OPEN ACCESS Materials
Materials 2015, 8, 5847-5861; doi:10.3390/ma8095277OPEN ACCESSmaterialsISSN ation Characteristics of Low-Heat Cement Substituted byFly Ash and Limestone PowderSi-Jun Kim 1 , Keun-Hyeok Yang 1, * and Gyu-Don Moon 21Department of Plant Architectural Engineering, Kyonggi University, Suwon, Kyonggi-do 16227,Korea; E-Mail: season@kgu.ac.kr2Department of Architectural Engineering, Kyonggi University, Graduate School, Suwon,Kyonggi-do 16227, Korea; E-Mail: mgd0123@kcl.re.kr* Author to whom correspondence should be addressed; E-Mail: yangkh@kgu.ac.kr;Tel.: 82-31-249-9703; Fax: 82-31-244-6300.Academic Editor: Jorge de BritoReceived: 5 July 2015 / Accepted: 24 August 2015 / Published: 1 September 2015Abstract: This study proposed a new binder as an alternative to conventional cement toreduce the heat of hydration in mass concrete elements. As a main cementitious material,low-heat cement (LHC) was considered, and then fly ash (FA), modified FA (MFA) byvibrator mill, and limestone powder (LP) were used as a partial replacement of LHC. Theaddition of FA delayed the induction period at the hydration heat curve and the maximumheat flow value (qmax ) increased compared with the LHC based binder. As the proportionand fineness of the FA increased, the induction period of the hydration heat curve wasextended, and the qmax increased. The hydration production of Ca(OH)2 was independentof the addition of FA or MFA up to an age of 7 days, beyond which the amount of Ca(OH)2gradually decreased owing to their pozzolanic reaction. In the case of LP being used as asupplementary cementitious material, the induction period of the hydration heat curve wasreduced by comparison with the case of LHC based binder, and monocarboaluminate wasobserved as a hydration product. The average pore size measured at an age of 28 days wassmaller for LHC with FA or MFA than for 100% LHC.Keywords: low-heat cement; fly ash; limestone powder; heat of hydration; pore size
Materials 2015, 858481. IntroductionCement, which is widely used as a construction material, forms hydrates such as calcium silicatehydrate (C–S–H), ettringite, and Ca(OH)2 through a hydration reaction in which hydration heat isproduced within the concrete because of an exothermic reaction. Since the thermal cracking of concretereduces its internal force, watertightness, and durability, an appropriate measure is required to controlthe heat of hydration. The factors that influence the hydration heat of concrete include the placementtemperature of the concrete and the hydration heat characteristics of the cement (hydration heat ofcement, hydration reaction rate of cement, and unit cement amount).In fields involving the construction of mass concrete structures, a pre-cooling method, which lowersthe temperature of the aggregate and mixing water, and a post-cooling method, which employs coolingpipes, is currently used to control the hydration heat of concrete. With respect to the material, efforts arealso being directed toward lowering the hydration heat of concrete. A typical method involves reducingthe use of cement by substituting ordinary Portland cement (OPC) with a mineral admixture, such as flyash (FA) or ground-granulated blast-furnace slag (GGBS). Because of its reduced OPC usage, concretethat uses a mineral admixture as a supplementary cementitious material (SCM) for cement exhibits notonly a low hydration heat but also a reduction in its greenhouse-gas emissions [1–4]. Thus, variousstudies have been conducted on blended cement that uses a mineral admixture as SCM for cement(binary or ternary blended cement). However, carbonation and low early strength have been identified asproblems for blended cement with a large amount of mineral admixture. In addition to the above methodwhich involves the substitution of the cement with a mineral admixture such as FA or GGBS, low-heatcement (LHC), which has a relatively low hydration heat compared with OPC, can be also used to reducethe hydration heat of the concrete. The hydration heats of the cement components, tricalcium silicate(C3 S), dicalcium silicate (C2 S), tricalcium aluminate (C3 A) and tetracalcium aluminoferrite (C4 AF) are510, 247, 1356 and 427 J/g, respectively, and the final hydration heat of the cement is directly affectedby the proportion of these components [5]. According to ASTM C 150 [6], the hydration heat of LHC(type IV) is defined as 250 J/g or less, and the C3 S, C2 S and C3 A contents are defined as less thanor equal to 35%, 40% and 7%, respectively. According to European standards EN 197-1 [7] and EN14,216 [8], LHCs are categorized into low-heat common cement and very-low-heat special cement, andtheir hydration heats are defined to be less than or equal to 270 and 220 J/g, respectively. Studies havebeen conducted on the hydration reaction and strength-development properties of LHC, but these arescarce compared with the studies on blended cement [9,10]. The emergence of mega-concrete structureshas generated demand for ultra-low-heat cement, which has a lower hydration heat than conventionalblended cement and LHC [9]. Low-heat blended cement, which employs a mineral admixture such asFA in place of LHC, has a lower hydration heat than OPC, and has also attracted much attention [11].This study aims to analyze the hydration rate, heat of hydration, hydration products, and porecharacteristics of binders that use FA and limestone powder (LP) as supplementary cementitiousmaterials of LHC.
Materials 2015, 858492. Materials and MethodsMaterials 2015, 82.1. Materials32.MaterialsandTable1 showstheMethodschemical and Bogue composition of the LHC, FA and LP by ASTM C 114 [12]. TheC3 S, C2 S, C3 A and C4 AF contents of the LHC are 31%, 48%, 3% and 11%, respectively. The C3 S and2.1. MaterialsC3 A contents are lower than the OPC content, whereas the C2 S and C4 AF contents are higher. The CaOand (SiOFechemicalof the compositionFA are 3.8%ofandrespectively,2 1Al2 O3 the2 O3 ) contentsTableshowsand Boguethe 91.4%,LHC, FAand LP by whichASTMcorrespondsC 114 [12]. to“Class[13].Table of2 showsphysicalcharacteristicsspecific gravityC2ASTMS, C3A Cand618C4AFcontentsthe LHCare 31%,48%, 3% andwhich11%, arerespectively.The C3SandThe CC”3S,ofandofCthelowerthanthe OPCcontent,whereastheC C204AF contentsarea higher.blainLHC, FA,areandLP byASTMC 188[14] andASTM[15].C4Figure1 showsscanning3A contents2S andThe CaOand (SiO(SEM)O3 FeFA arewhich3.8% isandrespectively,which2 Al2image3) MFA),FA91.4%,that hasbeen alteredusingcorrespondsto The“Classof ASTMC 618 [13].Table the2 showsphysicalwhicharea vibratorymill.FAC”particlesare spherical,whereasparticlesof thecharacteristicsMFA are mostlyirregularspecific gravity and blain of the LHC, FA, and LP by ASTM C 188 [14] and ASTM C 204 [15].in shape.Figure 1 shows a scanning electron microscope (SEM) image of modified fly ash (MFA), which is FATable1. usingChemicaland Bogueof rawweight,that has beenaltereda vibratorymill. compositionThe FA particlesare materialsspherical,(bywhereasthe%).particles of theMFA are mostly irregular in shape.MaterialsLHCMaterialsFALHCLPFALPChemical CompositionBogue Compositionand Boguecompositionof rawmaterials%).SiOTableO3ChemicalFe2 O3 CaOMgO KNa2 O TiOLOI(by* weight,C3 S C2 Al21.2O2 SO32SC3 AC4 AF25.33.13.462.5Chemical1.7 Composition0.57 0.10 0.091.90.831 Bogue48 Composition311SiOFeCaO 1.2MgO 1.1K2O 1.0Na2O 1.5TiO2 -SO3 2.23LOI *2O364.02 Al21.95.52O3 3.8-C3S -C2S C-3A C4-AF25.33.13.4 47.562.5 2.11.70.81117.78.20.6-0.57 -0.10 - 0.09 0.31.9 22.3- 31- 48-364.0 LHC:21.9 low-heat5.5 cement;3.8 FA:1.2fly ash;1.1LP: limestone1.01.5- * Loss2.23on ignition.powder;17.78.20.647.52.10.322.3LHC: low-heatFA: flycharacteristicsash; LP: limestoneofpowder;* Loss on ignition.Table cement;2. Physicalraw materials.MaterialsSpecificGravity(cm2 /g)Table2. PhysicalcharacteristicsofBlaineraw materials.LHC3.18Gravity .81MFA: modifiedfly ash. 3420MFA: modified fly ash.(a)(b)Figure1. Scanningelectronmicroscope(SEM)imagesimagesof (a) flymodifiedFigure1. Scanningelectronmicroscope(SEM)of ash(a) (FA)fly andash ed FA (MFA) with particle size of 20 μm.
Materials 2015, 858502.2. Preparation of SpecimensAs binders, LHC and supplementary cementitious materials which are FA, LP, and a mixture of FAand LP were used. There were nine types of binder, including LH100, which employed 100% LHC. Thebinder proportions are presented in Table 3.To analyze the effect of the fineness and amount of FA substitute on the hydration of the LHC, 20 wt %of the LHC was replaced by FA (LH80FA20), and 10, 20 and 30 wt % of the LHC was replaced byMFA (LH90MFA10, LH80MFA20 and LH70MFA30). The LP replaced 5, 10 and 15 wt % of the LHC(LH95LP5, LH90LP10 and LH85LP5). Additionally, a mixture of 15% FA and 5% LP was used as abinder (LH80MFA15LP5) to replace 20% of the LHC.Table 3. Binder proportions (by weight, tes were used as specimens for the thermogravimetric analysis (TGA), X-ray diffraction (XRD),and mercury intrusion porosimetry (MIP) measurements. When fabricating the pastes, the w/b(water/binder ratio) was set as 0.5. Following their formation, the pastes were cured in a chamber ata constant temperature and humidity of 23 C and 95% relative humidity (RH), respectively, for thedesired aging time, and subsequently dipped in isopropanol. All the specimens were dried in a vacuumdryer at a temperature of 40 C for one day before the measurements were performed. The pastes wereformed into a powder to use as specimens for the TGA and XRD measurements.2.3. Test MethodsUsing an isothermal calorimeter (TAM Air, TA instruments, New Castle, DE, USA), the hydrationheat characteristics (heat flow and heat of hydration) of the binders were measured by ASTM C1702 [16]. Distilled water was used as a reference substance. To fabricate the paste required for theisothermal calorimetry measurement, the w/b ratio was set at 0.5. Subsequently, 5 g of paste was placedinto a 20 mL glass sample vial, which was inserted into the isothermal calorimeter. The temperaturewas set at 23 C, and the measurement was performed continuously for 3 days. To calculate the boundwater and Ca(OH)2 contents of the binder with respect to the aging time, a TGA device (Thermo plusEVO2, Rigaku, Tokyo, Japan) was used. The reduction in weight was measured for the specimens thathad been aged for 3, 7 and 28 days. The temperature range and heating conditions used for the TGA
Materials 2015, 85851were 40–1000 C and 10 C/min, respectively. The bound-water and Ca(OH)2 contents were calculatedaccording to the specimen weights at each temperature; these were determined using the TGA resultsand the following equations [17,18]:Bound water pg/gq “W40 W480W480(1)W400 W480 74ˆ(2)W48018where Wn is the dry sample weight at a temperature of n C.XRD (Miniflex 600, Rigaku, Tokyo, Japan) was employed to analyze the hydration product of eachbinder. Dry powder passed through a 75-µm sieve was used as the XRD specimen. Step scanningwas conducted from 5 –65 , using a step interval of 0.02 and a scan speed of 1 /min. A pore-sizedistribution measurement was performed on the specimens that had been aged for 3, 7 and 28 days usingporosimetry equipment (Autopore IV 9500, Micrometrics, Norcross, GA, USA). The surface tension andcontact angle of the mercury used in the MIP measurements were 485 dynes/cm and 130 , respectively.Ca pOHq2 pg/gq “3. Results and discussion3.1. Isothermal CalorimetryThe heat flow curve of the LH100 specimen, consisting of 100% LHC, exhibits a shape similar to thatof OPC (Figure 2). Between approximately 1 and 2.5 h after the initiation of the hydration, an inductionperiod was observed for the LH100 specimen. Following this induction period, the acceleration periodcommenced, in which hydration occurred; the maximum heat flow value (qmax ) of the LH100 specimenwas 2.30 mW/g.On reaching qmax , the heat flow of the LH100 binder entered the deceleration period and subsequentlydecreased continuously. During the deceleration period, the amount of the calcium sulfate phasedecreased, and a “shoulder” relating to the secondary aluminate reaction was observed [19]. The heatflows of the LH80FA20 and LH80MFA20 binders, in which 20 wt % of the LHC was substituted byFA and MFA of various levels of fineness, respectively, are shown in Figure 3. The LH80FA20 andLH80MFA20 binders had longer induction times than the LH100 binder; they also took a longer timeto reach qmax and for the “shoulder” to appear. When FA was used as SCM, the delay phenomenon ofearly hydration (prior to 24 h) was exhibited because there was a decrease in the calcium concentrationof the pore solution owing to the reaction of the aluminate with the calcium and FA in the pore solution;thus, the nucleation of the C–S–H was delayed [18]. However, the qmax values of the LH80FA20 andLH80MFA20 binders were higher than that of the LH100 binder, and following the appearance ofthe “shoulder”, the heat flow values were continuously higher than that of the LH100 specimen. Thehydration of the cement was accelerated when FA was used as SCM because of the filler effect. WhenFA is used to replace cement, it fills the gaps between the cement particles. This yields additionalnucleation sites, and because the effective ratio of water to cement increases, the hydration of the cementaccelerates [20]. As shown in Figure 3, the qmax of the LH80MFA20 specimen (in which 20% MFAwas used as SCM, which had a higher fineness than the FA, while having the same substitution ratio)was greater than that of the LH80FA20 specimen (in which FA with a low-fineness was used as SCM).
Materials 2015, 8Materials 2015, 858526Theincreasedfinenessof theincreasedfinenessprovidedvalueincreasedas FAthe increasedfineness ofbecausethe FA theincreasedbecausethe increasedwasqmaxusedvalueas SCM).The qasmaxthemorenucleationsites,yieldinga largerfilleryieldingeffect andthe hydrationreactionalsohydrationaccelerateddue toalsothefinenessprovidedmorenucleationsites,a largerfiller effectand ].acceleratedto thenucleationand filler effect [17,20–22].Figure 2. Normalized heat flow during initial 24 h of hydration (LH100).Figure 2. Normalized heat flow during initial 24 h of hydration at flowsflowsofofLH80FA20LH80FA20andand especimensspecimensininwhichwhichthethe LHCLHC waswas substitutedsubstituted byby 10%,10%, 20%20% andTheand 30%30% )LH70MFA30)areareshownshownininFigureFigure4.4. AsAs the(LH90MFA10,the proportionproportion ofof od increased,increased, andand thereforforthethe“shoulder”toincreased,there waswas r”thetheincreasein theof FA,of Ca ofin theof cementtoappear.appear.WithWithincreasein amountthe amountof theFA,concentrationthe concentrationCa porein thesolutionpore –Hnuclei[23].However,astheproportionofMFAcement decreases, which delays the formation of C–S–H nuclei [23]. However, as the proportion ofincreased,the hydrationof theaccelerated;thus,thus,between17 and48 h48afterthe appearanceof theMFAincreased,the hydrationof LHCthe LHCaccelerated;between17 andh afterthe eFAFA increased,increased, thethe ct increased, and consequently, there was an acceleration in the hydration of the cement. When LP15),thesampledemonstratedwas used to replace the LHC (LH95LP5, LH90LP10 and LH85LP15), the sample demonstrated differentdifferentcharacteristicsheat-flow characteristicsto those whenobservedwhenusedFA aswasSCMused(Figureas SCM5).(Figure5). Comparedheat-flowto those observedFA wasComparedwith owvalueswereneitherincreasednorLH100 binder, when LP was used as SCM, the heat flow values were neither increased nor delayed afterdelayed after reaching q ; the plotted results demonstrated curves almost identical to that of LH100.reaching qmax ; the plottedmaxresults demonstrated curves almost identical to that of LH100.
Materials 2015, 8Materials 2015, 858537FigureFigure 4.4. NormalizedNormalized heatheat flowsflows ofof MFA30.LH70MFA30.Figure 5. Normalized heat flows of LH95LP5, LH90LP10, and LH85LP15.Figure 5. Normalized heat flows of LH95LP5, LH90LP10, and nder,theretherewaswasanmax andanincreaseincreaseininthetheqqmaxand aa reductionreduction inin thethe as SCM.The howed thatthe inductionFAwaswasusedusedas SCM.The howedthat theperiodof cementto betendsshortenedthe increaseinincreasethe amountof amountlimestoneThismayinductionperiod tendsof cementto be withshortenedwith thein theof powder.limestonepowder.beattributedthe fact thattotheC–S–HnucleiofcanC–S–Hbe acceleratedby theabsorptionThismay beto attributedtheformationfact thatoftheformationnuclei canbe physicalacceleratedby theofCaCO3 .absorptionBy examiningthe hydrationheat characteristicsaccordingto the amount accordingof LP usedtoasthea3. By examiningthe hydrationheat characteristicsphysicalof CaCOsubstitute,decreasedaccordingto thedecreasedincrementaccordingof LP to LHC(5%, 10%andtoamount oftheLPinductionused as aperiodsubstitute,the inductionperiodto theratioincrementof LPincreasedcompared with those of LH100.LHC andratiothe(5%,and 15%)and thewithqmaxthose15%)qmax10%increasedcomparedof LH100.Figure 66 showsshows thethe heatFigureheat flowflow measurementmeasurement resultsresults men,a abinderbinderinof thespecimenwaswhicha mixtureof ofFAFAandandLP LPwaswasusedas SCM.TheTheq maxqmaxinwhicha mixtureusedas SCM.of LH80MFA15LP5the LH80MFA15LP5specimensimilarto thatof theLH80MFA20specimen,for thethe LHC.LHC.wassimilarto thatof theLH80MFA20specimen,whichwhichusedused2020wtwt%% MFAMFA asas SCMSCM toreachreachqqmaxmax, ,andHowever,the 20binder.binder. WhenWhen thethe LHCLHC waswasthesubstitutedbybya tappearedbe moresubstitutedandandLP,LP,the thehydration-accelerationeffectappearedto betomorerapid rapidthanthanwhenthat whenthe wasLHCreplacedwas replacedFA alone.thatthe LHCby FAbyalone.
Materials 2015, 8Materials 2015, 858548Figure 6. Normalized heat flow of LH80MFA15LP5.Figure 6. Normalized heat flow of LH80MFA15LP5.Tableheat ofof 1-day1-day andand 3-day3-day cumulative eatheatofofhydrationhydration increasedincreased as the agingthethespecimens,agingtimetimeincreased.i
hydrate (C–S–H), ettringite, and Ca(OH) 2 through a hydration reaction in which hydration heat is produced within the concrete because of an exothermic reaction. Since the thermal cracking of concrete reduces its internal force, watertightness, and durability, an appropriate measure is required to control the heat of hydration. The factors that influence the hydration heat of concrete .
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