Composites: Part B 45 (2013) 63–69

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Composites: Part B

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The effect of fiber properties on high performance alkali-activated slag/silicafume mortars

Serdar Aydın ⇑, Bülent BaradanDepartment of Civil Engineering, 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 28 October 2011Received in revised form 19 August 2012Accepted 21 September 2012Available online 6 October 2012

Keywords:A. FibersB. Mechanical propertiesB. Fracture toughnessAlkali-activated cement

1359-8368/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.compositesb.2012.09.080

⇑ Corresponding author. Tel.: +90 232 301 7044; faE-mail address: serdar.aydin@deu.edu.tr (S. Aydın

The effects of length and volume fraction of steel fibers on the mechanical properties and drying shrink-age behavior of steel fiber reinforced alkali-activated slag/silica fume (AASS) mortars were investigatedwithin the scope of this research. Steel fibers with two different lengths of 6 mm and 13 mm, and fourdifferent volume fractions of 0.5%, 1.0%, 1.5% and 2.0% were used in the AASS mixtures. Also, a Portlandcement (PC) based 1.5% steel fiber (13 mm length) reinforced mortar was prepared for comparison. Testresults showed that mechanical performance of AASS mortars were significantly better than PC basedcontrol mortar. This superior performance of AASS mortar may be attributed to the higher bond proper-ties between the fibers and AASS matrix compared to PC matrix. The mechanical performance of AASSimproved dramatically parallel to the increment of fiber length from 6 mm to 13 mm. Also, the dryingshrinkage of AASS mortars decreased with the increasing fiber dosage.

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1. Introduction

Alkaline cements are comparatively new binders, alternative tothe traditional Portland cements, obtained through the alkalineactivation of different industrial by-products such as; blast furnaceslag and fly ash. During the past two decades, alkali-activatedcements and concretes have attracted strong interest all over theworld due to their advantages of low energy cost, high strengthand good durability compared to Portland cements [1,2].

Plain concrete is a brittle material with low tensile strengthproperties. This undesired behavior can be improved by inclusionof short discrete fibers into the matrix which prevent or control ini-tiation, propagation, or coalescence of cracks [3,4]. Steel, glass, car-bon, wood, synthetic and natural fibers are used for this purpose.The inclusion of fibers in concrete or mortar substantially improvesmany of its engineering properties such as; tensile strength, flex-ural strength, fracture toughness, resistance to fatigue, impact,wear, and thermal shock [4,5]. Fiber inclusion is also an efficientmethod to improve shrinkage behavior of concrete [6,7]. The mostimportant effect of fibers is prevention of crack propagation in con-crete. The extension and propagation of micro-cracks that occurdue to the internal stresses in concrete are prevented by stresstransfer capability of randomly distributed fibers. According totheir shape and quantity, fibers bear some of the stress that occurin the matrix themselves and transfer the other portion of stress tothe stable cement matrix portions [8–10].

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The performance of fiber reinforced concrete depends on theproperties of concrete and the fibers [4]. In steel fiber reinforcedconcretes, the most important factors affecting the concrete prop-erties are aspect ratio and dosages of fibers [8]. However, there arefew references concerning the influence of fibers on the mechani-cal properties of alkali-activated cements in the literature. Bernalet al. [6] investigated the effects of steel fiber incorporation onmechanical and fracture toughness properties of concretes basedon alkali-activated slag (AAS). As a result of their research, splittingtensile strength, flexural strengths and the properties related todurability performance were largely improved with the fiber addi-tion. However, compressive strength of AAS concrete reduced withthe fiber incorporation. Bernal et al. [11] studied mechanicalbehavior of pleated steel fiber incorporated AAS concrete. Themechanical strength of plain and fiber-reinforced alkaline con-cretes was found higher than conventional ordinary Portlandcement concrete with similar proportions of cement and fiber.Puertas et al. [2] studied the effects of polypropylene fiber inclu-sion on the properties of alkali-activated cement mortars. Incorpo-ration of polypropylene fibers did not positively affect themechanical behavior, modulus of elasticity and freezing/thawingresistance of alkali-activated mortars in this study. However, im-pact resistance of the mortars after wet/dry cycles increased withthe inclusion of polypropylene fibers. Alcaide et al. [12] showedthat, carbon fiber inclusion failed to improve strength of AAS mor-tar however it reduced the drying shrinkage. The inclusion of fiber-glass strands lowered AAS mortar shrinkage with no adverse effecton its mechanical properties [13]. Penteado Dias et al. [14] ob-served that, basalt fiber increased flexural and splitting tensile

64 S. Aydın, B. Baradan / Composites: Part B 45 (2013) 63–69

strength in alkaline concretes, and raised their fracture perfor-mance to substantially higher levels compared to Portland cementconcrete. Silva et al. [15] reported that adding wollastonite micro-fiber to alkali-activated matrices improved toughness of thecomposite.

1.1. Research significance

The aim of this study is to improve the mechanical properties ofAASS mortar by incorporation of steel fibers. The effect of fiberlength and dosage on mechanical properties and drying shrinkagehas been investigated. As a result of this study, an alternative highperformance composite to Portland cement based RPC (ReactivePowder Concrete) with a compressive strength value over200 MPa was produced by using 13 mm length steel fibers with1.5% dosage. This new type composite was produced by usingwaste materials (i.e ground granulated blast furnace slag and silicafume) and an activator solution. The utilization of waste materialsin RPC applications will also be helpful in reducing environmentalproblems. It will also reduce greenhouse gas emissions associatedwith the Portland cement production, and conserve existingnatural resources.

2. Materials and experimentation

Ground granulated blast furnace slag (GGBFS) has been pro-cured from Eregli steel plant, Turkey. The chemical compositionof GGBFS has been presented in Table 1. Specific gravity and spe-cific surface (Blaine) values of GGBFS were 2.88 and 410 m2/kg,respectively, and 90% of particles were below 45 lm. The basicitycoefficient [Kb = (CaO + MgO)/(SiO2 + Al2O3)] and the hydrationmodulus [HM = (CaO + MgO + Al2O3)/SiO2] of slag were 0.81 and1.33, respectively. A commercial silica fume (SF) was used in thisstudy with the chemical composition seen in Table 1. The specificsurface area and specific gravity of SF were 23360 m2/kg (BETnitrogen adsorption method) and 2.20, respectively. A commercialquartz sand, in four different size fractions (1–3 mm, 0.6–1.2 mm,0–400 lm and 0–75 lm), was used as aggregate.

Alkali-activated slag/silica fume (AASS) mortars were producedby the activation of GGBFS with technical grade sodium hydroxideand waterglass (liquid sodium silicate). Sodium silicate had a SiO2

content of 27% and Na2O content of 8%, and the silicate modulus(Ms) was 3.38. Sodium hydroxide and sodium silicate were mixedto provide 4% Na2O by weight of binder (GGBFS and SF) and Ms va-lue of 1.2. Activator solution was prepared 1 day before the castingof mortars.

Two different high strength brass-coated steel fibers were used,F6 (6 mm length) and F13 (13 mm length). The tensile strength anddiameter of these fibers were 2250 MPa and 0.16 mm, respectively.Aspect ratios (l/d) of these fibers were 37.5 and 81.25, respectively.

The specimens were identified with numbers designation: thefirst number indicates the fiber volume fractions (0%, 0.5%, 1.5%,or 2%) and the last number denominates the fiber length (6 or13 mm). Table 2 summarizes the mixture designs of plain AASSmortars (without fiber). In case of fiber inclusion, aggregateamounts were reduced in order to maintain 1 m3 of mortarvolume. Water to binder ratio of all mixtures was 0.17. GGBFS/SFratio of AASS mortars were four which was determined from the

Table 1Chemical compositions (%) of GGBFS and SF.

SiO2 Fe2O3 Al2O3 CaO MgO Na2O K2O SO3 LOI

GGBFS 40.20 1.68 11.66 35.90 5.88 0.30 1.47 0.90 0.88SF 96.10 – – – – – – – 1.81

preliminary studies. The lower ratios were resulted in rapid settingwhile the higher ratios were caused to lower mechanical perfor-mance. Mixtures were prepared by using an electrically drivenmechanical mixer conforming to the requirements of ASTM C305[16]. Initially, binder and aggregate were dry-mixed for a minuteand then the solution was gradually added during mixing. Themixture procedure continued for about 3 min. The flow test wasperformed according to ASTM C230 [17]. Finally, fresh mixtureswere cast into prismatic steel molds (40 � 40 � 160 mm). Thespecimens were kept in a humidity cabinet (�20 �C temperatureand 90% relative humidity) for 5 h and then they were subjectedto steam curing at 100 �C for 12 h, heating rate was of 22 �C/h.The heat-treatment cycle is given in Fig. 1. After steam curing,the specimens were demolded and tested.

Prismatic specimens were subjected to the flexural strength testaccording to ASTM C348 [18]. Fiber reinforced mortars were testedat a loading rate of 0.2 mm/min up to mid-span deflection of 3 mmunder closed loop control test procedure. Load–deflection curvesafter maximum load cannot be drawn for control mortars (withoutfiber) at this loading rate due to their brittle nature. Thus, controlmortars were tested at a loading rate of 0.02 mm/min. The speci-mens were loaded from their mid-span and the clear distancebetween simple supports was 130 mm. Toughness was calculatedby integrating the area under the load–deflection curve up to3 mm mid-span deflection. The compressive strength tests wereperformed following the flexural tests on two broken pieces leftfrom the flexural test according to ASTM C349 [19]. The loadedarea under compressive strength test was 40 � 40 mm and theheight of the specimens was also 40 mm. Each datum presentedis the average of the test results of at least three specimens forthe flexural strength and toughness, and at least six specimensfor the compressive strength.

Shrinkage values of AASS mortars were measured on25 � 25 � 285 mm prisms according to ASTM C596 [20]. The firstreadings were taken immediately after the heat treatment. Thelength change of specimens was recorded periodically up to6 months on two specimens for each mixture that were kept inlaboratory conditions (about 20 �C temperature and 50% relativehumidity).

3. Results

The discussion about the effects of length and volume fraction(Vf) of steel fiber on; workability, compressive strength, flexuralstrength and toughness of AASS mortars are presented below.

3.1. Workability

Spreading diameters of all mixtures are given in Fig. 2. 13 mmlength steel fibers caused to a greater decrease in the spreadingdiameter values compared to 6 mm length steel fibers as expected.Spreading diameter values of 6 mm fiber reinforced AASS mortarswere not changed beyond 1% of Vf. However, in case of 13 mmlength fiber inclusion workability decreased parallel to the increasein fiber dosage.

3.2. Compressive strength

The compressive strength and relative compressive strength ofall the mixtures tested in this investigation are shown in Fig. 3according to their fiber volumes and fiber lengths. The compressivestrength of mortars significantly increased with the fiber volumeand length as shown in Fig. 3. Moreover, the contribution of fiberlength to compressive strength seems to be more significant athigh fiber dosages. For example, the increase in compressive

Table 2Mixture proportions of AASS mortars (kg/m3).

GGBFS SF Quartz sand Waterglass NaOH Water

1–3 mm 0.6–1.2 mm 0–400 lm 0–75 lm

720.0 180.0 561.1 436.4 149.6 99.7 160.0 30.9 49.0

Fig. 1. Heat treatment cycle for steam curing.

0

20

40

60

80

100

120

140

160

180

0.0 0.5 1.0 1.5 2.0

Vf (%)

F6 F13

Flo

wdi

amet

er, m

m

Fig. 2. The influence of fiber length and content on workability.

S. Aydın, B. Baradan / Composites: Part B 45 (2013) 63–69 65

strength at 0.5% fiber dosage is about 30% for F6 and 35% for F13series. These values reach to 48% and 74% at 2% fiber content,respectively. Increase of compressive strength by fiber inclusion

132

172181177

0

20

40

60

80

100

120

140

160

180

200

220

240

0 0.5 1.0

Com

pres

sive

str

engt

h (M

Pa)

Vf (%

F6 F13

Fig. 3. The influence of fiber length and content o

may be attributed to the relatively small dimensions of these fi-bers, which gave these fibers the ability to delay the micro-crackformation and to arrest and prevent their propagation afterwardsup to a certain extent [4,21].

The compressive strength values of 1%F13 is approximatelyequal to 2%F6. In other words, to achieve about 190 MPa compres-sive strength value the required fiber amount for 13 mm lengthfiber is about half of the 6 mm length fiber inclusion. The increasein compressive strength is negligible over 1.5% fiber content forboth fiber lengths. Furthermore, the compressive strength valuesof 1.5%F13 and 2%F13 series exceed 200 MPa which is the repre-sentative strength value of RPC (Reactive Powder Concrete). How-ever, this value was not achieved by F6 even with 2% fiberinclusion.

3.3. Load deflection relationship

Load versus mid-span deflection curves of F6 and F13 mixtureswith different fiber contents are presented in Figs. 4 and 5, respec-tively. Flexural load–deflection curves were drawn using with onespecimen graph that represents closest to the average mechanicalperformance. Change in fiber contents influenced the behavior ofthe mixtures dramatically. All series containing steel fiber exhib-ited a deflection-hardening behavior that generates a higher loadcarrying capacity after the first cracking. Load carrying capacityand ultimate deflection values of control mortar (without fiber)have increased significantly by fiber inclusion. High load carryingcapacity after the peak load indicates an improved toughness dueto the reinforcing effect of the steel fibers. Instantaneous loaddecrements and increments were observed in the descendingbranch of steel fiber reinforced AASS for both fiber lengths. Thisbehavior is closely related to the length of fibers, orientation of fi-bers, gradual pulling out of the fibers and matrix–fiber bond. Thisbehavior is also reported for ultra-high performance fiberreinforced composites [22,23]. The maximum load and mid-spandeflection value at peak load have been increased by increase of

192 195193

223 229

0

20

40

60

80

100

120

140

160

180

1.5 2.0

Relative com

pressive strength (%)

)

n the compressive strength of AASS mortars.

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

0 0.5 1 1.5 2 2.5 3

Loa

d, N

Displacement, mm...

0%

0.5%

1.0%

1.5%

2.0% 0

1000

2000

3000

4000

0 0.1 0.2 0.3 0.4

Loa

d, N

Displacement, mm

0%

Fig. 4. Load–displacement curves of AASS with F6.

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

0 0.5 1 1.5 2 2.5 3

Loa

d, N

Displacement, mm

0% 0.5% 1.0% 1.5% 2.0%

0

1000

2000

3000

4000

0 0.1 0.2 0.3 0.4

Loa

d, N

Displacement, mm

0%

Fig. 5. Load–displacement curves of AASS with F13.

11.5

16.3

18.619.8 20.3

16.0

18.2

21.823.3

0

5

10

15

20

25

30

0 0.5 1.0 1.5 2.0

Fir

st-c

rack

str

engt

h(M

Pa)

Vf (%)

F6 F13

Fig. 6. The effect of fiber length and content on first-crack strength of AASS mortars.

66 S. Aydın, B. Baradan / Composites: Part B 45 (2013) 63–69

fiber dosage and length. AASS reinforced with 1% and 1.5% steel fi-ber exhibited a similar behavior for F6. This behavior was also validfor F13 with higher mechanical performance.

First-crack strength values of mortars have been determinedaccording to ASTM C1018 [24]. The inclusion of fibers increasedthe first-crack strength values parallel to the increase in fiber dos-age as shown in Fig. 6. At 0.5% and 1% fiber dosages, first-crackstrength values were similar for F6 and F13. Nevertheless, for thehigher fiber inclusion case (1.5% and 2%), the addition of 13 mmsteel fiber increased this property more than 6 mm fiber. Increasein first crack strength with 13 mm length fiber inclusion was alsoreported by Carvalho et al. [25].

3.4. Flexural strength

The variation of flexural strength and relative flexural strengthof all mixtures tested in this research are given in Fig. 7. The posi-tive effect of fiber content and length on flexural strength is muchmore significant than compressive strength case. However, theimprovement in flexural strength with increasing fiber content isnegligible beyond 1.5% for F6. Similar to the compressive strengthcase, contribution of fiber length to flexural strength is morepronounced at high fiber dosages. At 0.5% fiber dosage, F13 fiber

inclusion results in 15% flexural strength gain compared to F6 ser-ies while at 2% fiber inclusion case this value reach to 92%. Also, theflexural strength of 1%F13 was found significantly higher than2%F6.

12.017.1

20.924.1 25.2

19.7

32.4

39.8

48.4

0

50

100

150

200

250

300

350

400

450

0

10

20

30

40

50

60

0 0.5 1 1.5 2

Relative flexural strength (%

)F

lexu

ral s

tren

gth

(MP

a)

Vf (%)

F6 F13

Fig. 7. The effect of fiber length and content on flexural strength of AASS mortars.

S. Aydın, B. Baradan / Composites: Part B 45 (2013) 63–69 67

3.5. Toughness

There is an urgent need for high energy absorbing materials thatwill mitigate the hazards for structures subjected to dynamic loadssuch as; seismic, impact and blast. Thus, determining energyabsorption capacity of promising composites provides useful infor-mation for such applications [26]. The effect of fiber length andcontent on energy absorption capacity has been illustrated inFig. 8 using toughness values. These values represent the areasunder load–displacement curves of Figs. 4 and 5. The real and mostsignificant effect of fiber inclusion has been observed in energyabsorption capacity. The toughness of fiber incorporated seriesincreased up to 12,500% compared to control mortar without fiber.This significant increase in toughness can be attributed to the abil-ity of the steel fibers to arrest cracks at both micro- and macro-levels. At micro-level, fibers inhibit the initiation of cracks, whileat macro-level fibers provide effective bridging of stresses. Thisbehavior leads to increase in toughness and ductility [3,27]. Tough-ness values of AASS mortars with F13 fibers are more than F6 seriesby 110–202%. Also, the toughness of 0.5%F13 is higher than 2%F6.In other words, to achieve the same toughness value, requiredamount of F6 fibers is at least four times higher than the amount

224

4611

7727

13908

21486

0

5000

10000

15000

20000

25000

30000

0 0.5 1

Tou

ghne

ss (

Nm

m)

Vf (%

F6 F13

Fig. 8. The effect of fiber length and con

of F13 fibers. If loss in workability can be tolerated, this means aserious economic benefit in the production of composites.

3.6. Drying shrinkage

Drying shrinkage values of F6 and F13 fiber incorporated AASSmortars versus time are presented in Fig. 9. As shown in Fig. 9 fiberinclusion positively affected the drying shrinkage behavior of AASSmortars. Drying shrinkage values of plain AASS mortars have beenreduced up to 24% by fiber inclusion. However, drying shrinkagesof F6 and F13 fiber incorporated AASS mortars were found similarat same fiber dosages.

3.7. Comparison with PC based binders

PC based RPC’s with and without steel fibers were produced forcomparison purposes. Ordinary Portland cement (CEM I 42.5R)with Blaine fineness of 369 m2/kg has been used with the dosageof 720 kg/m3 equal to the GGBFS content of AASS series. SF contentand water/binder ratio of this mixture were also same with AASS. A50 kg/m3 polycarboxylic-based superplasticizer was used to pro-vide the similar flow value with AASS mixtures. The fiber content

1125212590

23655

28155

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

11000

12000

13000

1.5 2

Relative toughness (%

)

)

tent on toughness of AASS mortars.

Fig. 9. The effect of length and fiber content on drying shrinkages of AASS mortars.

Table 3Properties of AASS and PC mortars with and without fiber.

0%F 1.5%F13

AASS PC AASS PC

Flow diameter (mm) 156.0 169.0 121.0 126.0Compressive strength (MPa) 131.8 133.2 223.0 214.0Flexural strength (MPa) 12.0 17.2 39.8 33.3First crack strength (MPa) 11.5 17.2 21.8 19.2Toughness (N mm) 224 156 23,655 19,810

68 S. Aydın, B. Baradan / Composites: Part B 45 (2013) 63–69

and length of RPC were equal to AASS 1.5%F13. The mechanicalproperties of plain and 1.5%F13 fiber reinforced PC based mortarsare presented in Table 3 with AASS mixtures. It should be statedthat, PC based plain mortar without fibers showed an instanta-neous failure at the peak load. Thus, post-crack portion of theload–deflection curve could not be obtained for this mixture.

The compressive strength of PC based plain mortar was approx-imately equal to plain AASS mortar. However, the flexural strengthof plain AASS mixture was significantly higher than the plain PCmixture as seen in Table 3. The flexural and compressive strengthvalues of PC and AASS have significantly increased by fiber inclu-sion. However, increase ratio in flexural strength for AASS mortarwith fiber inclusion is higher than PC based mixture. The increaseratios were 2.3 for AASS and 0.9 for PC. This indicates better bond-ing properties between fiber and mortar phase for AASS basedbinders. SEM micrographs also confirmed better bond propertiesin the interfacial zone between fibers and matrix phase for AASSbinders compared to PC based binders (Fig. 10). The better bondcharacteristics of alkali activated binders compared to PC have alsobeen reported previously [28,29]. The formation of the dense and

Fig. 10. SEM micrographs of fiber–matrix

uniform transition zone has been attributed to the water reducingfunction of Na2SiO3 and high initial concentration of (SiO4)4� in thepore solution by Shi and Xie [28], and the reaction between thealkali activator and the surface of the other material [29]. Almostsame flexural performance of PC based binder with 1.5%F13 hasbeen recorded by AASS with 1.0%F13 as a result of the better bondbetween steel fibers and mortar phase. In other words, AASS basedbinders may provide similar performance with a lower fibercontent. This means more economic and environmental fiberreinforced composites with AASS can be produced.

4. Conclusions

Based on the experimental results of this investigation the fol-lowing conclusions can be drawn:

– Longer fibers resulted in better mechanical performance interms of compressive strength, flexural strength and toughnessin AASS mortars. AASS mixture that has a 1%, 13 mm lengthfiber has similar compressive strength and higher flexuralstrength values than 6 mm length fiber reinforced AASS mixturewith a dosage of 2%. Also, the toughness of 13 mm fiber rein-forced AASS mortar with 0.5% dosage is higher than 6 mm fiberreinforced AASS mortar at the dosage of 2%. This result is anindication of serious economic and environmental benefit.

– The contribution of fiber length to the mechanical performanceof AASS mixtures is more pronounced at higher fiber dosages.

– All mechanical properties of AASS mortars increase with thehigher fiber dosages. However, increase in compressivestrength, flexural strength and toughness for 6 mm length fiberis negligible beyond 1.5% fiber dosage. Also, compressivestrength increase for 13 mm length fiber is not significantbeyond 1.5% fiber dosage. AASS mortars that have compressivestrength values over 200 MPa can be produced by using 13 mmlength fiber with a 1.5% fiber content.

– The drying shrinkage values of AASS mortars decreased with theincrease of fiber dosages independent from the fiber length.

– AASS mortars present significantly higher mechanical perfor-mance than PC based mortar at the same fiber dosage. Similarmechanical performance for 1.5% steel fiber reinforced PC mor-tar can be obtained by using 1% steel fiber reinforced AASS mor-tar. This superior performance may be attributed to higher bondproperties between fiber and AASS mortar interfacial zone com-pared to PC mortar.

– As a general conclusion, alkali-activated slag/silica fume basedhigh performance fiber reinforced composites may be a promis-ing low-cost alternative material compare to conventional Port-land cement based fiber reinforced composite.

transition zone (a) AASS and (b) PC.

S. Aydın, B. Baradan / Composites: Part B 45 (2013) 63–69 69

Acknowledgements

This study is a part of a project supported by the Scientific Re-search Council of Dokuz Eylul University (2009.KB.FEN009). Theauthors gratefully acknowledge for this financial support. Inaddition, the authors thank Mr. Mehmet YERL_IKAYA from BEKSA-DRAMIX, and Mrs. Selma CESUR from KARÇ_IMSA for supplyingmaterials.

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