassanAl-Chaar3
1 DepartmentofCivilandEnvironmentalEngineering,MichiganStateUniversity,3546EngineeringBuilding,
E.Lansing,MI48824,USA
2 MetnaCorporation,1926TurnerStreet,Lansing,MI48906-4051,USA
3 ConstructionEngineeringResearchLaboratory(CERL),USArmyEngineerResearchandDevelopment
Center,Champaign,IL61822,USA
* Correspondence:[email protected]
(cid:1)(cid:2)(cid:3)(cid:1)(cid:4)(cid:5)(cid:6)(cid:7)(cid:8)(cid:1)
(cid:1)(cid:2)(cid:3)(cid:4)(cid:5)(cid:6)(cid:7)
Received:27February2018;Accepted:4April2018;Published:12April2018
Abstract: Cementitiouscomposites,includingferrocementandcontinuousfiberreinforcedcement,
are increasingly considered for building construction and repair. One alternative in processing
of these composites is to infiltrate the reinforcement (continuous fibers or chicken mesh) with a
flowablecementitiousslurry. Therelativelyhighdensityofcementitiousbinders,whencompared
withpolymericbinders,areasetbackineffortstointroducecementitiouscompositesaslower-cost,
fire-resistant,anddurablealternativestopolymercomposites. Aerationoftheslurryisaneffective
meansofreducingthedensityofcementitiouscomposites. Thisapproach,however,compromises
themechanicalpropertiesofcementitiousbinders. Anexperimentalprogramwasundertakenin
order to assess the potential for production of aerated slurry with a desired balance of density,
mechanical performance, and barrier qualities. The potential for nondestructive monitoring of
strengthdevelopmentinaeratedcementitiousslurrywasalsoinvestigated. Thisresearchproduced
aeratedslurrieswithdensitiesaslowas0.9g/cm3withviablemechanicalandbarrierqualitiesfor
productionofcomposites. Themicrostructureofthesecompositeswasalsoinvestigated.
Keywords: aeratedcementslurry;foamingagent;microstructure;thermalconductivity;sorptivity;
density;compressivestrength;lightweightstructures;composites
1. Introduction
Continuousfiberreinforcedcementcompositesandferrocementareexamplesofcompositesused
inbuildingconstructionandrepairapplications[1–4]. Whencomparedwithpolymercomposites,
cementitiouscompositesofferimprovedfireandmoistureresistance,andeconomics. Thedensityof
cementitiousmatrices,however,ishigherthanthatofpolymericmatrices.
Oneapproachtoproductionofcementitiouscompositesinvolvesinfiltrationofthereinforcement
systemwithacementitiousslurry.Aerationoftheslurryofferstheopportunitytoreducethedensityof
cementitiousmatrices. Whileaerationtendstocompromisethestrengthofcementitiousmaterials,the
aeratedmatricesmaystillbeabletomeetthedemandsontheirmechanicalperformanceinthecontext
ofcompositeswithrelativelyhigh-volumefractionsofcontinuousreinforcementwithhighspecific
surfacearea. Thesedemandsaredifferentfromthoseplacedonconcreteinconventionalreinforced
concretestructures. Thecombinationofaerationandreinforcementofhighspecificsurfaceareaand
closespacingcouldalsoprovidedesiredworkabilityattributes(e.g.,easeofscrewapplicationand
cutting)thatwouldmakesomewoodconstructiontechniquesapplicabletothematerial.
Materials2018,11,597;doi:10.3390/ma11040597 www.mdpi.com/journal/materials
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Efforts to reduce the density of slurry need to address (besides mechanical performance and
interactionswithdifferentreinforcementsystemssuchaswiremeshreinforcement)twoaspectsofthe
slurrybehaviorthatareofpracticalsignificanceinthisapplication: (i)sorptivityforprotectingthe
insulationandtheinteriorofthebuildingagainstmoisturetransport;and(ii)thermalconductivityfor
addingvaluetowardsenhancingtheenergy-efficiencyofthebuilding.
Thesynergybetweenfibersandorganicpolymershasbeenkeytotheemergenceofcomposites
aswidelyusedstructuralmaterials. Inthissynergisticaction,fibersaccountforthedistinctlyhigh
strengthandmodulusofcomposites. Organicpolymers,ontheotherhand,provideforstresstransfer
tofibers,andstressredistributionamongfibersuponearlyruptureofsomestatisticallyweakerones[5].
Thesecontributionsofthepolymermatrixrelyheavilyupontheirstraincapacityanddesiredadhesion
tofibers. Giventhebrittlenatureofpolymermatricessuchasepoxy;itistheirrelativelylowelastic
modulusthatisresponsiblefortheirelongationcapacity. Thepolymermatrixalsocontributesbarrier
qualitiestocomposites. Thegenerallyopenmolecularstructuresofbothorganicpolymersandfibers
areresponsiblefortherelativelylowdensityofcompositesthatbenefitstheir‘specific’strengthand
modulus. Weightsavinghasbeenavitalconsiderationintransitionfrommetalstocompositesin
aerospaceandotherapplications[5].
Theworkreportedhereinfocusesondevelopmentofaninorganicmatrixwithreduceddensity
and elastic modulus that suits utilization as matrix in continuous fiber reinforced composites that
have similarities to ferrocement products. The matrix developed here is an aerated slurry that
offersdesiredrheologicalattributesforinfiltratingfabricandmeshreinforcementsystems. Aerated
cementitious materials (e.g., aerated concrete) have been developed mostly to provide thermal
insulationqualities[6–8]. Thisworkfocusedonthedevelopmentofaeratedslurrieswitharelatively
lowdensityandmodulusandviablestrengthforuseasmatrixinstructuralcompositesasinvestigated
inourpreviousstudies[9–13]. Thisaeratedslurry-infiltratedmeshisdevelopedasaconstruction
materialthatoffersqualitiesintermediatebetweenwoodandconcrete. Itisintendedtoprovidea
desiredbalanceofrelativelylowdensity,ductility,toughness,strength,workability,moistureandfire
resistance,anddurabilityunderweatheringexposure.
2. MaterialsandMethods
Theaeratedslurrywaspreparedbymixingthefoamingagent(saponin). Saponinsarenatural
surfactants found abundantly in various plant species. More specifically, they are amphipathic
glycosides comprising one or more hydrophilic glycoside moieties combined with a lipophilic
triterpene derivative [14]. Figure 1 shows a saponin molecule obtained from the residues of sisal
defibering. Ithasbeenusedinformulationofdetergents[15].
Figure1.Saponinmoleculeextractedfromsisalwaste[16].
Aerationofcementitiousmaterialscanbeaccomplishedviastabilizationofentrappedairusing
surfactants [17–19], or by the addition of fine powder that generates gas by undergoing chemical
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reactions with cementitious materials. Aerated concrete has been produced with a wide range of
densities(300to1800kg/m3). Thefocusofthisworkisonachievingdensitiesbelow1000kg/m3
thataregenerallyviewedasinsulatingmaterials[20,21]. Saponin(ahydrolyzedproteinextracted
fromplants)[22–24]wasusedinthisworkasasurfactantforproductionofaeratedslurry. Saponin
wasmixedwiththemixingwaterofslurry,andagitatedtoformfoam,whichwasthenmixedwith
Portlandcementtype1toproducetheaeratedslurry. Asasurfactant, saponinlowersthesurface
tensionofwater. Surfactantsaremoleculeswithpolarandnonpolarendsthatattachtowaterand
air,respectively. Surfactantsaremoleculeswithpolarandnonpolarendsthatattachtowaterandair,
respectively. Theorientationofsurfactantmoleculesinbulksolutionisrandom. Thoseoccurringatthe
air/liquidinterfacesoradsorbedoncementparticles,however,havepreferredorientationsthattend
tominimizetheunfavorableinteractionsbetweentheliquidphaseanddifferentmolecularsectionsof
thesurfactant. Figure2showsthealignmentofamonolayerofsurfactantmoleculesattheinterface
betweenairandsurroundingliquidphase. Thehydrophobictailsofsurfactantmoleculesstickoutof
thesolutiontoreducethedistortionofwatermolecules,andthuslowertheoverallfreeenergyofthe
system[25,26]. Themutualrepulsionbetweenthehydrophilicheadsofsurfactantmoleculesreduces
theattractionofthebulkliquidphase,producingalowersurfacetension. Becauseoftheelectrostatic
componentoftherepulsionforceofionicsurfactants, theireffectivenesstoreducesurfacetension
is more significant than that provided by nonionic surfactants [27]. The nature and concentration
of surfactants determine the physical and chemical properties of the air bubble/liquid interfaces,
including surface tension (equals to free surface energy) and stability. The electrostatic and steric
repulsionsbetweensurfactantshelpstostabilizetheairbubblesthatformwithintheliquid[28,29].
Thehydrophilicendsofthesurfactantmoleculesarealsoelectrostaticallyattractedtocementparticles,
thisisalsoafactorinstabilizingtheairbubbleswithincementpaste(Figure3).
Figure2.Surfactantmoleculesatthewater–airinterface[25,30].
Figure3.Interactionbetweenairbubblesandcementparticles[25].
Saponinandwaterweremixedat1200rpmrotationalspeed,usingaCraftsman®mixingblade
attachedtoadrill(Figure4),toproducefoamedwater. Thefoamedmixingwaterwasthenaddedto
cementatwater/cementratioof0.45to0.6. Mixingwasaccomplishedinamortarmixerfor2min.
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Thewater/cementratioofdifferentaeratedslurrieswasadjustedinordertoproduceadesiredfresh
mixrheologyforinfiltratingmultiplelayersofchickenmesh. Therequiredfreshmixrheologycould
bedefinedbyaviscosityofabout1900cPandayieldstrengthofabout70. Theresultingaeratedslurry
wasplacedin50mmcubemoldsandkeptinsealedconditionatroomtemperaturefor24h. Thecube
specimens were then demolded and cured at 95 ± 5% relative humidity and room temperature
for seven days. The aerated slurry mix proportions considered in this experimental program are
introducedinTable1. Forthenon-aeratedslurry(with0%saponincontent),water/cementratiowas
between(0.45–0.55).
Figure4.Foamgenerationinwaterviahigh-speedmixing.
Table1.Theaeratedslurrymixesproportionsconsideredinthisinvestigation.
Mix SaponinDosage(byWeightofCement) Water/CementRatio
1 0.005%
2 0.01% 0.45
3 0.02%
4 0.005%
5 0.01% 0.50
6 0.02%
7 0.005%
8 0.01%
9 0.015%
0.55
10 0.02%
11 0.025%
12 0.03%
13 0.02%
0.6
14 0.025%
Figure5showsexamplesoftheaeratedslurrycubespecimensthatweretestedincompression
formeasurementofthecompressivestrengthofaeratedslurry(atsevendaysofage). Thedensityof
aeratedslurrywasmeasuredbydividingtheair-driedweighttothevolumeofthesespecimens.
Figure5.Aeratedslurryspecimens.
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The aerated slurry would be the primary protection of the building interior and also the
indigenousinsulationtobeusedwithinthestructuralpanelsagainstweatheringeffects. Moisture
wouldbetransportedthroughtheaeratedslurryskins(withchickenmeshreinforcement)viacapillary
sorption. Anexperimentalstudywasthusundertakeninordertomeasuretheeffectsofaerationon
thecapillarysorptivityoftheslurry. SorptivitytestswereperformedperASTMC1585;thespecimens
usedforthispurposewerecylindersof100mmdiameterand50mmthickness. Aschematicofthetest
setupisshowninFigure6a,andapictureofmultiplespecimensduringsorptivitytestingisshownin
Figure6b. Thespecimensweredemoldedafter24hofstorageinsealedconditionandwerecuredat
roomtemperatureuntilthetestage. Thistestmethodinvolvesexposureofoneflatsurfaceofspecimen
towater,whichtheremainingsurfacessealedagainstmoistureloss. Massgainovertimeisrecorded
asameasureofmoisturesorption. Sorptivityisexpressedintermsofthesorptionrateofmoistureinto
theaeratedslurry. Thistestwascontinuedfortwodaysinordertogainfurtherinsightintoschematic
time-historyofcapillarysorption[31,32].
Figure6.Sorptivitytestsetup.(a)Schematics;(b)pictureofmultiplespecimensduringthetest.
The ultrasound pulse velocity (UPV) of aerated slurry was measured nondestructively using
a portable equipment (58-E4800 UPV tester, CONTROLS S.p.A, Milan, Itlay). The UPV test setup
is shown in Figure 7. In this test, an ultrasonic pulse is generated and transmitted to the surface
of concrete through the transmitter transducer. The time taken by the pulse to travel through the
aeratedslurry,t ,ismeasuredbythereceivertransducerontheoppositeside. The54kHztransducers
us
werepositionedatthecenterofeachopposingface. Thepropagationtimeoftheultrasonicwaves
transmittedthroughthe150mmlongcylindricalspecimenswasmeasuredwithaccuracyupto0.1s.
Athincouplant(solidbluekaolin-glycerolpastewasusedattheinterfacebetweentransducersandthe
aeratedslurryspecimensurfacestoensuregoodcontact. Thepulsetraveltime(t)fromthefrontsideto
therearsidewasautomaticallyrecorded. Theultrasoundpulsevelocitywasmeasuredapproximately
50haftermixingoftheaeratedslurry[33–35]. Thespecimensusedforperformanceoftheultrasound
pulsevelocitywerepreparedfromaeratedslurrymixesofsimilarproportionsasthoseusedforother
experiments; themixesusedforpreparationoftheultrasoundpulsevelocityspecimens,however,
werenotthesameasthoseusedforpreparationofthespecimensusedinothertests. Allspecimens
werecuredatroomtemperatureand95±5%relativehumidity.
Thethermalconductivityofaeratedslurrytestwasmeasuredatsevendaysofageinaccordance
toASTMC177[36]. Aeratedslurrycementspecimenswereovendriedfor24hatatemperatureof
105±5◦C.Figure8showsthethermalconductivitytestingconfigurationandtestsetup.Thespecimen
wasplacedbetweenhotandcoldplateswith40and18◦C,respectively,tosimulateoutdoorandindoor
temperatures. Temperaturesonthehotandthecoldplatesaswellastheheatflowwererecorded
versustimeover24h. Theresults,aftertheprocessreachedequilibrium,wereusedtocalculatethe
thermalconductivityofaeratedslurry.
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Figure7.Ultrasoundpulsevelocitytestsetup.
Figure8.(a)Thermalconductivitytestingdiagramand(b)testsetup.
Furthermore,aeratedslurrysamplesweresubjectedtoScanningElectronMeasurement(SEM)
observationtoevaluatemicrostructuralfeatures. SEMobservationswerecarriedoutonJCM-5000
NeoScope™(JEOLLtd.,Tokyo,Japan)atanacceleratingvoltageof10–15kVusingasecondaryelectron
(SE)detector. Theinvestigationswereconductedonfracturesurfacesofthepasteofthesamplesafter
28days. SamplesweresputteredwithgoldbeforetheSEMmeasurements.
3. ExperimentalResultsandDiscussion
3.1. CompressiveStrength
Table 2 presents the measured values of seven-day compressive strength and density for the
aeratedslurrymixdesignsintroducedearlierinTable1. Lowervaluesofdensitytendtocorrespond
withlowervaluesofcompressivestrength. Thisisbothbecauseoftheriseinaircontentandalso
theincreaseinwater/cementratioforachievingviablefreshmixrheology. Mix13withdensityof
0.9g/cm3and5.4MPacompressivestrengthatsevendaysprovidesaviablebalanceofdensityand
strength for the targeted ferro-cement application. This paper strongly emphasizes the density of
aeratedslurryinordertoenhancetheefficiencyofseismicdesign[37,38]andalsotoenablemanual
installationofthebuildingstructure.
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Table2.Mixdesignsandperformancecharacteristicsoftheaeratedslurry.
Mix Seven-DayCompressiveStrength,MPa Density,g/cm3
1 10.7 1.9
2 8.2 1.5
3 6.3 1.4
4 14.1 1.2
5 10.5 1.81
6 9.2 1.3
7 13.3 1.6
8 11.1 1.7
9 9.4 1.3
10 6.4 1.17
11 2.4 0.65
12 1.2 0.8
13 5.4 0.9
14 7.1 1.12
3.2. Sorptivity
ThesorptivitytestresultsarepresentedinFigure9asthecapillaryriseofmoistureversustimefor
aeratedslurriespreparedwithdifferentdosagesofthefoamingagent(saponin),withwater/cement
ratioof0.55. Thetwohigherdosagesoffoamingagent(0.015%and0.02%)areobservedtoproduce
lowersorptionratesandcapacities. Thisisapositivetrend,indicatingthatloweringthedensityofthe
slurryviaaerationwouldactuallyimproveitsbarrierqualitiesforprotectingthebuildinginterioras
wellasthenaturalinsulationagainstweatheringeffects.
Figure9.Capillarysorptionofaeratedslurriesversusthesquarerootoftime.
The initial sorption rate (S) is the slope of the sorption curve shown in Figure 9 up to 6 h;
i
thesecondarysorptionrateistheslopeofthecurveafteroneday. Boththesecalculationsaremade
usingalinearregressionanalysis(ASTMC1585)[39]:
√
I = S × t+b
i
The resulting values of initial and secondary sorption rate are presented in Table 3 together
with the corresponding values of the aerated slurry density. These results confirm that lowering
thedensityofaeratedslurryfrom1.7to1.17–1.3g/cm3leadstoasignificantdropintheinitialand
secondary sorption rates of the slurry. This can be explained by the fact that aeration introduces
isolatedairbubblesintotheslurry, whichdisruptthecontinuityofcapillaryporesthroughwhich
sorptionoccurs[40–45]. AnoverallsorptivityvalueisalsopresentedinTable3,whichistheslopeofa
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regressionlinefitintothewholedatapoints(usingtheaboveequation). Theoverallsorptivityvalues
furtherconfirmsthedropinsorptionratewithreductionofthedensityofaeratedslurry.
Table3.Sorptionratesanddensitiesofslurriespreparedwithdifferentdosagesofthefoamingagent.
DosageofFoamingAgent% 0.01% 0.015% 0.02%
√
Initialsorptionrate,mm/ s 0.0242 0.0188 0.0132
√
Secondarysorptionrate,mm/ s 0.0044 0.0013 0.0019
R2(Regressionvalue) 0.951 0.950 0.958
Density,g/cm3 1.7 1.3 1.17
Sorptivity,mm/min0.5 0.75 0.5 0.34
In order to confirm the finding that aeration actually reduces the sorption rate and extent of
slurry(i.e.,enhancesitsbarrierqualities),testswerealsoperformedonaslurrywithoutanyaeration.
The sorption test data presented in Figure 10 confirm that aeration reduces the rate and extent of
moisturesorptionintotheslurry. AsschematicallydepictedinFigure11,introductionofisolatedair
bubbleforcestortuoussorptionpathsthroughcapillarypores,whichreducethesorptionrateofthe
slurry. Theexperiencewithentrainedairbubblesindicatesthatindividualairbubblesremainlargely
emptyofwaterevenunderlong-termexposuretomoistconditions. Thisphenomenon,aswellasthe
reductionintherateofmoisturesorption,explainthedropintheextentofmoisturesorptionwith
introductionofindividualairbubblesviaaerationofslurry.
Figure10.Sorptivityofnon-aeratedversusaeratedslurries.
Figure11.Sorptionpathsintonon-aeratedandaeratedslurries.(a)Non-aerated;(b)aerated.
Opticmicroscopeimageswereobtainedfromsectionsofaeratedslurriespreparedwithdifferent
dosagesofthefoamingagentinordertounderstandthemorphologyofairbubblesandexplaintheir
effects on compressive strength. Figure 12a,b show microscopic images of slurries prepared with
0.005%and0.02%concentrationsofthefoamingagent,respectively.Ariseinthefoamingagentdosage
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isobservedtoincreasethesize(aswellasthevolumefraction)ofairbubble. Itshouldbenotedthat
smaller and regularly formed air bubbles produce higher compressive strengths than coarser and
irregularlyformedairbubbles[46]. Mechanicalpropertiesarestronglyinfluencedbythedistribution
ofporeswithinthehardenedaeratedslurry[47,48]. Thesphericalanddistributednatureoffoamsin
Figure12bwithhigheraircontentledtoaviablelevelofcompressivestrengthwhichwasnotlower
thanthatprovidedbytheaeratedslurryshowninFigure12bwithlowercontentofirregularly-shaped
airbubbles. Thisobservationconfirmsthatmicrostructuralpropertiesareprimaryfactorsinfluencing
the material properties of aerated slurry [38]. An example optic microscope image of an exterior
surfaceofanaeratedslurryofhighercompressivestrengthisshowninFigure13,wherefinerand
moreuniformlydispersedairbubblescanbeobserved.
Figure12. Opticalmicroscopeimagesofsectionsofaeratedslurrieswithdifferentdosagesofthe
foamingagent(saponin).(a)0.005%foamingagent;(b)0.02%foamingagent.
Figure 13. A typical microscopic image of the exterior surface of an aerated slurry with higher
compressivestrength.
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3.3. SEMObservations
Thesphericalgeometryofairvoidsisanimportantfactorinfluencingthestructuralandfunctional
propertiesofaeratedbinders[49,50]. Inaddition,thevoidsshouldbedistributedevenlyinthemassto
producehomogenousbindersofimprovedperformance. Largervoids(macro-pores)wouldlower
thedensityofaeratedslurrybutcouldcompromiseitsmechanicalperformance. Dependingonthe
type and dosage of the foaming agent, aerated cement slurries may incorporate both micro- and
macro-pores [51]. Macro-pores could be formed as a result of the merger of micro-pores. This is
because expansion of the matrix upon micro-pore formation generates pressure at the interfaces
betweenmicro-pores[52]. Figure14ashowsanSEMimageofanaeratedslurryafterhydration.
Figure14.SEMimagesofthefracturedsurfacesoffoamcementsmadefromslurriesafterhydration
for28dayswithadensityof(a)1.0g/cm3(b)0.75g/cm3(c)1.3g/cm3.
Asidefromthegelpores(<10nm)andcapillarypores(10nmto10µm),hollowshellporeshave
beenproposedasathirdcategoryofintrinsicporesinbulkofhydrationproducts[52]. Hollowshells
haverangeinsizefrom1toaround20µm, aboutthesizeofsmallercementgrains, embeddedin
cementgelandchanneledtotheoutsidethroughcapillaryandgelpores.
Anidealmicrostructureofaeratedcementminimizestheextentofwatertransportbyuniformly
distributingthediscretemicro-sizeporesgeneratedbythefoamingagentwithinthecementslurry.
Thecoalescenceofmanyirregular-shapedpores, however, maycreateadisturbedmicrostructure,
triggeringahighdegreeofwatermobility. Inordertoverifythis,twofracturedsamplesofaerated
slurrieswith0.75and1.3g/cm3bulkdensitieswereexaminedusingascanningelectronmicroscope
(Figure14b,c,respectively). Theimageontheright,fortheslurrywith1.3g/cm3bulkdensity,shows
amicrostructureofuniformlydistributeddisjunctivepores. Incontrast,adifferentmicrostructurewas
observedfortheaeratedslurrywith0.75g/cm3bulkdensity(left)whereananomalouscoalescence