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Comparing the performance ofdifferent commercial clays in fly ash-modified mortarsNishant Garg a & Kejin Wang aa Department of Civil, Construction, and EnvironmentalEngineering, Iowa State University, Ames, IA, USAVersion of record first published: 27 Nov 2012.

To cite this article: Nishant Garg & Kejin Wang (2012): Comparing the performance of differentcommercial clays in fly ash-modified mortars, Journal of Sustainable Cement-Based Materials, 1:3,111-125

To link to this article: http://dx.doi.org/10.1080/21650373.2012.745217

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Comparing the performance of different commercial clays in flyash-modified mortars

Nishant Garg* and Kejin Wang

Department of Civil, Construction, and Environmental Engineering, Iowa State University, Ames,IA, USA

(Received 29 September 2012; final version received 18 October 2012; accepted 29 October2012)

Three different commercially available clays (Actigel, Concresol, and Metamax)were compared based on their performance and effect on fly ash-modified mortarsprepared with a consistent w/b of 0.45 and s/b of 2.75. Three fly ashes (FA1–3)from different sources were selected to evaluate the performance of these three clays.Ternary mortar blends were formed at various addition and replacement levels ofthese clays to study their performance in terms of the heat of hydration, thermal set-ting time, air content, flow ability, and compressive strength development up to90 days. Results indicated that slight addition of Actigel or Palygorskite significantlyaffects the rheology as well as decreases the setting time of fresh mortar, the claybeing able to alter the fresh state properties to a great extent. All the clays acceler-ated the cement hydration process as well as decreased the flow ability of mortars.Metamax or metakaolin was found to be the most reactive clay among the threeclays in terms of its pozzolanic behavior.

Keywords: mineral clays; fly ash; ternary blended mortars; Palygorskite; metakaolin

1. Introduction

The increasing concern for CO2 emis-sions from cement production has shiftedthe focus of current cement and concreteresearch toward a sustainable approach.Addition or replacement of cement withsupplementary cementitious materials(SCMs) is the most common approachemployed today for achieving sustainableconcretes with the desired fresh, mechani-cal, and durability properties. However,as the replacement level of fly ash (FA)in the binder increases, the early agestrength of concrete begins to decline [1].Hence, many researchers have designedternary blends [2,3] and even quaternaryblends [4] to minimize the quantity of

cement and optimize the usage of SCMsin a given binder system.

Addition of clays to cement systemswas initially considered to be deleteriousand often resulted in decreased perfor-mance [5]. But with growing andimproved mineral processing technolo-gies, various commercial clays that arebeneficial to concrete are now availablein the market. Various naturally occurringas well as processed fine clays now serveas potential mineral additions in cement-based systems to improve their overallperformance. A recent study by Lind-green et al. [6] evaluates the effect ofsuch clays or sheet silicates on the micro-structure and porosity of cement-based

*Corresponding author. Email: ngarg@chem.au.dk

Journal of Sustainable Cement-Based MaterialsVol. 1, No. 3, September 2012, 111–125

ISSN 2165-0373 print/ISSN 2165-0381 online� 2012 Taylor & Francishttp://dx.doi.org/10.1080/21650373.2012.745217http://www.tandfonline.com

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systems. Clays are often found toimprove the fresh concrete properties, asthey have the potential to improve thegreen strength of concrete [7] and havethe ability to improve the rheology ofcement-based materials [8], and oftenpossess pozzolanic properties [9].

In this study, three different commer-cial clays namely, Actigel, Concresol,and Metamax, were evaluated to observetheir effect on fly ash-modified mortars.While Actigel and Concresol can be clas-sified together as recently developedcommercial clays primarily for rheologi-cal purposes, metakaolin is different inthe sense that its effectiveness in applica-tion as a reactive SCM is already estab-lished [10]. A few informative reviewarticles [11,12] demonstrate the extensivework that has been done on metakaolinin the past two to three decades but theformer two recent clays (Actigel andConcresol) in the market have not beensystematically investigated. Their grow-ing application in the self-consolidatingconcrete industry [13,14] requires furtherexamination which is undertaken in thisstudy.

This study is focused on comparingthe performance of the above-mentionedclays by measuring various propertieslike the heat of hydration, air content,flow ability, and compressive strength ofmortar mixtures containing three differenttypes of FAs. Various ternary blendscomprising of cement, FA, and clay weredesigned to evaluate the performance ofselected clays in such ternary systems. Asecondary aim was to determine if theseclays would be able to compensate thestrength loss induced from the high levelof FA replacement. First, Actigel’s rheo-logical effect was studied in detail bytesting the effect of its addition at lowpercentages (0.5 and 1.0%) in the threedifferent and diverse FAs. Second, all thethree clays at low addition levels (1.5and 3.0%) were compared to each otherin mortar blends. Third, Metakaolin alone

was chosen to experiment further athigher replacement levels (5, 10, and15%) based on its recommended dosagefor an effective pozzolanic behavior.

2. Experimental details

2.1. Materials

2.1.1. Ordinary Portland cement

An ASTM Type I/II ordinary Portlandcement (OPC) was used whose chemicalcomposition is given in Table 1. TheOPC used in this study was the finestbinder among all the SCMs employedwith only 6.55% binder by mass retainedon a 45 μm sieve. Figure 1 shows theparticle size distribution of OPC in com-parison with the three FAs used.

2.1.2. FAs

Three FA from different sources wereused, and their chemical composition isgiven in Table 1. FA1 has the highestalkali content (2.80%), the highestSiO2 +Al2O3 + Fe2O3 content (85.9%),and the lowest calcium content (1.46%).FA2 has the highest LOI (11.58%) whichcorresponds to its highest carbon content(10.50%) in the series. FA3 has a highamount of iron content (18.2%) but otherthan that there is no other major differ-ence between the FAs except that theyrepresent a wide spectrum in the diversityof available FAs. In terms of particlefineness, FA3 is the finest FA in the ser-ies, as seen in Figure 1.

2.1.3. Clays

Three commercially available clays orsheet silicates – Actigel, Concresol, andMetamax were obtained for comparison.Their physical properties and chemicalproperties are shown in Tables 2 and 3,respectively. Actigel is basically anextracted and refined version of theclay Palygorskite which is a hydrous

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magnesium aluminum phyllosilicate or(Mg,Al)2SiO4(OH)·4H2O. It has a uniquemorphology as its fine particles are spher-ical in nature from which very fine nee-dles appear to be extruding, as seen by

Scanning Electron Microscopy (SEM)analysis previously [15]. That is why it isalso considered as a naturally occurringnanoclay with its bristles having diameterof only a few nanometers. The secondclay, Concresol, is a ground clay blend ofvarious plastic Westerwald clays fromGermany used as a fine filler for mortarand concrete with CE certificationaccording to DIN EN 13139, 12620, and1097-7. Hans-Werner and Harald [16]utilized this clay additive for theenhancement of no-slump concrete prop-erties and found that it improves freshgreen strength as well as later agecompressive strength. The third clay,Metamax or high-reactivity metakaolin(HRM), can be defined as the poorlycrystalline transition phase obtained afterkaolinite is calcined at a high temperature

Table 1. Chemical composition of OPC and FAs.

Compound (%) Cement FA1 FA2 FA3

SiO2 20.2 53.5 48.3 46.0Al2O3 4.7 25.2 19.3 17.8Fe2O3 3.3 7.2 4.99 18.2SO3 3.3 0.32 0.75 2.59CaO 62.9 1.46 8.14 8.40MgO 2.7 1.43 2.26 0.95Na2O – 0.49 1.21 0.59K2O – 3.51 1.15 2.16Eq. Na2O 0.54 2.80 1.79 2.01LOI 1.1 4.87 11.58 1.49Carbon – 5.30 10.50 0.55% Retained on #325 sieve 6.55 20.15 20.95 13.25

Figure 1. Particle size distribution of OPCand FAs.

Table 2. Physical properties of clays [18–20].

S. no.Commercialname of clay

Averageparticlesizerange(μm)

Specificsurfacearea(m2/g)

Specificgravity Mineralogical designation

1. Acti-Gel 208 0.5–30.5 150 2.62 Palygorskite or purified hydrousmagnesium aluminumphyllosilicate

2. Concresol 105 1.8–146 23 2.72 Kaolinite (45%), illite (20%) andsilica (35%)

3. MetaMax HRM 1.8–294 13 2.6 Purified calcined kaolinite

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range of 700–800 °C [17] resulting in afine material that conforms to the ASTMC 618, Class N (natural) pozzolan speci-fications.

Figure 2 shows the particle size distri-bution (PSD) of the three clays. Based onthe PSD, Metamax is the finest of thethree clays followed by Concresol, andthen Actigel. However, it must be notedthat the given average particle size rangeor PSD for Actigel or Palygorskite corre-sponds to its aggregated particles not thedispersed surface needles or bristleswhose diameter is in the range of nano-scale. Without any dispersion, the parti-cles of Palygorskite tend to agglomeratetogether.

The physical properties of clays inTable 2 show that Actigel has the maxi-mum surface area of 150m2/g, followed

by Concresol and then Metamax. Thechemical composition of clays in Table 3shows the typical elements that are pres-ent. While Actigel has a combination ofsilica, alumina, magnesium, and probablysome unidentified elements, Metamax isdominated largely by only silica and alu-mina. Concresol has the highest amountof silica in it followed by alumina andtrace quantities of other elements.

Fine aggregate used was river sandwhich passed all ASTM C-136 specifica-tions, and had a Fineness modulus of3.09, absorption of 1.14%, and bulk spe-cific gravity of 2.62. Tap water controlledat 70 ± 3°F was used as mixing water forall mixes.

2.2. Mix proportions

Table 4 shows the 12 variations of a totalof 36 (12� 3) mortar mixtures designedwith three different FAs. All the bindersconsisted of 40% of FA (212 kg/m3)replacing cement by weight while cementcontent varied depending upon whether amix had clay addition or replacement. Aconsistent sand-to-binder ratio of 2.75and water-to-binder ratio of 0.45 wasadopted in all mixes.

2.3. Test methods

These mortars were prepared accordingto ASTM C305. A modification wasintroduced in the mixing procedure bypre-dispersing the clays in water for aminute in a Hobart Mixer running at theslow speed i.e. with the paddle revolutionrate of 140 ± 5 r/min and a planetarymotion of approximately 62 r/min. Freshmortar mixes were tested for their aircontent with a HM-34 Chace Air Indica-tor Kit. Their heat of hydration up to25 h was recorded in an eight-channelsemi-isothermal calorimeter set at a refer-ence temperature of 20 °C. Flowability ofthese fresh mortars was evaluated byusing a flow table as per ASTM C230.

Figure 2. Particle size distribution of clays[14,18].

Table 3. Chemical composition of clays[18,21].

Compound(%) Actigel Concresol Metamax

SiO2 49.57 69.88 53Al2O3 9.44 23.53 43.8Fe2O3 3.31 1.97 0.43SO3 – – 0.03CaO 1.88 0.19 0.02MgO 8.81 0.49 0.03Na2O 0.59 0.06 0.23K2O 0.66 2.16 0.19Eq. Na2O 1.02 0.08 0.028LOI < 0.50 – 0.46

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Then 2″� 2″� 2″ mortar cubes werecast, and cured at 23 ± 2 °C and ≥ 95%RH in a curing room for 7, 28, 56, and

90 days to be tested for their compressivestrength as per ASTM C109.

Table 4. Mixing proportions of cementitious materials (kg/m3).

S. no. Mix ID OPC Actigel (A) Concresol (C) Metakaolin (M)

1 40FA 318.1 – – –2 40FA+ 0.5A 318.1 2.7 – –3 40FA+ 1.0A 318.1 5.3 – –4 40FA+ 1.5A 318.1 8.0 – –5 40FA+ 1.5C 318.1 – 8.0 –6 40FA+ 1.5M 318.1 – – 8.07 40FA+ 3.0A 318.1 15.9 – –8 40FA+ 3.0C 318.1 – 15.9 –9 40FA+ 3.0M 318.1 – – 15.910 40FA+ 5M 291.6 – – 26.511 40FA+ 10M 265.1 – – 53.012 40FA+ 15M 238.6 – – 79.5

Figure 3. Effect of Actigel addition on rate of heat generation of fly ash-modified mortars.

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3. Results and discussion

3.1. Effect on heat of hydration

Figure 3 shows the generation of heat ofhydration of different mortars containing0, 0.5, 1.0, 1.5, and 3.0% of Actigel. Itcan be seen that with increasing additionlevel of Actigel, the rate of heat evolutionincreases. This behavior gives an indica-tion that presence of this clay increasesthe cement hydration reaction by evolv-ing more heat which is probably due toits filler effect. The fine clay particlespre-dispersed in the water, may haveserved as additional nucleation sites inthe cement matrix for the growth ofhydration products, as previouslyexplained by Gutteridge and Dalziel [22].

Figure 4 shows the heat of hydrationresults for the fly ash-modified mortars

containing Concresol additions at 1.5 and3.0% level. Concresol too affected therate of heat development although it wasnot as pronounced as in the case of Acti-gel. As is seen in Figure 4(d), there is nosignificant effect on rate of heat genera-tion due to addition of Concresol at thegiven percentages.

Figure 5 shows the effect of Metamaxaddition at 1.5 and 3.0% in fly ash-modi-fied mortars. Again, similar to the case ofConcresol, Metamax at these low per-centages does not have any significanteffect on the rate of heat generation ofthe given mortars. Only in the case ofFA2 (Figure 5(b)) an increase in heatgeneration peaks was seen which is notconsistent with other FAs, which may bedue to some experimental error.

Figure 4. Effect of Concresol addition on rate of heat generation of fly ash-modified mortars.

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The effect of Metamax replacementof the binder at higher levels on theheat generation in fly ash-modified mor-tars is summarized in Figure 6. It canbe seen that with increasing replacementlevel of Metamax, the peak of heatgeneration is reducing which seems tobe inconsistent with the data obtainedin Figure 5 at first. However, keepingin mind that addition and replacementare different approaches of utilization ofmineral admixtures in a binder system,this phenomenon can be better under-stood. A closer look at the trendsobtained in Figure 6 reveals that theinduction period is still reduced withincreasing Metamax replacement level,which indicates that the inclusion ofmetakaolin in the system is accelerating

the hydration process. However, thepeaks of heat generation are reducedbecause as the quantity of metakaolinin system is increased with increasingpercentage replacement level, the quan-tity of available cement to hydratedecreases (according to Table 4).

This means that while the reactivealuminosilicate phases in metakaolincan accelerate the hydration reaction,reduce the setting time of the mortar,the net generated heat will decreaseonce there is comparatively lessercement available to react with water ina given binder system. The clinker/water ratio decreased with increasingmetakaolin replacement in these set ofmortar mixes, resulting in such a phe-nomenon.

Figure 5. Effect of Metamax addition on rate of heat generation of fly ash-modified mortars.

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On comparing the clays among eachother, Actigel was found to be the mostdominant in inducing an exothermic reac-tion in the cement hydration processwhich can be related to its fine particlesize when in dispersion. Generally, it wasseen that with increasing addition level,the peak of heat of hydration increasedtoo, irrespective of the clay under consid-eration. The impact of Metamax ormetakaolin on the total heat evolved cor-related well with the calorimetric resultsobtained earlier by Frías et al. [23].

3.2. Effect on thermal setting time

Figure 7 shows the effect of Actigeladdition on the thermal setting time of

mortars. The thermal setting time wascalculated based on the methoddescribed previously by Ge et al. [24].This method consists of taking the firstderivative of the rate of heat generation(q) curve with respect to time (t), givingd(q)/d(t). The derivative is plottedagainst the time and the point when thecurve reaches its peak i.e. the time whenthe maximum heat is evolved is denotedas the initial setting time. And the timewhen this curve drops down to zero isthe time when there is no more heatgeneration from the cementitious mate-rial indicating that final set has occurred.As is seen in Figure 7, increasing addi-tion level of Actigel, decreases theoccurrence time of the peak of heat of

Figure 6. Effect of Metamax replacement level on rate of heat generation of fly ash-modifiedmortars.

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hydration, thus the reduction in both ini-tial and final setting time of fly ash-modified mortars follows the same trend.

There is slight decrease in settingtimes of the fly ash-modified mortars onaddition of Concresol and Metamax aswell which follows the trends and obser-vations recorded in the previous sectionon rate of heat generation (seeSection 3.1). Replacement of FA withhigh amounts of Metamax results in aclear linear decrease in setting time withincreasing replacement percentage as seenin Figure 7(d). This shows that the highlyreactive Metakaolin accelerates the set-ting process of the blended cement dueto its fine particle size, and the effect onsetting time is more pronounced at higherclay % replacement levels.

3.3. Effect on air content

The air content of mortar mixtures with1.5 and 3.0% addition level of three claysand 5, 10, and 15% replacement level ofMetamax was measured and is comparedin Figure 8. It is evident from the resultsobtained that the addition of claysincreases the amount of air voids in agiven mortar mixture. In the decreasingorder of air content, mortars with Actigelhad the highest air voids in them, fol-lowed by the mixes containing Concresoland Metamax. At higher levels ofmetakaolin replacement, the amount ofair voids increased with increasingreplacement level. This can be explainedas due to the absorption characteristics ofthese clays, that is, as their fine particle

Figure 7. Effect of clays on setting times of fly ash-modified mortars.

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size absorbs the lubrication water in thesystem, air is filled in the voids previ-ously occupied by free water. Actigelresults in floc formation due to theopposing surface charges [21] on its nee-dle-shaped bristles, resulting in increasedviscosity. This behavior is betterunderstood by analyzing the flow tabledata in Section (3.4) which shows thataddition of clays results in stiffer mix-tures with lesser free water and more airin their void system.

3.4. Effect on flowability

Figure 9 shows the flowability of thesame mortar mixtures to better illustratethe inverse relationship that was obtainedbetween the air content and flow. Addi-tion of the clay Actigel or Palygorskitesignificantly reduced the mortar flow,again due to the effect of its high wateradsorption and thixotropic behavior [21].

Lindgreen et al. [6] have previouslyreported an adsorption rate of Palygorsk-ite clays as high as 373% which alsoexplains why there is a lack of water inthe mortar systems when higher percent-ages of Actigel are added. Tregger et al.[25] studied the same three clays andthey found that the magnesium alumino-silicate clay or Palygorskite had the high-est green strength as it increased thestiffness of fresh concrete significantly.Usage of this clay at as high as 3.0% ofaddition level, thus, resulted in mortarmixes which were too stiff to have anyflow at all.

Concresol also reduced mortar flowability noticeably which can be attributedto the increase of fine particles in themortar mixtures. The mixtures havingMetamax had flow ability comparable tothe control mixtures without clay addi-tion. Kuder and Shah [26] had previouslyfound that clays like Concresol and Meta-

Figure 8. Effect of clays on air content of fly ash-modified mortars.

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max can be used to enhance the freshstate rheological properties of stiffcementitious materials and were found tobe an economical alternative as comparedto cellulose ethers. Replacement ofcement with Metamax at higher percent-ages resulted in significant reduction inflow due to excessive amount of fines inthe system and very little free water forlubrication. Thus, a high-range waterreducer may be beneficial when highamounts of Metamax or a similar clay isemployed in the cement system.

3.5. Effect on compressive strength

It was found that Actigel or Palygorskiteat lower addition levels (0.5–1.0%) hadlittle to no effect on the compressivestrengths of mortars and hence they arenot being published in this paper. Actigelis most effective for altering only thefresh state properties as observed in pre-vious Sections. Figure 10 summarizes the

compressive strength development ofmortars modified with clays.

The left column of Figure 10 showsthe 1.5 and 3.0% addition levels of thethree clays and right column shows the 5,10, and 15% replacement level of Meta-max. It is clear that strength of all mixescontaining Concresol and Metamaxincreased directly with curing age andaddition percentage. However, only incase of Actigel, the strength reduced sig-nificantly at higher percentages due to thehigh water adsorption property of Acti-gel; this resulted in dry mixes with zeroworkability and high air voids (see Sec-tions 3.3 and 3.4), and hence samplecubes could not be compacted wellenough for representative strength testing.The cubes for compressive strength wereseverely honeycombed on their surfaceand large air pockets appeared to be visi-ble throughout the cubical specimen.

Since Actigel is made of magnesiumaluminosilicate bristles which are about

Figure 9. Effect of clays on flowability of fly ash-modified mortars.

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1.5–2 μm in length with an averagediameter of 30Å, it ends up in a card-like microstructure which is difficult tobreak down, hence resulting in a mixwhich has increased yield stress andviscosity [21]. This makes it immenselydifficult to work with when used at

higher addition level, especially in theabsence of any super plasticizer orwater-reducing agent. A possible reme-diation to this effect could have beenachieved if additional water was addedto nullify the high surface adsorptioncharacteristic of this clay.

Figure 10. Effect of clays on compressive strength of fly ash-modified (40% replacement)mortars.

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Concresol was found to increase theoverall compressive strength for all flyash-modified mortars by around 5–10%,especially at the 3.0% addition levelwhich is consistent with the fact that itserves as a filler and provides additionalnucleation sites for cement microstruc-tural growth, as reasoned previously bySchellhorn and Hans-Werner [27].

Overall, it can be seen that Metamaxhas the highest reactivity among the claysused as it improves the strength of all thefly ash-modified mortars by 10–15%.This result correlates well with thealready-proven fact that metakaolin orcalcined kaolinite is the most reactive ofall clays when applied as mineral admix-tures [28]. Also, this is why it was cho-sen to be evaluated at higher replacementpercentage of 5–15% to see if it can miti-gate the high volume replacement of FAin the cement system. Metakaolin has areactive pozzolanic behavior in a cementsystem resulting in the gradual consump-tion of portlandite with age, as shownearlier by Wild and Khatib [29]. InFigure 10 right column, it is seen that thehighest replacement percentage of 15% isquite dominant in terms of compressivestrength at the early age of 28 days whileat later ages, metakaolin seems to be dor-mant in its pozzolanic reaction. This dor-mancy is evident as the 90-day strengthfor all metakaolin-replaced mixtures didnot have any significant improvementover the control, as strengths of all mixesconverged at the final curing age. Poonet al. [30] had previously shown that therate of pozzolanic reaction of metakaolinpeaks at 14 days and begins to slowdown after 28 days, which is similar tothe trends obtained in this study.

4. Conclusions

The following conclusions can be drawnbased on the various results obtainedfrom the tests of mortars modified withFA and different addition/replacementpercentages of clays:

(1) All clays tend to increase the rateof heat generation and decrease thetime of occurrence of peak of thisheat evolution, indicating that theyaccelerate the cement hydrationprocess due to their fine nature andfiller effect. Accordingly, the ther-mal setting time of mortars is alsoreduced after addition of clays.

(2) Actigel or Palygorskite has a highadsorption characteristic whichmay be harmful to a cement sys-tem if higher addition levels areemployed. It is recommended toadd extra free water or high-rangewater reducers to compensate forthis effect.

(3) Addition of clays tend to increasethe amount of air voids in the sys-tem as the free or lubricatingwater in the voids is reduced dueto the increased amount of fineparticles in the system.

(4) All the clays reduce the flowabilityof the mortars, with Actigel orPalygorskite being the most domi-nant in inducing the stiffnesseffect. Optimum usage of Actigelcan help in improving shape sta-bility of concrete to be employedin slip-form paving applications.

(5) Actigel has little or no effect onthe compressive strength develop-ment of mortars, while Concresolimproves the strength between 5and 10% of control, and Metamaxshows the most reactive pozzola-nic behavior at early ages and upto 28 days.

(6) In ternary blends, combination ofMetakaolin and FAs can be opti-mum as the pozzolanic reactionfrom the clay is dominant at earlyages and that of FAs is at laterages. Thus, the clay and FA willinduce additional alumina incorpo-ration in the structure of C–S–H,resulting in longer silica chains,and higher strength.

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Thus, various fresh state and hardenedstate properties measured were found tobe in correlation with each other, givingexpected results. Further research ondurability aspects may be conducted ifthe usage of such commercial clays hasto become widespread.

AcknowledgmentsThe present research is sponsored by the OakRidge Associated Universities (ORAU) –Tennessee Valley Authority (TVA) (GrantNo. 7-22976). Authors are thankful to claymineral manufacturing corporations forproviding samples for research purposes.Assistance from Wei Zhang, ISU in labtesting is appreciated. The views expressedherein are entirely authors’ own and in noway reflect the opinions of the commercialentities involved.

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