Construction and Building Materials 23 (2009) 1223–1231

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Construction and Building Materials

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Mechanical properties of reactive powder concrete containing mineraladmixtures under different curing regimes

Halit Yazıcı *, Mert Yücel Yardımcı, Serdar Aydın, Anıl S�. KarabulutDepartment of Civil Engineering, Engineering Faculty, Dokuz Eylül University, Buca 35160, _Izmir, Turkey

a r t i c l e i n f o a b s t r a c t

Article history:Received 23 June 2008Received in revised form 23 July 2008Accepted 7 August 2008Available online 14 September 2008

Keywords:Reactive powder concreteFly ashGround granulated blast furnace slagSilica fumeToughness

0950-0618/$ - see front matter � 2008 Elsevier Ltd. Adoi:10.1016/j.conbuildmat.2008.08.003

* Corresponding author. Tel.: +90 232 4127044; faxE-mail address: halit.yazici@deu.edu.tr (H. Yazıcı).

Mechanical properties (compressive strength, flexural strength, and toughness) of reactive powder con-crete (RPC) produced with class-C fly ash (FA) and ground granulated blast furnace slag (GGBFS) wereinvestigated under different curing conditions (standard, autoclave and steam curing) in this study. Testresults indicate that, compressive strength of RPC increased considerably after steam and autoclavingcompared to the standard curing. On the other hand, it was observed that steam and autoclave curingdecreased the flexural strength and toughness. Increasing the GGBFS and/or FA content improved thetoughness of RPC under all curing regimes considerably. Furthermore, SEM micrographs revealed densemicrostructure of RPC.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Reactive powder concrete (RPC) is a new generation concreteand it was developed through microstructure enhancement tech-niques for cementitious materials. As compared to ordinary ce-ment-based materials, the primary improvements of RPC includethe particle size homogeneity, porosity, and microstructures. Themechanical properties that can be achieved include the compres-sive strength of the range between 200 and 800 MPa, fracture en-ergy of the range between 1200 and 40,000 J/m2, and ultimatetensile strain at the order of 1% [1,2]. This is generally achievedby micro-structural engineering approach, including eliminationof the coarse aggregates, reducing the water-to-cementitiousmaterial ratio, lowering the CaO to SiO2 ratio by introducing thesilica components, and incorporation of steel micro-fibers [3]. Itwas reported that RPC has a remarkable flexural strength and veryhigh ductility. Its ductility is about 250 times higher than that ofconventional concrete [1,2]. Low permeability, dense micro-struc-ture and superior mechanical properties (very high compressivestrength, flexural strength, fracture energy and toughness) definethe RPC as an ultra-high performance concrete [4]. Nowadays,RPC seems to be a promising material for special pre-stressedand precast concrete members. This material can therefore be usedfor industrial and nuclear waste storage facilities [1–4]. Althoughproduction costs of RPC are generally high, some economicaladvantages also exist in RPC applications. It is possible to reduce

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or eliminate passive reinforcement using with steel fibers. And,due to ultra-high mechanical performance of RPC, the thicknessof concrete elements can be reduced, which results in materialsand cost savings.

Chan and Chu [3] reported that incorporation of silica fume inRPC matrix remarkably enhances the steel fiber–matrix bond char-acteristics due to the interfacial-toughening effect upon fiber slip.Massidda et al. [5] studied the effects of autoclaving under satu-rated vapor at 180 �C on the physical and mechanical propertiesof reactive-powder mortars reinforced with brass-coated steel fi-bers. Autoclaving generally has beneficial effects on the mechani-cal properties both in terms of flexural and compressive strength.High pressure steam curing for 3 h of specimens pre-cured atambient temperature for 3 days, yielded flexural strength of30 MPa and compressive strength of 200 MPa. Shaheen and Shrive[6] investigated freeze–thaw resistance of RPC. Test results showedthat RPC has excellent freeze–thaw resistance with no sign of dam-age up to 600 cycles according to ASTM C 666 test procedure. Rou-geau and Borys [7] showed that ultra-high performance concretecan be produced with ultra-fine particles other than SF such asfly ash, limestone microfiller or metakaolin. Furthermore, Kejinand Zhi [8] showed that the maximum heat of cement hydrationin binary/ternary cement (fly ash and/or GGBFS) concrete de-creased with supplementary cementitious material (SCM) replace-ments. As a result, SCM concrete generally has a lower risk ofthermal cracking than Portland cement (PC) concrete.

Cement dosage of RPC is generally as high as 800–1000 kg/m3

to achieve ultra-high strength under very low water/cementratios. A high amount of cement not only affects the production

Table 2Mixture proportions of RPC

Material CTRL G10F10 G10F20 G10F30 F20 G40

Cement (kg/m3) 830 664 581 498 664 498SF (kg/m3) 291 205 157 141 195 173GGBFS (kg/m3) – 83 83 83 – 332FA (kg/m3) – 83 166 249 166 –1–3 mm Quartz (kg/m3) 489 521 534 530 516 5410.5–1 mm Quartz (kg/m3) 244 260 266 264 257 2690–0.4 mm Quartz (kg/m3) 244 260 266 264 257 269Water (kg/m3) 151 151 151 151 151 151SP (L/ m3) 55 35 34 33 38 35Water from SP 33 21 20 20 23 21Water/cement 0.18 0.23 0.26 0.30 0.23 0.30Water/powder 0.13 0.15 0.15 0.16 0.15 0.15Water/powdera 0.16 0.17 0.17 0.18 0.17 0.17CaO (Mol) 9.40 8.38 7.82 7.27 8.29 7.55SiO2 (Mol) 7.22 6.43 6.00 6.06 6.36 6.29Steel fiber (kg/m3) 234 234 234 234 234 234Flow table (mm) 115 115 113 113 114 117Molar CaO/SiO2 1.30 1.30 1.30 1.20 1.30 1.20

a Calculated with total water (water + water from SP).

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costs, but also has negative effects on the heat of hydration andmay cause shrinkage problems. Mineral admixtures can be a fea-sible solution to overcome these problems in RPC. The mainobjective of this research is to determine the effect of mineraladmixtures on the mechanical properties of the RPC. Furthermore,this research aimed to reduce consumption of cement and silicafume in order to lower the material costs and to decrease the neg-ative impacts (heat of hydration, shrinkage and environmentalproblems). Portland cement and silica fume was replaced withGGBFS and/or FA at different proportions and mechanical perfor-mance determined after different curing regimes. Test resultsindicate that low cement RPC has satisfactory performance com-pared to the conventional RPC the matrix phase of which consistof cement and silica fume. In other words, it seems that greeningthe RPC is also possible using with high amount of mineraladmixtures.

2. Experimental

The RPC considered here is prepared by the following ingredi-ents: Ordinary Portland cement (CEM-I 42.5-R); quartz powder(0–0.4 mm) and quartz sand (0.5–1.0 and 1.0–3.0 mm, with a spe-cific gravity of 2.65), silica fume (SF), a polycarboxylate-basedsuperplasticizer (SP) in conformity with ASTM C 494-81 type Fand brass-coated steel micro-fibers (6 mm long with the diameterof 0.15 mm, the aspect ratio and tensile strength of the fibers is 40and 2250 MPa, respectively). The physical, chemical and mechani-cal properties of cement, silica fume, fly ash and slag are presentedin Table 1.

Table 2 summarizes the mixture designs of RPC produced inthis study. As can be seen from Table 2, abbreviations were usedfor mixtures according to GGBFS and/or FA content. FA andGGBFS were denoted by F and G. FA or GGBFS ratios by cementweight were also given in the abbreviations. For instance,G10F20 means cement was replaced with 10% GGBFS and 20%

Table 1Physical, chemical and mechanical properties of cement, silica fume, fly ash and slag

Chemical composition (%)

Cement Silica fume (SF) Fly ash (FA) Slag (GGBFS)

SiO2 20.10 92.26 42.10 39.66Al2O3 5.62 0.89 19.40 12.94Fe2O3 2.17 1.97 4.60 1.58CaO 62.92 0.49 27.00 34.20MgO 1.14 0.96 1.80 6.94Na2O 0.30 0.42 – 0.20K2O 0.85 1.31 1.10 1.44SO3 2.92 0.33 2.40 0.72Cl� 0.001 0.09 – –L.O.I. 3.84 – 1.30 1.20I.R. 0.63 – – –F.CaO (%) 0.52 – 4.30 –

Physical properties of cementSpecific gravity 3.13Initial setting time (min) 130Final setting time (min) 210Volume expansion (mm) 1.00

Specific surface(m2/kg)Cement (Blaine) 380SF (nitrogen Ab.) 20,000FA (Blaine) 290GGBFS (Blaine) 396

Compressive strength of cement (MPa)2 days 29.97 days 43.228 days 51.9

FA. Moreover CTRL shows Portland cement RPC that contain onlycement and SF as a binder without FA or GGBFS. Replacement ra-tios presented here were chosen according to results of previousstudy [9].

For each type of the proposed mixture proportions of RPC, dryingredients (i.e. cement, SF, FA and GGBFS, quartz powders, quartzsand and silica fume) were first mixed for about 3 min at low andhigh speed in Hobart mixer. Water and superplasticizer wereadded and re-mixed for about 5 min at high speed. Subsequently,fibers were added and additional mixing was applied for about2 min. The specimens were kept in the moulds for 16 h at roomtemperature of about 20� C. After that RPC specimens were re-moved from the steel molds. One-third of the RPC specimens werecured in water at 20� C. The other one-third of specimens wereautoclaved under 2.0 MPa pressure for 8 h (210� C). Temperatureand pressure reached to their maximum values in 2.5 h. Remain-ing specimens were exposed to steam curing at 100� C for 3 days.Heating rate of steam cure treatment was 11� C/h. This extended(3 days) high temperature (100� C) steam curing which is differ-ent from conventional curing process were preferred due to thehigh amount of reactive cementitious materials in RPC. Studiesshowed that high mechanical properties can be achieved underthese conditions at early ages [1–3]. Cwirzen et al. [10] also indi-cated that heat treatment densified the microstructure of the RPCmatrix. The specimens, which were subjected to heat treatment,were kept in laboratory conditions for cooling before testing inthis study.

Prismatic specimens (40 � 40 � 160 mm) were used to deter-mine the flexural strength and toughness. Flexural specimens weretested at the loading rate of 0.1 mm/min up to mid-span deflectionof 2.5 mm under closed loop control test procedure. The specimenswere loaded from their mid span and the clear distance betweensimple supports was 130 mm. Toughness was regarded as the areaunder the load–deflection curve up to 2.5 mm mid-span deflection.The compressive strength test was performed following to the flex-ural tests. The two broken pieces left from flexural test were sub-jected to compressive strength test. The loaded area undercompressive strength test is 40 � 40 mm and the height of thespecimens is also 40 mm. The moduli of elasticity values weredetermined on 100 � 200 mm cylinders. Each data presented hereare the average test results of three specimens. On the other hand,flexural load–deflection curves were drawn using with one speci-men graph that represents closest to the average mechanicalperformance.

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3. Results and discussion

Test results are presented in the following paragraphs.

3.1. Compressive strength of RPC

The compressive strength of mixtures after different curing con-ditions (standard curing for 2 and 28 days, steam and autoclavecuring) is presented in Fig. 1. The compressive strength of all thesemixtures is over 200 MPa after 28-day standard curing. It is obvi-ous that RPC containing GGBFS and/or FA also showed satisfactoryresults. GGBFS and/or FA replacement reduced the compressivestrength slightly after 2-day standard water curing. When thereplacement level reached to the 40% compressive strength reduc-tion is higher. This can be explained by the nature of pozzolanicreaction. On the other hand, it is obvious that the early compres-sive strength of RPC over 100 MPa and reached up to the163 MPa even under 2-day standard curing. This behavior is quitedifferent from conventional concrete and can be attributed to thevery low water/binder ratio and high amount of binder of RPCwhich cause the binder grains close to the each other and reducedporosity. Note that detailed previous study [9] showed that FAcaused more compressive strength reduction than GGBFS espe-cially over 20% replacement level. Furthermore, generally binarycombination (GGBFS + FA) showed better performance than onlyFA replacement. This finding was not valid for GGBFS. In this caseGGBFS is much better than using binary replacement. In otherwords, to avoid important mechanical loss with high amount ofFA, binary combinations were also designed in this study presentedin Table 2.

Steam and autoclaving improved the compressive strength ofRPC considerably. Moreover, autoclaved and steam cured sampleswere only 2 and 4-day old on testing day. From the point of min-eral admixtures, compressive strength of G10F10, F20 and G40 isclose to the strength of control mixture (Portland cement RPC)after autoclave curing. Greater values also exist after steam curing.This finding is also valid for control mixtures which have onlycement and SF as a binder. It is well known that steam curing at65–85� C for a few hours (for example 3–12 h) normally causesultimate strength reduction due to rapid reactions compared tothe standard curing in water at 20� C. Compressive strengthimprovement in RPC can be explained by pozzolanic reactions at

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Mi

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eng

th, M

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Fig. 1. The influence of GGBFS and/or FA content

elevated temperatures in which less reactive form of silica in SF,FA and GGBFS shows pozzolanic reaction that is normally remainsunreacted inert filler under standard curing. Note that control mix-ture also showed similar behavior. This can be attributed to the sig-nificant amount of SF in the control mixture. According to thereplacement level, improvement in compressive strength is be-tween 21% and 35% for autoclaving compared to the 28-day watercuring. This increment is 14–26% after steam curing. Moreover,steam/autoclave strength ratio is over 93%.

Normally the cement content of RPC is very high and the water/binder ratio of it is very low, which causes rapid hydration reac-tion, high heat of hydration and shrinkage. These problems maybe solved with mineral admixtures. The hydration mechanism ofmineral admixtures is different from that of cement. The mineraladmixtures reacts with water and then with calcium hydroxideto form cement hydration product through pozzolanic reaction toform extra C–S–H gel in the paste and slow down the strengthdevelopment at early age [11–14]. All these properties of GGBFSand/or FA may be advantage for RPC. Zhanga et al. [15] indicatedthat mineral admixtures greatly reduced the hydration heat andthe exothermic rate and prolonged the arrival time of the highesttemperature, particularly when two or three types of mineraladmixtures were added at the same time (double adding and tripleadding) in high strength concrete.

The 28-day strength in standard curing can be achieved inabout 24 h with autoclave curing [16]. However, the bond strengthbetween the concrete and the reinforcement is usually much lower(by about 50%), and the material tends to be more brittle than or-dinary concrete [17]. Under the conditions of high temperature andpressure, the chemistry of hydration is substantially altered. C–S–Hforms but is converted to a crystalline product a-calcium silicatehydrate (a-C2S) which cause an increase in porosity and reductionin strength. However, in the presence of silica, a-C2S converts totobermorite (C5S6H5) on continued heating thus high strengthcan be obtained. On the other hand, prolonged autoclaving maycause the formation of other crystalline calcium silicate hydrateswith a strength reduction. It is believed that the complete conver-sion to tobermorite is not desirable and that there is an optimumratio of amorphous to crystalline material for maximum strength[18–21]. However, with a small addition of silica fume, the cementstrength increases and the pore structure is densified. In addition,silica fume particles fill micro- and submicro-meter level pores in

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G10F30 F20 G40

xtures

In water (2 days) In water (28 days)Steam curing Autoclave curing

and curing regime on compressive strength.

1226 H. Yazıcı et al. / Construction and Building Materials 23 (2009) 1223–1231

paste and limit the particle size of hydrates that known as a spacefilling effect [22].

Preliminary test results [9] showed that GGBFS and/or FA can beused as an alternative fine silica sources in this study. Due to thisfinding SF content of the mixtures presented here was decreasedwith increasing mineral admixtures content. Test results alsoshowed that decreasing the SF content also decreased the SP de-mand considerably. In other words, using the GGBFS and/or FA asa silica source in the RPC has many beneficial affects (economy,decreasing shrinkage, heat of hydration, etc.). It can be seen thatfrom Table 2, SF content was reduced with the increasing GGBFSand/or FA replacement. SiO2 and CaO contents were calculatedfor each ingredient. SF content has been calculated to keep the mo-lar CaO/SiO2 ratio constant (1.20–1.30). Molar ratio of 1.30 waschosen for less than 40% replacement level and 1.20 for 40%replacement level according to the preliminary tests. For example,SF content of control mixture is 291 kg/m3 and 141 kg/m3 forG10F30 mixture. As can be seen in Fig. 1, decreasing SF contentwith increasing mineral admixture replacement generally did notcause mechanical properties loss.

3.2. Flexural properties of RPC

Flexural strength of mixtures after different curing conditionsis presented in Fig. 2. GGBFS and/or FA replacement generally pos-itively affected the flexural strength of RPC in all curing regimes.This behavior can be attributed to the improvement in bondstrength. Flexural strength improvement according to the GGBFSand/or FA replacement in 28-day standard, steam and autoclavecuring are between 7% and 35%, 12% and 36%, and 9% and 18%,respectively. On the other hand, although steam and high pressuresteam curing increased the compressive strength significantly,improvement in flexural behavior is not in the same extent. Thisis probably due to the weaker bond between the fibers and matrixafter these curing regimes [12]. Steam curing generally reducedthe flexural strength compared to the 28-day standard curing.Flexural strength loss after steam curing is between 11% and33% according to mixture type. However, flexural strength ofsteam cured G10F20 mixture is 3% greater than standard curedones. Mechanical properties of autoclaved specimens were gener-ally very close to the 28-day standard cured series except G10F30mixture.

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CTRL G10F10 G10F20

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Fle

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Fig. 2. The influence of GGBFS and/or FA conte

Flexural strength/compressive strength ratio is 14% for standardcured Portland cement RPC. This ratio is between 15% and 19%according to mineral admixtures replacement level in the samecuring regime. However, the flexural strength/compressivestrength ratio of Portland cement RPC is 9% and increased to the11–12% using with GGBFS and/or FA in the case of steam curing.These ratios are 10% and 12–13% for autoclaving, respectively.

Flexural load–deflection curves of 2-day standard cured mix-tures containing different amounts of GGBFS and/or FA are pre-sented in Fig. 3. To avoid the confusion, G10F10 and G10F20mixtures which generally showed similar behavior with other bin-ary combination (G10F30) are not presented in Fig. 3. It can be seenfrom Fig. 3 that generally GGBFS and/or FA replacement has posi-tively affected the flexural behavior of RPC. The maximum bendingloads of F20, G10F30 mixtures are greater than Portland cementRPC (control). This behavior can be attributed to the improvementin matrix phase, which also improves the bond strength betweenmatrix and the fibers. The displacement at maximum load is be-tween 0.14 and 0.34 mm. After the peak load, gradual load decre-ment was observed in all series. It can be noted that residualload at 2.5 mm displacement is over 2500 N.

Load–deflection curves of the 28-day standard cured specimensare given in Fig. 4. It can be seen from Fig. 4 that RPC containingGGBFS and/or FA showed higher performance than Portland ce-ment RPC under flexural loading. High post-peak load carryingcapacity shows well toughness and reinforcing effect of the steelfibers. Sudden load decrements and increments were observed inthe descending branch of the RPC containing mineral admixtures.This behavior is probably related to the length of fibers (6 mm),orientation of fibers, gradual pulling out of the fibers and ma-trix–fiber bond. This behavior is also reported in the literaturefor ultra-high performance fiber reinforced composites [22]. How-ever, sudden load drops were not observed in 2-day water curing.This is probably due to the lower bond strength between fibers andmatrix in this early age. Furthermore, residual load at 2.5 mm dis-placement is 2000 N for control mixture and increased with min-eral admixture replacement up to 3500 N. Residual load alsoincreased with 28-day water curing compared to the 2-day watercuring. The displacement at maximum load is between 0.26 and0.45 mm.

Fig. 5 shows load–deflection curves of steam cured RPC. It isobvious that mineral admixture replacement positively affected

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G10F30 F20 G40

tures

In water (2 days) In water (28 days)

Steam curing Autoclave curing

nt and curing regime on flexural strength.

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Fig. 3. The load–displacement relationship of 2-day water cured mixtures according to the GGBFS and/or FA content.

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Fig. 4. The load–displacement relationship of 28-day water cured mixtures according to the GGBFS and/or FA content.

H. Yazıcı et al. / Construction and Building Materials 23 (2009) 1223–1231 1227

the performance of RPC under bending. This behavior is clearer forsteam curing compared to the standard water curing. It seems thatnegative effect of steam curing on flexural performance of RPC canbe decreased using with GGBFS and/or FA. Furthermore, residualload at 2.5 mm displacement is 2500 N for control mixture and in-

creased with mineral admixture replacement up to 3150 N. Thedisplacement at maximum load is between 0.12 and 0.44 mm.

The load–deflection curves of autoclaved RPC are presented inFig. 6. There is an important difference between Portland cementRPC and RPC containing mineral admixtures under flexural load

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Fig. 5. The load–displacement relationship of steam cured mixtures according to the GGBFS and/or FA content.

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Fig. 6. The load–displacement relationship of autoclave cured mixtures according to the GGBFS and/or FA content.

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after autoclave curing. In other words, GGBFS and/or FA improvedthe flexural performance of autoclaved RPC considerably. It is obvi-ous that the negative effect of autoclaving on bond between matrixof RPC and fibers can be decreased using with mineral admixtures.The displacement at maximum load is between 0.29 and 0.45 mm.

The magnitude of sudden load decrements and increments indescending branch of RPC decreased after autoclave curing com-pared to the other curing regimes. This can be explained by thedecreasing bond strength due to autoclaving which causes gradualpulling out of the fibers instead of pulling out of suddenly.

H. Yazıcı et al. / Construction and Building Materials 23 (2009) 1223–1231 1229

Toughness of mixtures after different curing conditions is pre-sented in Fig. 7. GGBFS and/or FA replacement generally positivelyaffected the toughness in all curing regimes. This behavior can beattributed to the improvement in bond strength between matrixphase and fibers. Toughness values increased 18–46% accordingto mineral admixture replacement compared to the Portland ce-ment RPC under standard curing. This ratio is between 24–44%for steam curing and 23–39% for autoclaving. In other words, usingGGBFS and/or FA improved the toughness of RPC considerably. Theother factor which affects the toughness of RPC is curing regime.The maximum toughness performance was observed from stan-dard water curing for both Portland cement RPC and RPC contain-ing mineral admixtures. While compressive strength of autoclavedand steam cured specimens were considerably higher than stan-dard cured ones, the highest toughness values were determinedafter the 28-day standard curing. Steam curing reduced the tough-ness compared to the 28-day standard curing. This decrement isbetween 10% and 34% according to mixture type. On the otherhand, autoclaving slightly reduced the toughness compared tothe 28-day standard curing. In this case decrement ratio is between4% and 18%. This behavior is probably due to the weaker bond be-tween the fibers and matrix after steam curing and autoclaving[16]. It is obvious that steam curing cause much more bondstrength loss than autoclaving in RPC.

3.3. Influence of the GGBFS and/or FA replacement on modulus ofelasticity of RPC

Moduli of elasticity of some selected autoclaved mixtures weredetermined and are shown in Fig. 8. It can be seen that modulus ofelasticity of RPC decreased slightly when the replacement levelreached to 30%. This can be attributed to the decreasing compressivestrength level. The maximum decrement (18%) was observed in 40%GGBFS replacement compared to the control mixture. Decrement inmodulus of elasticity values was also reported for high strength con-crete containing mineral admixtures by Nassif et al. [23].

3.4. Microstructure of RPC

Microstructure of the selected RPC mixtures has been investi-gated by using JEOL JSM 6060 electron microscope (SEM). The

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Fig. 7. The influence of GGBFS and/or FA conte

samples for SEM analysis were prepared by taking small piecesfrom the prismatic specimens. Original microstructure and mor-phology of the RPC were observed on fractured surfaces using withsecondary electron imaging. The general micro-structural featuresof RPC were determined by using backscattered electron (BSE)imaging. Samples were coated with gold. The SEM study was car-ried out by using an accelerating voltage of 20 kV.

Limited micrographs are presented here to give only an opinionabout microstructure of RPC. Microstructure investigations re-vealed the very dense microstructure of RPC in this study. Verylow water/binder ratio causes the cement grains closer to eachother. Fig. 9a shows BSE image of autoclaved Portland cementRPC. The matrix phase predominantly consists of outer product.There are a large number of small grains in these areas which havebeen termed undesignated product or groundmass. There is consid-erable infilling of the large capillary pores by this undifferentiatedproduct during hydration. Portland cement contains four mainphases: impure C3S (alite), impure C2S (belite), impure C3A (alumi-nate) and impure Ca2(Al, Fe) (Ferrite solid solution, Fss). Duringgrinding, facture generally occurs through the phases rather thanbetween them, so that the resulting cement grains almost alwayscontain more than one phase [24]. Unhydrated cement grains alsoexist (light grey areas in Fig. 9a). There are spherical pores in RPCone of which is visible at the bottom of Fig. 9a. These pores formedpossibly due to the side effect of high amount of superplasticizerhave different diameter giving wide range between 10 and300 lm. These pores generally were empty in standard or steamcured samples however filled with tobermorite- or jennite-likestructures in autoclaved ones. Similar findings are also valid RPCcontaining mineral admixtures. To obtain the morphology, espe-cially after autoclaving, secondary electron imaging was also usedin this study. Some other type of pores which are in groups andhas different shape observed in FA replacement. This is presentedin Fig. 9b. EDS analysis showed that pore groups like this is unre-acted spherical FA grain which is cut in sample preparation process.

SEM microphotograph of autoclaved Portland cement RPC andF20 mixture was presented in Fig. 10 which was taken in second-ary mode. It can be seen from the Fig 10 that spherical pores havebeen filled with needle-like tobermorite and jennite-like structuresafter autoclaving in both control and F20 mixtures. Energy disper-sive spectroscopy (EDS) analysis in Portland cement RPC showed

G10F30 F20 G40

xtures

In water (2 days) In water (28 days)Steam curing Autoclave curing

nt and curing regime on toughness of RPC.

Fig. 10. SEM images of autoclaved (a) control mixture (spherical pores filled with jennite-like structures) (b) F20 mixture needle-like tobermorite.

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Fig. 8. The effect of GGBFS and/or FA content on the moduli of elasticity of the mixtures (autoclave curing).

Fig. 9. SEM images of (a) autoclaved control mixture (pores in the matrix) (b) steam cured F10G20 mixture (pores in the matrix).

1230 H. Yazıcı et al. / Construction and Building Materials 23 (2009) 1223–1231

that Ca/Si, S/Ca and Al/Ca ratios of this type of jennite are 1.85,0.003 and 0.036, respectively. EDS analysis in FA mixture showed

that Ca/Si, S/Ca and Al/Ca ratios of this type of tobermorite are1.02, 0.03 and 0.05, respectively.

H. Yazıcı et al. / Construction and Building Materials 23 (2009) 1223–1231 1231

4. Conclusions

Test results showed that RPC containing high volume mineraladmixtures have satisfactory mechanical performance. Althoughthe cement and silica fume contents of these mixtures importantlylower than conventional RPC, compressive strength exceeded200 MPa after standard water curing. Autoclave and steam curingseems very effective ways to increase the compressive strengthof RPC. This can be attributed to the improvement of hydrationprocess under these curing regimes. In this case compressivestrength is over 234 MPa after steam curing and greater than250 MPa after autoclaving. Furthermore, these mixtures have alsoimportant environmental benefits. Decreasing cement content re-duces heat of hydration and shrinkage which are normally impor-tant problems for conventional RPC.

On the other hand, steam and autoclave curing caused somereduction in flexural strength compared to the 28-day standardcuring. This is probably due to the decreasing bond strength be-tween matrix and fibers. This behavior is much more importantin steam curing than autoclaving. GGBFS and/or FA replacementdecreased this negative effect in both steam and autoclaving.GGBFS and/or FA improved the flexural performance of RPC underall curing regimes. Toughness values increased importantly usingwith these mineral powders.

Test results showed that GGBFS and/or FA can also be used as afine silica source for RPC. In other words, SF can also be reduced byincreasing GGBFS and/or FA content. This gives also importantadvantages (economy, reduced heat of hydration, shrinkage andsuperplasticizer demand) like cement replacement.

Mineral admixtures decreased the modulus of elasticity of RPCespecially over 30% replacement levels. Although some sphericalentrained air pores exist, SEM investigations revealed the densemicrostructure of RPC. Tobermorite- and jennite-like structureswere observed in autoclaved specimens.

Acknowledgements

This study is a part of the project supported by the Scientificand Technological Research Council of Turkey (TÜB_ITAK, ProjectNo.: 104I085). The authors gratefully acknowledge to TÜB_ITAK.In addition, the authors thank to Mr. Mehmet Yerlikaya from BEK-SA-DRAMIX, Mr. Okan Duyar from BASF-YKS and Mr. HakanS�envardarli from KARÇ_IMSA for materials support.

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