Development of Alkaline-Activated Self-Leveling Hybrid Mortar Ash-Based Composites

22 Pages·2017·8.25 MB·English

materials
Article
Development of Alkaline-Activated Self-Leveling
Hybrid Mortar Ash-Based Composites
LuísUrbanoDurloTambaraJúnior ,MalikCheriafandJanaídeCavalcanteRocha*
LaboratoryofWasteValorizationandSustainableMaterials(ValoRes),DepartmentofCivilEngineering,PPGEC,
FederalUniversityofSantaCatarina(USFC),CampusTrindade,88040-900Florianópolis,SC,Brazil;
[email protected](L.U.D.T.J.);[email protected](M.C.)
* Correspondence:[email protected];Tel.:[phone]
(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:1September2018;Accepted:22September2018;Published:26September2018
Abstract: Thisstudyinvestigatedthereactivitypropertiesofself-levelinghybridalkali-activated
cements,suchasordinaryPortlandcement(OPC)anditsresidualprecursors,coalbottomash(BA),
andricehuskash(RHA).DuetotherelativelylowreactivityofBA,binarymixeswereproducedwith
OPCusingcontentsof2.5–30%inthetreatedBAsamples. Furthermore,ternarymixeswereprepared
inproportionsof25%,50%,and75%withRHAasareplacementmaterialfortheOPC(mixwith
90%:10%BA:OPC).Forallofthemixesthespreadingbehaviorswerefixedtoobtainaself-levelling
mortar, anddimensionalchanges, suchascurlingandshrinkage,wereperformed. Mortarswith
30%OPCreachedacompressivestrengthof33.5MPaandflexuralstrengthof7.53MPa. Ascanning
electronmicroscope(SEM)andX-raypowderdiffraction(XRD)wereusedtoindicatetheformation
ofN-A-S-Handa(N,C)-A-S-Hgel,similartothegelwithtraceofcalcium. Thebestperformancewas
achievedwhenthebinarymixproduced10%OPC.AhybridmortarofOPS-BApresented10times
lowersusceptibilitytocurlingthananOPCmortar. Theresultsshowedthatbothashesreducedthe
shrinkageandcurlingphenomena.
Keywords: hybridcements;self-levellingmortar;ashes;alkali-activation
1. Introduction
Alkali-activatedmaterials(AAM)arewidelyknownasaneco-efficientcementduetoitslow
CO emissions and reuse of waste residue, achieving physical and mechanical properties similar
2
to ordinary Portland cement (OPC) [1–4]. An aluminosilicate mineral and an alkaline solution
are required for activation [5]. Industrial waste, as the fly ash (FA), blast-furnace slag (BFS), and
bottom ash (BA) are commonly used as aluminosilicate minerals in alkaline activation [6–9]. The
developmentofalkali-activatedsystemswiththeuseofashfromcoal-firedthermalplantshasproven
tobepromising. However,theuseofBAisapotentialalternativetotheincreasingconsumptionof
FAasasupplementaryadditiontoOPCclinker,orduringhighlevelsofproductionwhenmaterials
becomeunderusedandblendedintopondsasBA.
Some recent studies have demonstrated that BAs can be used as an alternative source of
aluminosilicates in mixes for alkaline activation [6–15]. To ensure the sufficient reactivity of this
precursor material, a larger amount of amorphous silica and relatively small particle size are
required[14–16]. However,todevelopsagreatermechanicalstrengthAAM-baseddependsonash
reactivity[6,11,12],andahigherrateofdissolutioninalkalinemedium[5]. Duetoalessamorphous
content,glassyfilmcanshowattheparticlesurface,thushinderingtheinitialdissolution[15]. The
reactionscouldbeimprovedbyashesmilling[14,16]inadditiontoachangeintheSiO /Al O molar
2 2 3
ratio[17]. IfthedissolutionandsolidificationprocessofcalciumisfasterthanSi/Al,thenacceleration
hardens[17–19].
Materials2018,11,1829;doi:10.3390/ma11101829 www.mdpi.com/journal/materials
Materials2018,11,1829 2of22
Rice husk ash (RHA) is generally produced by the controlled burning of rice husks [20].
Amorphous silica is the predominant component, but its content varies according to the ash
treatment [21,22]. Their use in AAM is principally an alternative source of soluble silica [23–25].
Kusbiantoro et al. [26] observed that the presence of RHA in alkali-activated concretes, with
replacement of up to 7% of ash, increased the compressive strength and delayed the dissolution
andthepolycondensationofalumina,thusaffectingthegelationprocessandincreasestheSi/Almolar
ratioandthepolymericandthepolysialatestructuresleadingtoanincreaseinstrength.
Nonetheless, Nazari et al. [27] noticed that the binary mixtures of FA and RHA (containing
20–40%ofRHA)hadalargeramountofRHAreducedalumino-silicategelsphasesformation. Higher
NaOH molarity accelerates dissolution, althought if it accelerates above 12 M then obstruction
of polycondensation occurs. The authors related better mechanical performance with higher
materialfineness.
Xu and Van Deventer [28] claim that Al and Si dissolve quicker when used in NaOH when
comparedtoKOH,resultinginamoreresistantstructure,duetoanentropyincreaseatthesystem.
However,thereareimprovementsofalkaliactivationwhencombinedwithactivators(e.g.,hydroxide
andsilicate). Nucleationturnsslowlywhenusedwithhydroxides. Whenusedwithsilicate,however,
there is a higher presence of soluble silicon, which accelerates the nucleation process, acting as a
“seeding”agent,risingthedissolutionofvitreousphaseoftheashes[29].
Palomoetal.[12]describedthreetypesofalkaline-activatedcements: Cementrichincalcium,
wheretheprecursorhasCaOlevels>10%,inwhichthemainproductsofthereactionareC-A-S-H
gels;cementwithlowcalciumcontent,inwhichthemainproductsofthereactionareN-A-S-Hgels;
andthehybridcements,withlowlevelsofPortlandcementinconjunctionwiththealuminosilicate,in
whichtheproductsofthereactionarecomplex,includingC-A-S-HandN-A-S-Hgels.
Similarly,Garcia-Lodeiroetal.[30]establishedamodeldetailingahybridof70%FA–30%OPC,
activated with NaOH and a water-glass. Based on the existence of an initial dissolution of the
alumino-silicateandcalciuminthesolution,SiandAlinalumino-silicateandofCa-OandSi-Ointhe
OPCbrokechains. Whenthesolutionbecomessaturated,aprecipitationofN-A-S-Handformation
ofC-S-Hgelsoccur. Overtime,Sidissolves,transformingN-A-S-Hchainsfromtype1(Si/Al=1)to
type2(Si/Al=2). Inaddition,thepresenceofCaandAlionsintheaqueousmediumthatreactwith
C-S-HgeltoformaC-(A)-S-H(2D).AportionofCathatdoesnotreactbefore,interactswithN-A-S-H,
forming(N,C)-A-S-Hgel[31].
Quetal.[32]studiedtheeffectofthecuringtemperatureonhybridcementscontainingFA,BFS,
and30%OPC.TheyfoundthatthedominantreactionproductsformedwereC-(A)-S-HandC-A-S-H
gels. Additionally,theyconcludedthatcuringtheproductsat85◦Cacceleratedthereaction,resulting
in higher mechanical strength in the initial days of curing. Yip et al. [33] reported the coexistence
ofC-S-HandN-A-S-Hgelsinhybridcements,sincethemediumisnottooalkaline. Furthermore,
Angulo-Ramírezetal.[34]reportedthepresenceofmainlyC-S-HandC-A-S-Hgelsinhybridcements
with20%OPCand80%BFS.
Therearehighexpectationsassociatedwiththeuseofhybridcements,sincetheycanreplace
theOPCcementsinmortarsandconcretes[35]. Nevertheless,fewstudieshavebeenconductedto
characterizethepropertiesofhybridcementscontainingcoalBA.Thecuretimeandtemperatureaffect
theformationofthestructureobtainedfromalkalineactivation,wherehighreactiontemperatures
increasethemixesmechanicalstrength[36]. Thepotentialfortheapplicationofhybridcementto
produceaself-levellingcompound(SLC)wasassessedinthisstudy.FormortarscomprisedofSLC,the
propertiesrequiredare: Fluidity,lowviscosity,spreading,fastdrying,dimensionalstability,surface
resistance, and durability [37]. In other words, a self-leveling material needs to present resistance
to segregation, the capacity for fluidity, and self-levelling. For the characterization of the fluidity,
monopointtests,suchasmini-slumptest,arewidelyused. Alkali-activatedproductsusuallypresent
highviscosity,resultingfromtheuseofanalkalinesolution. Thismayhaveapositivecontributionin
termsofavoidingthesegregationofself-levellingproducts. However,therearerecurringproblems,
Materials2018,11,1829 3of22
includinghighshrinkageandcurling,especiallywhenappliedinmaterialswiththinthicknesses. The
shrinkagephenomena,ontheotherhand,isrelatedtohardenedgelsofalkaline-activatedsystems[38].
Taking into consideration the reactivity properties of BA and RHA, the valorization of these
wastescouldbeachievedthroughtheiruseinalkali-activation. Therefore,ourstudyproducedbinary
andternaryhybridsystemsusingBA,RHA,andOPC.Thesampleswereevaluatedbaseduponthe
settingtimeofthefreshmaterial,aswellastheirmechanical,microstructural,anddurableproperties.
Therefore,thestudysoughttoimprovethetechnologyofself-levelingAAMscreeds.
2. MaterialsandMethods
BA,classF,wascollectedfromtheJorgeLacerdathermoelectricplant,inSouthBrazil. Toimprove
thereactivityandtheformationofanamorphousphase,theashwasgroundinamillfor450min,and
treatedat600◦Cfor1hinamufflefurnace. TheBAwasusedasthemainsourceofaluminosilicate
thatproducedalkali-activatedmortars. TheOPCusedrepresentedacementwithoutsupplementary
materials,typeIIIaccordingtothedescribedonBrazilianStandardNormalizationNBR5733-91High
early strength Portland cement—Specification, and NBR NM 22 Portland cement with material
additionspozzolan—Chemicalanalysis—Methodarbitration[39,40].RHAoriginatedfromacontrolled
combustioninafluidizedbedatthePileccothermoelectricplantinAlegrete, inSouthBrazil. The
RHAwasusedtoreplacetheOPCandincreasethereactivesilicaintheternarymixes. Thechemical
compoundswereanalyzedwithanenergydispersiveX-rayspectroscopy(EDX700Hs,Shimadzu,
Tokyo,Japan).Thestrengthactivityindex(pozzolanicactivity)wasmeasuredaccordingtothestandard
NM22[41].Particlesizedistributionwasdeterminedwithlasergranulometry,usingaMastersizer2000
(MicrotracS3500,Largo,FL,USA).Fourier-transformInfrared(FTIR)spectroscopywascarriedout
usinganAgilentCary600(Agilent,SantaClara,CA,USA)atfrequenciesbetween4000and400cm−1.
AccordingtoNBR5752,thepozzolanicactivityoftheRHAwasdetermined[42]as122%. The
strengthactivityindexwastheratioofthecompressivestrength,withtheadditionandthecompressive
strengthofcontrol(withoutRHA).Thestrengthresultsweregreaterthan75%andthustheRHAwas
foundsuitableforadditiontoconcrete. Inordertoincreasethesurfacearea,theBAwasground. The
grindingtimewasestablishedasatleast6h[43],untilallthegroundmaterialwas<45µm. TheBA
wasthenpreparedbygrindingtheparticlesinagrindingmillfor7.5handthenputinamufflefurnace
in1hat600◦Cforcalcination. TheBAcalcinationwasperformedinordertoreducethecontentof
unburnedmaterial,duetothehighashcontentfoundinBraziliancoal.
ThesodiumhydroxidesolutionwaspreparedbydissolvingNaOHpelletswith97%(P.A.)in
distilledwater,keepingaconstantmolarconcentrationof14forallmixes. Thesolutionofsodium
silicate(SiO =26.5%;Na O=10.6%;H O=62.9%;anddensityof1.39g/cm3)wasmixedwiththe
2 2 2
hydroxidesolution,keepingaproportionof1:2ofhydroxidetosilicate. Mixesofthesolutionswere
prepared24hbeforetheproductionofthemortarandhadafinalpHof13. Forthemortarsproduction,
naturalfinesanddimension(1.2/0.15)mmwithafinenessmodulusof1.83,andaspecificgravityof
2.60g/cm3 wasused. Themortarsweremixedinmechanicalplanaterymixer,withamixingtime
of2minat150rpmand1minat300rpm. Asuperplasticizeradditive,withasolidscontentof0.22,
based on polycarboxylate ether, was used to obtain workability (flow value > 250 mm), required
forself-levelingmortars. Afterthemortarproduction,heatcuringwasperformedfor24hat80◦C.
Samples were then kept in a room with controlled temperature (23 ± 2 ◦C) and relative humidity
(RH=60±5%)untilthecompletionofthetests.
Materials2018,11,1829 4of22
2.1. PreparingAlkali-ActivatedMortars
We studied eight alkali-activated mortars with different mixes. In the binary mixtures, BA
waspartiallyreplacedwithOPCinproportionsof0%,2.5%,5%,10%,and30%. Thesemixeswere
numbered1(Reference),2,3,4,and5,respectively. Theternarymixtureswerebasedon90%BAand
10%OPC,withtheOPCbeingpartiallyreplacedwithRAHinproportionsof25%,50%,and75%;these
wereidentifiedasmixes6,7,and8,respectively. Thebinderandsandratioremainedconstantat1:2
(bymass)andthewater/cementratiowaskeptconstantat0.55(wt%). Acorrectionofdensitywas
necessarytoequivalentvolumeinsystemswhichadoptahighervolumeofmineralwithdifferent
specificgravity. EachmixtureproportionsandthemolarratioareshowninTable1.
Table1.Mixtureproportions.
MortarMixProportions(per100gofPrecursor) MolarRatio
Mix Samples
BA OPC RHA SH1 SS2 Sand SP3 Si/Al Na2O/SiO2 H2O/Na2O
1 OPC0(REF) 100 - - 60 30 200 1.0 2.57 0.27
2 OPC2.5 97.5 2.5 - 60 30 200 1.0 2.60 0.27
3 OPC5 95.0 5.0 - 60 30 200 1.4 2.64 0.27
OPC10/RHA
4 90.0 10.0 - 60 30 200 1.2 2.71 0.28 12.26
0
5 OPC30 70.0 30.0 - 60 30 200 1.4 3.08 0.30
6 RHA25 90.0 7.5 2.5 60 30 200 1.2 2.81 0.27
7 RHA50 90.0 5.0 5.0 60 30 200 1.2 2.91 0.26
8 RHA75 90.0 2.5 7.5 60 30 200 1.2 3.02 0.25
1Sodiumhydroxidesolution.2Sodiumsilicatesolution.3Superplasticizer.
2.1.1. WorkabilityoftheAlkali-ActivatedMortars
The workability was measured using a mini slump cone (19 cm × 38.1 cm × 57.2 cm). This
test was performed according to a previously described procedure [44]. The measurements were
performedimmediatelyafterthemixtureofthemortar,andthespreadingwasnotedbywayoffour
perpendicularmeasurements. Theflowvaluewasrecordedastheaveragevalue. Themaintenanceof
thespreadingpropertywasassessedoveraperiodof2h,atintervalsof30min.
2.1.2. MechanicalStrengthofAlkali-ActivatedMortars
Compressiveandflexuralstrengthtests(Solotest,SãoPaulo,Brazil)wereperformedaftermolding
the mortars in prisms with dimensions of 4 × 4 × 16 cm, according to Brazilian Standard NBR
13279[45]. Thesamplesweretestedat1and28daysafteractivation. Anaverageofn=3samples
toflexuraltestsandn=6tocompressivetests. Theelasticanddynamicmodulusweredetermined
accordingtothestandardmethods,NBR8522[46]andASTMC597[47],respectively. Thedynamic
moduluswasobtainedwithPunditLabmodel6.0apparatus(Procec,Zurich,Switzerland),featuringa
20mmdiameterand200kHzfrequency. Accordingtothemanufacturer,theaccuracyoftheequipment
was0.001%.
Todeterminethewaterabsorptionviacapillarity,weusedcylindricalmortarssamplescasted
inamold5cm×10cmdimensions,throughthevariationofawatercolumnbytime,inaMariotte
tube[48]. Thesorptivity(absorptionversussquarerootoftime)wasobtained. Thewettingangles
ofthemixesweredeterminedthroughtheabsorptionoftwodifferenttypesofliquids: Deionized
waterandethylalcohol. TheopenporositywasdeterminedaccordingtotheBrazilianstandardNBR
9778[49]usingsamplesof5cm×10cm,andperformingtheatsampleagesof1and28days.
Materials2018,11,1829 5of22
2.1.3. ResistancetoAcidAttack
Toassessthemortarsdurability,twotestswereconducted: Theacidattacktestandtheimmersion
anddryingtest. Theprismaticsampleswithdimensionof4cm × 4cm × 16cmweretestedafter
28days. Theacidattackwasconductedbyimmersingthesamplesin1Nsolutionsofhydrochloric
acid(HCl)andaceticacid(HAc),separately,forfourcyclesofsevendays. Theexperimentdetermined
themassloss,acidattackresistance,andcompressionstrength. Thewettinganddryingtestwasalso
carriedout28daysafterthemortarsactivation. Theimmersionanddryinginvolved28cycleswhere
eachcycleconsistedofkeepingthesamplessaturated,indistilledwater,for16h,followedbydrying
inanovenat50◦Cfor8h.
2.1.4. DimensionalChanges
The linear shrinkage was measured by drying according to NBR 15261 [50]. Samples with
dimensions of 2.5 cm × 2.5 cm × 28.5 cm were used to measure the linear variation. The curling
phenomenonwasevaluatedbymeasuringtheverticaldisplacementofamortarsampleplacedina
33cm×33cm×3cmmold,usingfourLVDT(Linearvariabledifferentialtransformers)sensorsat
themortarscreeds,atdistancesof2.5and4.5cmfromtheedges,andtwosensorsinthecenterofthe
mortars[51]. Mortarsweremixedinportableverticalshaftmixer. Lastly,masslosswasmonitored
withaloadingcellplacedunderthescreeds.
2.2. PreparingAlkali-ActivatedPastes
Aselectivechemicalattackofalkali-activatedpasteswasperformedafter28daysofheattreatment
(24hat80◦C).Duetotheashdissolutioninthealkalineactivation,aconcentrationof1:20HClwas
usedtodeterminethequantityofreactedcement,thusevaluatingthedegreeofreaction[6]. The1:20
HClsolutionwaspreparedusingHClwith37%ofdistillateddeionizedwater.
Besidesthat,weinvestigatedsettingtimeandmicrostructure. X-raydiffractogramsofthesamples
in powder form were obtained using a Phillips X-pert CuKa diffractometer (Philipps Analyticlal
XRay, Almelo, The Netherlands, 40 kV, 30 mA), with scan step size of 2◦ s−1, at 2θ angle range in
3–55◦ divergenceslitof1◦ andreceivingslitof0.2-mm. Forthescanningelectronmicroscope(SEM)
analysis,sampleparticleswereobservedunderaJeolJSM-6390SL(JEOLUSAInc.,PeabbodyMA,
USA) (15.0 kV) and energy-dispersive X-ray spectroscopy (EDS) probe was also performed at the
CentralLaboratoryofElectronMicroscopy(LCME-UFSC).AfourierTransformInfraredspectroscopy
(FTIR)wasperformedforcharacterizemicrostructure,usingaJASCOFT-IR4200(JASCOCoorporation,
Tokyo,Japan)atfrequenciesbetween400and4000cm−1. ThemeasurementswereperformedwithKBr
pellets. Forthepastestheproportionsofcementmaterialandalkalineactivator(ratiobinder:solution)
andsuperplasticizerwerekeptthesame. ThesettingtimewasdeterminedusingaVicatapparatus
(Solotest,SãoPaulo,Brazil)applyingthesamecureconditionsusedformortarcasting.
2.3. MicrostructureAnalysis
Allmortarsamplesweresubjectedtomicrostructuralanalysisafter28days,inordertoverify
theeffectoftheproportionofOPCintheBAreplacement,aswellastheinfluenceofreactivesilica.
Powderedmaterial(<150µm)wasanalyzedwithX-raypowderdiffraction(X-pertsystemPhilips
AnalyticalXRay,Almelo,TheNetherlands). FragmentswereusedtoobtainSEMimages(JEOLUSA
Inc.,Peabbody,MA,USA),withthesamedevicesusedtothepastesinvestigation.
Below,Figure1summarizesourexperimentalprocedure.
Materials 2018, 11, x FOR PEER REVIEW 5 of 21
33 cm × 33 cm × 3 cm mold, using four LVDT (Linear variable differential transformers) sensors at
the mortar screeds, at distances of 2.5 and 4.5 cm from the edges, and two sensors in the center of the
mortars [51]. Mortars were mixed in portable vertical shaft mixer. Lastly, mass loss was monitored
with a loading cell placed under the screeds.
2.2. Preparing Alkali-Activated Pastes
A selective chemical attack of alkali-activated pastes was performed after 28 days of heat
treatment (24 h at 80 °C). Due to the ash dissolution in the alkaline activation, a concentration of 1:20
HCl was used to determine the quantity of reacted cement, thus evaluating the degree of reaction [6].
The 1:20 HCl solution was prepared using HCl with 37% of distillated deionized water.
Besides that, we investigated setting time and microstructure. X-Ray diffractograms of the
samples in powder form were obtained using a Phillips X-pert CuKa diffractometer (Philipps
Analyticlal XRay, Almelo, The Netherland, 40 kV, 30 mA), with scan step size of 2° sec−1, at 2θ angle
range in 3°–55° divergence slit of 1° and receiving slit of 0.2-mm. For the scanning electron microscope
(SEM) analysis, sample particles were observed under a Jeol JSM-6390SL (JEOL USA Inc, Peabbody
MA, USA) (15.0 kV) and energy-dispersive X-ray spectroscopy (EDS) probe was also performed at
the Central Laboratory of Electron Microscopy (LCME-UFSC). A fourier Transform Infrared
spectroscopy (FTIR) was performed for characterize microstructure, using a JASCO FT-IR 4200
(JASCO Coorporation, Tokyo, Japan) at frequencies between 400 and 4000 cm−1. The measurements
were performed with KBr pellets. For the pastes the proportions of cement material and alkaline
activator (ratio binder:solution) and superplasticizer were kept the same. The setting time was
determined using a Vicat apparatus (Solotest, São Paulo, Brazil) applying the same cure conditions
used for mortar casting.
2.3. Microstructure Analysis
All mortar samples were subjected to microstructural analysis after 28 days, in order to verify
the effect of the proportion of OPC in the BA replacement, as well as the influence of reactive silica.
Powdered material (<150 µm) was analyzed with X-ray powder diffraction (X-pert system Philips
Analytical XRay, Almelo, The Netherland). Fragments were used to obtain SEM images (JEOL USA
Inc, Peabbody MA, USA), with the same devices used to the pastes investigation.
Materials2018,11,1829 6of22
Below, Figure 1 summarizes our experimental procedure.

Figure1. Experimentalprocedureschema: (a)mini-slumpflowandself-levellingmortar(spread);

(b)linearshrinkagebartestswithbinarysamples(up)andternarysamples(down);(c)heatcureof
screedsofandcurlingtest.
3. Results
3.1. PrecursorsCharacteristics
Below,Table2summarizestheresultsobtainedforthechemicalandphysicalcharacteristicsof
theprecursorminerals.
Table2.Chemicalandphysicalcharacteristicsoftheprecursormaterials.
TotalOxide
(wt%) SiO2 Al2O3 Fe2O3 CaO K2O SO3 TiO2 LOI1
BA 40.82 37.46 5.71 1.73 5.20 0.29 1.90 6.67
RHA 92.00 ND2 0.17 0.60 1.72 ND2 0.11 3.50
ParticleSize
SpecificGravity BlaineFineness
D10(µm) D50(µm) D90(µm) (g/cm3) (m2/kg)
BA 1.545 8.597 22.733 2.43 715.5
OPC 3.561 15.822 60.120 3.05 500.5
RHA 2.166 10.497 41.208 2.12 1174.0
1Lossonignition;2ND:Notdetected.
TheBAandRHAcontained>40%reactiveSiO andthecontentofvitreousphasewas>50%[6,7].
2
TheaverageparticlesizeswerebelowthediameterofOPC—8.59µmand10.49µmforBAandRHA,
respectively. ThespecificareasoftheRHAandBAasheswerecomparedwithOPC(500.5m2/kg),
withvaluesof1174and715.5(m2/kg),respectively. Figure2showstheX-raydiffractogramofBA
aftertreatmentandtheRHA.RHAashcontainsmainlyamorphousmaterials,withcrystallinephases
ofcristobalite(PDF#39-1425)andquartz(PDF#46-1045). TheRHAparticleshadanirregularshape
andwerelamellarandfibrous,withpresenceofelongatedparticles,duetotheunburnedmaterial,
Materials 2018, 11, x FOR PEER REVIEW 6 of 21
Figure 1. Experimental procedure schema: (a) mini-slump flow and self-levelling mortar (spread); (b)
linear shrinkage bar tests with binary samples (up) and ternary samples (down); (c) heat cure of
screeds of and curling test.
3. Results
3.1. Precursors Characteristics
Below, Table 2 summarizes the results obtained for the chemical and physical characteristics of
the precursor minerals.
Table 2. Chemical and physical characteristics of the precursor materials.
Total Oxide wt (%) SiO2 Al2O3 Fe2O3 CaO K2O SO3 TiO2 LOI 1
BA 40.82 37.46 5.71 1.73 5.20 0.29 1.90 6.67
RHA 92.00 ND 2 0.17 0.60 1.72 ND 2 0.11 3.50
Particle Size
Specific Gravity Blaine Fineness
D10 (µm) D50 (µm) D90 (µm)
(g/cm3) (m2/kg)
BA 1.545 8.597 22.733 2.43 715.5
OPC 3.561 15.822 60.120 3.05 500.5
RHA 2.166 10.497 41.208 2.12 1174.0
1 Loss on ignition; 2 ND: Not detected.
The BA and RHA contained > 40% reactive SiO2 and the content of vitreous phase was > 50%
[6,7]. The average particle sizes were below the diameter of OPC—8.59 µm and 10.49 µm for BA and
RHA, respectively. The specific areas of the RHA and BA ashes were compared with OPC (500.5
m2/Kg), with values of 1174 and 715.5 (m2/kg), respectively. Figure 2 shows the X-ray diffractogram
of BA after treatment and the RHA. RHA ash contains mainly amorphous materials, with crystalline
phases of cristobalite (PDF#39-1425) and quartz (PDF#46-1045). The RHA particles had an irregular
shape and were lamellar and fibrous, with presence of elongated particles, due to the unburned
material, with lengths of around 150 µm (Figure 3). BA mainly contains quartz crystals, mullite, and
Mheatmeriaatlsit2e0,1 w8,1it1h,1 m82o9stly spherical particles and unburnt carbon (loss on ignition (LOI) = 6.67%). 7of22
The grinding of the BA particles promoted a reduction in the internal porosity, the formation of
irregular particles, and the presence of microspheres (Figure 3a). The RHA particles exhibit irregular,
withlengthsofaround150µm(Figure3). BAmainlycontainsquartzcrystals,mullite,andhematite,
lamellar, and fibrous materials related to the presence of unburned material (Figure 3b).
withmostlysphericalparticlesandunburntcarbon(lossonignition(LOI)=6.67%).
C
Q - Quartz
M
Q M - Mullite
H - Hematite
C - Cristobalite
Q
Q
Q
C
Rice Husk Ash
Q M H
M H MQ M
M
M Q Bottom Ash
5 10 15 20 25 30 35 40 45 50 55 60 65 70
2θ (degrees)

Materials 2018, 11, x FOR PEER RFiEgVuIrEeW2 . XRDpatternsofbottomashandricehuskash. 7 of 21
Figure 2. XRD patterns of bottom ash and rice husk ash.

(a) (b) (c)
FFiigguurree 33.. SSiinngguullaarr eelleeccttrroonn mmiiccrroossccooppee ((SSEEMM)) iimmaaggeess:: ((aa)) BBoottttoomm aasshh aafftteerr ggrriinnddiinngg aanndd ccaallcciinnaattiioonn aatt
660000 °◦CC ((××1510500)0;) (;b(b) )RRiciec ehhuusksk asahsh (R(RHHAA) )(×(×50500)0; )(;c)( cR)HRHA A(×(1×50105)0. 0).
FTThIeRg srpinedctirnogscoofpthye wBaAs fpaacriltiitcaletesdp urosmingo taend Aagreildeunct tCioanryin 60th0e ininsttreurnmaelnpto arto sfriteyq,uthenecfioersm baettiwoneeonf
4ir0r0e0g uanladr p40a0rt iccmle−s1,. aFnigdutrhee 4p srhesoewnsc ethoef mFTicIRro srpeshuelrtess f(oFri gthuer em3ian)e.rTahlse (RtrHeaAtepda artnicdl eusnetxrheaibteitdi rBrAeg uanladr,
RlaHmAel)l.a Tr,haen wdifidber obuansdm laotceartieadls aret l3a6te0d0 ttoo t3h2e00p rcemse−n1 cfoero bfoutnhb suarmnepdlems watearsi adlu(eF itgou trhee3 Ob)-.H bond. The
band FcTloIRses ptoe c1t6ro0s0c ocmpy−1w caosrrfeascpiloitnadteedd utosi nthgea nanAgguillaern tdCefaorrym6a0t0ioinn sotrfu tmhee nHt-aOt-fHre qbuoenndc ieins btheetw reaewn
m40a0t0eraianlds. 4T0h0ec mba−n1d. Fpirgeuserent4 asth 1o2w00s tahnedF 8T0I0R crmes−u1 wltsafso druthe etom tihnee rsatrlset(cthreinatge dviabnradtiuonnt roefa atemdoBrpAhaonuds
SRiH-OA-S).i,T whheiwchi dsehobwanedd lao hcaigtehdera cto3n6t0e0ntto to3 2R0H0Acm d−u1e ftoor itbso ttrhansasmmipttleasncwea. sThdeu esmtoaltlh beaOnd-H clboosen dto. T50h0e
cbman−1d foclro BseAt ow1a6s0 d0ucem t−o1 ccroyrsrteasllpinoen dSei-dOt-oSit.h Tehaen bgaunladr adte f5o6r0m–5a5ti0o ncmo−f1 tchaenH b-eO a-Hssobcoiantdedin wtihteh rtahwe
omctaathereidarlso.nT phreebseanntd inp rtehsee nmtualtli1t2e.0 0Thaen dBA80 h0ecamt −tr1eawtmasednut eatt o60t0h e°Cst rinetccrheiansgedv itbhrea tiniotnenosfitaym oof rtphheo Ou-s
HSi -bOa-nSdi, awndhi tchhes pheoawk eadt 1a5h51ig chmer−1c aopnpteenartetdo bReHcaAusdeu teheto stirtosntrgaensts mviibtrtaanticoen. wThase tshme aCll-Ob abnodndcl.o seto
500cm−1forBAwasduetocrystallineSi-O-Si. Thebandat560–550cm−1canbeassociatedwiththe
octahedronpresentinthemullite. TheBAheattreatmentat600◦CincreasedtheintensityoftheO-H
bandandthepeakat1551cm−1appearedbecausethestrongestvibrationwastheC-Obond.
3450 560
1635
800
3450 1635 1090 470
560
800
1635
3450 1090 470
800
1090 470
4000 3500 3000 2500 2000 1500 1000 500
Wavenumber (cm-1)
BA untreated BA treated RHA

Figure 4. Fourier-transform Infrared (FTIR) spectra for precursor minerals: Bottom ash (BA)
untreated, BA treated, after grinding and calcination at 600 ºC (1 h); Rice husk ash (RHA), from
commercial supplier.
3.2. Workability of the Alkali-Activated Mortars
We obtained hybrid alkali-activation mortars with self-leveling properties. All mixtures were
tested with a superplasticizer based on polycarboxylate, except for OPC 10, which was also prepared
with an additive based on naphthalene. There was a trend toward greater flowability for mixes with
lower OPC content. The self-leveling properties were acquired with 1.0–1.4% of superplasticizer.
Materials 2018, 11, x FOR PEER REVIEW 7 of 21

(a) (b) (c)
Figure 3. Singular electron microscope (SEM) images: (a) Bottom ash after grinding and calcination at
600 °C (×1500); (b) Rice husk ash (RHA) (×500); (c) RHA (×1500).
FTIR spectroscopy was facilitated using an Agilent Cary 600 instrument at frequencies between
4000 and 400 cm−1. Figure 4 shows the FTIR results for the minerals (treated and untreated BA and
RHA). The wide band located at 3600 to 3200 cm−1 for both samples was due to the O-H bond. The
band close to 1600 cm−1 corresponded to the angular deformation of the H-O-H bond in the raw
materials. The band present at 1200 and 800 cm−1 was due to the stretching vibration of amorphous
Si-O-Si, which showed a higher content to RHA due to its transmittance. The small band close to 500
cm−1 for BA was due to crystalline Si-O-Si. The band at 560–550 cm−1 can be associated with the
octahedron present in the mullite. The BA heat treatment at 600 °C increased the intensity of the O-
Materials2018,11,1829 8of22
H band and the peak at 1551 cm−1 appeared because the strongest vibration was the C-O bond.
3450 560
1635
800
3450 1635 1090 470
560
800
1635
3450 1090 470
800
1090 470
4000 3500 3000 2500 2000 1500 1000 500
Wavenumber (cm-1)
BA untreated BA treated RHA

Figure 4. Fourier-transform Infrared (FTIR) spectra for precursor minerals: Bottom ash (BA)
untreFaigtuedre, B4.A Ftorueraiteerd-tr,aanfstfeorrmgr iInndfrianrgeda n(FdTIcRa)l csinpaectitoran faotr 6p0r0ec◦uCrso(1r hm);inReriacles: hBuostktoams has(Rh H(BAA)), from
untreated, BA treated, after grinding and calcination at 600 ºC (1 h); Rice husk ash (RHA), from
commercialsupplier.
commercial supplier.
3.2. WorkabilityoftheAlkali-ActivatedMortars
3.2. Workability of the Alkali-Activated Mortars
Weobtainedhybridalkali-activationmortarswithself-levelingproperties. Allmixtureswere
We obtained hybrid alkali-activation mortars with self-leveling properties. All mixtures were
testedwithasuperplasticizerbasedonpolycarboxylate,exceptforOPC10,whichwasalsoprepared
tested with a superplasticizer based on polycarboxylate, except for OPC 10, which was also prepared
withanadditivebasedonnaphthalene. Therewasatrendtowardgreaterflowabilityformixeswith
with an additive based on naphthalene. There was a trend toward greater flowability for mixes with
lower OPC content. The self-leveling properties were acquired with 1.0–1.4% of superplasticizer.
lower OPC content. The self-leveling properties were acquired with 1.0–1.4% of superplasticizer.
OPC5requiredthesamesuperplasticizercontentasOPC30sample,duetotheirlowerworkability.
Self-l evelingpropertieswerealsoobtainedforallternarymixesbyadding1.2%ofadditive(bymass)
tothebinder. Noexudationorsegregationwasobservedinthefinalmini-slumptest.
Mortarsamplesthatreachedaninitialflowvaluewere250mm. After30min,thebinarymix
spreadswere: 280,225,and225forOPC2.5,OPC5andOPC10,respectively. Forternarymixes,the
spreadsafter30minwere225±25mm. Reductionsofmorethan60%,comparedtoinitialflowvalue,
occurredafter60min.
The final setting after heat alkali-activation were 18 to 20 min, a rapid hardening to
self-levelingproduct.
3.3. MechanicalStrengthandDurabilityoftheAlkali-ActivatedMortar
Table3summarizesthestrengthvaluesandphysicalparameters(sorptivity,porosity,andwet
angle)foundatday1andday28.
ThedevelopmentofcompressivestrengthforthedifferentamountsofaddedOPCindicatethat
themeanvaluesdidnotdiffersignificantly. Lowstrengthwasnotedforthesamplescontainingonly
BA,duetothelowdegreeofreactivityobservedintheselectiveacidattackonthepastes. ForOPC0
andOPC2.5,therewasnosignificantdifferenceinthestrengthforlaterages. Whencomparingthe
averagevaluesofOPC2.5andOPC5,nosignificantdifferenceafteronedayofcuringwasrevealed.
Therefore,itwasverifiedthathigherlevelsofcementinfluencedthedevelopmentofthecompressive
strength. Theflexuralstrengthresultsforthebinarysamplesdemonstratedsimilarbehavior,with
valuesfivetimeslowerthanthosewiththecompressivestrength. Therewasadelaytothereactionof
bottomashes(OPC0).
WhenRHAwasusedasreplacementmaterialinthemixOPC10inproportionsof0%,25%,50%
and75%,therewasareductioninthestrength[12]. TheelongatedRHAparticlesindicatealower
Materials2018,11,1829 9of22
degreeofashdissolution. ThereductionintheamountofCaOpresentalsodecreasedthereactivity.
HighercompressivestrengthvaluescanbeobservedwithloweramountsofRHA,indicatingthat
RHAhadlowreactivity. Theflexuralstrengthwaslowerthanthatofthebinariesmixes,duetothe
degreeofreaction.
Table3. Strengthsvaluesandphysicalparametersat1and28daysformixesofordinaryPortland
cement(OPC)andRHA.
Mixes Compressive FlexuralStrengh Sor√ptiviy WetAngle(◦) OpenPorosity E(GPa)
Strengh(MPa) (MPa) (cm min)
Day1 28Days Day1 28Days Day1 28Days Day1 28Days Day1 28Days 28Days
1.92 2.58 1.35 1.60
OPC0 0.0158 0.0159 89.76 89.76 25.30 22.74 —
(1.12) (0.13) (0.20) (0.22)
21.27 21.59 4.61 7.38
OPC2.5 0.0205 0.021 80.84 80.84 20.05 15.22 14.68
(1.64) (2.40) (0.12) (0.29)
22.34 23.22 3.89 7.98
OPC5 0.0361 0.031 76.04 76.04 22.91 14.08 14.78
(1.31) (2.40) (0.18) (0.20)
26.01 28.13 4.86 7.75
OPC10 0.0409 0.0084 88.65 88.65 15.80 15.62 16.05
(1.69) (2.21) (0.29) (0.49)
28.00 33.46 4.67 7.53
OPC30 0.0445 0.0215 85.28 85.28 15.89 12.17 18.73
(1.10) (2.08) (0.49) (0.23)
11.46 20.14 3.47 4.05
RHA25 0.0363 0.0306 89.95 89.92 12.21 10.9 13.91
(0.68) (2.11) (0.21) (0.29)
6.13 14.30 2.56 3.60
RHA50 0.0471 0.0197 89.91 89.97 13.20 12.22 12.62
(0.25) (1.52) (0.12) (0.11)
5.76 12.74 2.48 3.26
RHA75 0.0369 0.0149 89.82 89.98 14.62 12.93 10.51
(0.24) (0.62) (0.13) (0.32)
Itwasfoundthatatthebinarymixesdynamicmodulusat28days,highervalueswereobtained
byincreasingOPCcontentinthemixes; forternarymixes,thevaluesdecreasedwithhigherRHA
content. Dynamicmodulusofelasticitywas10.5to18.5GPa.
Forallmixes,theinitialsorptivityafterday1washigherthanthatobservedafter28days. Withan
increaseintheOPCcontentinthebinarymixesafteralkaline-activation,anincreaseintheabsorption
canbeobservedafterday1ofaging,whichwasassociatedwithincreasedporosity. After28days,
weobservedthatforthelowerlevelsofBAreplacementinOPC(OPC2.5andOPC5samples)there
washighersorptivitywhencomparedwithOPC10andOPC30. Thiswasduetotheformationof
(N,C)-A-S-H gels in the activations when a higher OPC content was present, thus resulting in the
fillingofthepores.
In addition, we observed that all ternary mixes exhibited higher sorptivity when compared
withthereferencesample. Similarbehaviorwasevidentafter1and28dayswithareductioninthe
sorptivity. RHA75showedlowsorptivity,primarilyduetotheinertnatureoftheRHA,whichshowed
lowerreactivityandfilledthecapillarypores.
Everysamplehadahigheropenporosityafterday1, comparedwiththeresultafter28days.
AnincreaseintheOPCcontentsresultedinloweropenporosityandabsorption,duetoanincrease
inthe(N,C)-A-S-Hgelformation. Inaddition,theternarymixesshowedloweropenporosityand
absorption,whichweattributedtothefillereffectduetopresenceofRHA.
Durability
Figure5showsthatthemasslosswashigherforthebinarymixescomparedwiththeternary. This
isduetotheattackontheOPC,the(N,C)-A-S-Hgel,andthealkalineactivationBAgel(N-A-S-H).Itcan
benotedthatthemixescontaining2.5%and5%ofOPCshowedasignificantdegreeofdegradation,
indicatingthatOPCcontentaffectedtheresistanceoftheacidattack. InthemortarscontainingRHA,
thedegreeofdegradationwaslower. Comparatively,theresultsshowthatthePortlandcementmortar
(PCM),withthesamew/cratio,degradedmoreaggressivelythantheAAMs. FortheAAM,nosigns
ofsurfacedeteriorationwerefoundinthevisualinspectionoftheprisms. TheOPCspecimenswere
severelydeterioratedafterthe28cycles.
Materials 2018, 11, x FOR PEER REVIEW 9 of 21
For all mixes, the initial sorptivity after day 1 was higher than that observed after 28 days. With
an increase in the OPC content in the binary mixes after alkaline-activation, an increase in the
absorption can be observed after day 1 of aging, which was associated with increased porosity. After
28 days, we observed that for the lower levels of BA replacement in OPC (OPC 2.5 and OPC 5
samples) there was higher sorptivity when compared with OPC 10 and OPC 30. This was due to the
formation of (N,C)-A-S-H gels in the activations when a higher OPC content was present, thus
resulting in the filling of the pores.
In addition, we observed that all ternary mixes exhibited higher sorptivity when compared with
the reference sample. Similar behavior was evident after 1 and 28 days with a reduction in the
sorptivity. RHA 75 showed low sorptivity, primarily due to the inert nature of the RHA, which
showed lower reactivity and filled the capillary pores.
Every sample had a higher open porosity after day 1, compared with the result after 28 days. An
increase in the OPC contents resulted in lower open porosity and absorption, due to an increase in
the (N,C)-A-S-H gel formation. In addition, the ternary mixes showed lower open porosity and
absorption, which we attributed to the filler effect due to presence of RHA.
Durability
Figure 5 shows that the mass loss was higher for the binary mixes compared with the ternary.
This is due to the attack on the OPC, the (N,C)-A-S-H gel, and the alkaline activation BA gel (N-A-S-
H). It can be noted that the mixes containing 2.5% and 5% of OPC showed a significant degree of
degradation, indicating that OPC content affected the resistance of the acid attack. In the mortars
containing RHA, the degree of degradation was lower. Comparatively, the results show that the
Portland cement mortar (PCM), with the same w/c ratio, degraded more aggressively than the AAMs.
MFaoterr itahlse2 A01A8,M11,, 1n8o29 signs of surface deterioration were found in the visual inspection of the prism10s.o Tf2h2e
OPC specimens were severely deteriorated after the 28 cycles.
14 14
a) b)
12 12
%)10 %)10
( (
ss 8 ss 8
o o
s L 6 s L 6
s s
a 4 a 4
M M
2 2
0 0
0 5 10 15 20 25 30 0 5 10 15 20 25 30
Time (days) Time (days)
PCM OPC2.5 OPC5 OPC10 OPC30 PCM OPC2.5 OPC5 OPC10 OPC30
c)14 d)14
12 12
%)10 %)10
ss ( 8 ss ( 8
o o
s L 6 s L 6
s s
a 4 a 4
M M
2 2
0 0
0 5 10 15 20 25 30 0 5 10 15 20 25 30
Time (days) Time (days)
RHA0 RHA25 RHA50 RHA75 RHA0 RHA25 RHA50 RHA75
FFiigguurree5 5.. MMaassss lloossss ooff bbiinnaarryy aanndd mmoorrttaarrssd duurriinngga acciidda attttaacckk:: ((aa)) BBiinnaarryy aafftteerr HHAAcc eexxppoossuurree;;( (bb))B Biinnaarryy
aafftteerrH HCClle exxppoossuurree;;( c(c))T Terenrnaaryrya aftfetrerH HAAcce xepxpoosusurer;e(; d(d))T eTrenranrayrya fatfetreHr HCClel xepxopsousruer.e.
FFiigguurree6 6s shhoowwsst thheer reessuultltssa afftteerrt thheeH HAAcca annddH HCClle exxppoossuurree.. TThheefl fleexxuurraall ssttrreennggtthh wwaass ssttrroonnggllyy
aafffefeccteteddb ybyth tehaec iadciadtt aactkta,cdku, edtuoet hteo fothrme afotiromnaotfiocnra cokf scirnatchkes mino rtthaer, masosrhtoawr, nasin sFhiogwurne 6ina .FHigouwreev 6ear,.
fHoroawlleavlekra,l finoer -aalcl taivlkaatelidnes-aamctpivleast,endo ssaimgnpilfiecsa, tnivoe sicgonmifpicraetsisvive ecostmrepnrgetshsilvoes sstwreansgotbhs leorsvse wda(Fs iogbusreer6vbe)d.
Materials 2018, 11, x FOR PEER REVIEW 10 of 21
(Figure 6b).
100 92 90 90 96 89
81 82 83
80 67 73 67 67 76
62 63
%) 60 52
k ( 40
20
0
PCM OPC 2.5 OPC 5 OPC 10/ OPC 30 RHA 25 RHA 50 RHA 75
RHA 0
Sample
HAc attack HCl attack

(a)
100 99 95 99 99 98 98 99 98
86
90
77
80
68
70
60
%) 50
k ( 40
30
20
10
0
PCM OPC 2.5 OPC 5 OPC 10/ OPC 30 RHA 25 RHA 50 RHA 75
RHA 0
Sample
HAc Attack HCl Attack
(b)
FiguFriegu6r.e R6e. sRisetsaisntacnecteo toa caidcida tatattcakck:: (a(a)) FFlleexxuurraall ssttrreennggtthh; ;(b(b) )CComomprpersesisvseiv setrsetnrgetnhg. tKh .vKaluvea l(u%e): ( %):
K=K(( R= e(s(Risetsainstcaenacet 2at8 2d8a dyasy−s-RReessisitsatanncec eafateftre cryccylec)l/eR)e/sRisetasnisctea natc e28a dta2y8sd) ×a y1s0)0.× 100.
The results of the wetting and drying tests carried out on the mortars are shown in Table 4. It
can be observed that 28 cycles of wetting and drying did not cause degradation of the PCM samples.
The alkali-activation had an influence on the flexural strength, mainly in the case of binary mixes.
Only the alkali-activated sample containing the highest percentage of Portland cement showed lower
results after wetting and drying cycles.
Table 4. Rate of degradation of binary and ternary mortars of alkali-activated mortars.
Sample Kf Remark kc Remark
PCM −0.016 No effect −0.149 No effect
OPC 2.5 0.333 Influence −0.107 No effect
OPC 5 0.431 Influence −0.436 No effect
OPC 10 0.249 Influence −0.056 No effect
OPC 30 0.375 Influence 0.095 Influence
RHA 25 0.170 Influence −0.125 No effect
RHA 50 0.033 Influence −0.011 No effect
RHA 75 0.002 Influence −0.033 No effect
K value: K = ((Resistance at 28 days-Resistance after cycle)/Resistance at 28 days).
3.4. Dimensional Changes
3.4.1. Linear Shrinkage
The results for linear shrinkage variation are shown in in Figure 7. We found that binary samples
had a lower linear shrinkage than ternary samples. In binary mixes, high contents of OPC raised the
linear shrinkage, due to the amount of Portland paste, which increased gel formations. RHA was