Construction and Building Materials 58 (2014) 31–37

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Construction and Building Materials

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Recycling of waste PET granules as aggregate in alkali-activated blastfurnace slag/metakaolin blends

http://dx.doi.org/10.1016/j.conbuildmat.2014.02.0110950-0618/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +90 388 225 4023; fax: +90 388 225 4229.E-mail addresses: sakcaozoglu@gmail.com, sakcaozoglu@nigde.edu.tr

(S. Akçaözoglu).

Semiha Akçaözoglu a,⇑, Cüneyt Ulu b

a Department of Architecture, Faculty of Architecture, Nigde University, Nigde 51240, Turkeyb Department of Civil Engineering, Faculty of Engineering, Nigde University, Nigde 51240, Turkey

h i g h l i g h t s

� Recycling of waste PET aggregate in alkali activated slag and slag/metakaolin blended mortar was investigated.� The strength values of alkali-activated mortars decreased as the amount of waste PET aggregate increased.� Metakaolin addition caused to decrease of the strength values of alkali activated slag mortars.� PET aggregate can be used in AAS mortar up to 80% replacement of slag aggregate to produce structural lightweight concrete.

a r t i c l e i n f o

Article history:Received 7 January 2014Received in revised form 3 February 2014Accepted 6 February 2014Available online 28 February 2014

Keywords:RecyclingWaste PETAlkali-activatorLightweight concreteBlast furnace slagMetakaolinLiquid sodium silicate

a b s t r a c t

In this study the utilization of waste PET aggregate in alkali-activated slag and slag/metakaolin blendedmortar was investigated. Sodium hydroxide (NaOH) pellets and liquid sodium silicate were used as acti-vators. Eighteen different mortar mixtures were prepared for the laboratory tests. In the reference mix-ture, unground slag (max size of 4 mm) was used as aggregate. In PET aggregate mixtures, slag aggregatewas replaced with waste PET aggregate, in amount of 20%, 40%, 60%, 80% and 100% by volume. The water-binder (w/b) ratio and aggregate-binder ratio used in the mixtures were 0.50 and 2.75, respectively. Theunit weight, compressive strength, flexural tensile strength, ultrasonic wave velocity and water absorp-tion and porosity ratios of the mixtures were measured. The test results showed that, using PET aggregatecontributed to decrease of unit weight of alkali-activated mortars due to the low density of PET aggregate.Although the strength values of the specimens decreased depending on increasing waste PET aggregateamount, the compressive strength values of the alkali-activated slag mortars containing waste PET aggre-gate were satisfactory. In addition, alkali-activated slag mixtures containing 60% and 80% waste PETaggregate were drop into structural lightweight concrete category in terms of unit weight and strengthproperties. However, the compressive strengths of alkali-activated slag/metakaolin blended mixtureswere lower than alkali-activated slag mixtures at the same cure condition. It is concluded from the testresults that there is a potential for the use of waste PET as aggregate in the production of alkali-activatedslag mortar. Because of using waste materials as binder and aggregate for mortar production in this study,alkali-activated slag mortar with PET aggregate is thought to be a good alternative for recycling of wastematerials.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

There is an increasing attention about alkali-activated bindersall over the world due to the potential environmental and energeticbenefits, high strength and superior durability compared to ordin-ary Portland cements (OPC). Alkali-activated binders are classifiedas the third generation binder after lime and OPC and they have

emerged as an alternative to OPC through the use of a clinker-freebinder matrix such as alkali-activated slag, fly ash or metakaolin[1–6]. Alkali-activated binders are characterized by high mechani-cal performance at early ages, high stability in aggressive environ-ments, resistance to elevated temperatures, low energyconsumption and low CO2 emission compared OPC [4,7,8]. Becauseof these advantages, alkali-activated materials are increasinglybeing employed in various applications, such as transportation,industrial, agricultural, residential and mining [6,9,10].

The performance of alkali-activated materials is strongly influ-enced by the characteristics of the precursors and the activation

Table 1Chemical properties of GGBFS and MK.

Oxide(%)

SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O SO3 LOI

GGBFS 36.70 14.21 0.98 32.61 10.12 0.42 0.76 0.99 –MK 51.6 40.18 1.21 1.15 0.20 0.98 0.63 0.03 –

32 S. Akçaözoglu, C. Ulu / Construction and Building Materials 58 (2014) 31–37

conditions [1]. The precursors for alkali-activated binders areindustrial wastes such as blast furnace slag or fly ash which con-tains high amounts of alkali soluble silica and alumina or metaka-olin [11]. Alkali activation of slag is a model of (Si + Ca) system,having C–S–H as the main reaction products, and geopolymer isa kind of (Si + Al) cement with metakaolin and fly ash as mainmaterial, and the reaction products are zeolite like polymers [12].The second group named as geopolymer since they have polymericstructure [6,13]. In recent years, metakaolin has been largely inves-tigated as a geopolymer precursor [14–17]. In addition to thesestudies, there are some investigations about alkali-activated slag/metakaolin blends also available [18–20].

In spite of the fact that blast furnace slag may show little or nocementing properties, alkali-activated slag (AAS) concretes can de-velop very high strength in the presence of a proper alkali activator[21]. AAS based concretes can exhibit similar or better propertiesto OPC concrete in terms of strength and durability in aggressiveenvironment, in addition the high temperature behavior of AASconcrete is superior to their OPC equivalent [11,22–24]. The bondbetween paste and aggregate or reinforcement in AAS concrete ismuch better than OPC concrete [5]. However, AAS concretes oftenshow rapid setting time and exhibit considerably higher shrinkagethan OPC concrete [25–28].

Many factors such as the nature and fineness of slag, the typeand dosage of activators, timing for the addition of activators,chemical and mineral admixtures, addition of lime, water/binderratio, curing conditions and aggregate volume have an effect onthe properties of AAS concretes [5,8,29–31]. The alkali activatornature is the most significant factor [28]. When sodium hydroxide(NaOH) and liquid sodium silicate (Na2SiO3) mixture is used asactivator, the material formed is amorphous and cementitious,but its structure and composition is different from the productformed when NaOH is used alone [6]. Liquid sodium silicate pro-vides the best activation compared to sodium carbonate (Na2CO3)and sodium hydroxide activators in terms of compressive strength.The activation sequence of three alkali activators was following:Na2SiO3�nH2O + NaOH� Na2CO3 > NaOH [27,28]. AAS concretesshow higher compressive strength values than OPC concretes andthe other activated concretes [4].

Hydration process of AAS systems and OPC systems are similarto each other, but mechanisms of hydration are different [21]. Themain reaction product of AAS is C–S–H gel similar to that foundin Portland cement but with lower Ca/Si ratio (around 0.7)[4,5,8,12,32]. The hydration process of AAS is influenced by theNa2O content and the silica modulus (Ms). If higher silicate mod-ulus activator with a higher dosage of Na2O is used, higherstrength is provided [2,8,33]. The strength of AAS systems is alsoaffected by curing condition. Since the reaction activation energyof AAS is higher than that of Portland cement, high temperaturecuring has benefits to improve the strength of AAS concretes[34,35].

Because of the production of OPC in terms of the energy con-sumption and the emission of CO2 has led to the search for moreenvironmentally viable alternatives to Portland cement, blast fur-nace slag and other industrial wastes have been used for mineraladmixture or aggregate in concrete production. The use of blastfurnace slag in concrete production provides ecological advantagesapart from the energy savings and contribution to the strengthproperties of concrete. Even though the cement industry utilizeslarge quantities of blast furnace slag, there is a still great volumeavailable for use as an alternative binder [26]. Since quantity ofblast furnace slag is not enough to spend all stocks and grindingprocess is also required energy, using blast furnace slag as fineaggregate can be helpful to solve this problem [36]. In additionusing slag aggregate can also improve some strength and durabilityproperties of concrete [37–40].

Another waste material which can be used as aggregate in con-crete production is waste Poly-ethylene Terephthalate (PET) bot-tles granules. The amount of PET bottles in the world hasincreased drastically depending on increasing usage. This situationhas caused to the emergence of solid waste problem. In order tofind a solution to this problem, in recent years some studies aboutrecycling of PET wastes have been carried out. Using waste PETbottles as aggregate in OPC concretes or mortars has attractedthe interest of researchers for a recycling application. Some physi-cal and mechanical properties of OPC concretes or mortars withwaste PET aggregates are examined in the studies [41–48]. How-ever there is no study about using waste PET aggregate in alkali-activated concretes or mortars.

The aim of this study is to investigate the usability of waste PETbottles granules as aggregate in the alkali-activated slag and slag/metakaolin blended mortar production. The use of waste PET asaggregate can be a feasible and economical recycling method. Ifproduced alkali-activated mortar including waste PET aggregatehas satisfactory strength properties, it can be a good alternativefor recycled construction materials. Construction industry is oneof the areas of solid wastes can be used in large quantities in orderto reduce the large amounts of natural resources using. The utiliza-tion of waste materials in construction industry may also be help-ful in reducing environmental problems such as reduction oflandfill disposal and safeguard of non-renewable raw materialsand prevention of environmental pollution. In addition usingGGBFS and metakaolin instead of Portland cement for alkali-acti-vated concrete production can be helpful to reduce greenhousegas emissions emerged with Portland cement production.

2. Properties of materials used

2.1. Ground granulated blast furnace slag (GGBFS)

GGBFS was obtained from the Iskenderun Iron-Steel Factory in Turkey. The den-sity and specific surface (Blaine) values of GGBFS were 2.81 g/cm3 and 4250 cm2/g,respectively. The slag activity index at 7 and 28 days were 61.5% and 86.5%, respec-tively [49]. Its chemical oxide composition is presented in Table 1.

2.2. Metakaolin (MK)

A high-reactivity metakaolin (MK) named Metamax was used for replacementGBFS. Metakaolin used meets all of the specifications of ASTM C-618 [50] Class Npozzolans. The density of metakaolin was 2.5 g/cm3. The chemical oxide composi-tion of MK is presented in Table 1.

2.3. Alkali activator

The alkali activators used were laboratory-grade >97% pure NaOH pellets andliquid sodium silicate (LSS). LSS had a SiO2 content of 23.57% and Na2O content of11.45%, and the silicate modulus (Ms) was 2.06. Sodium hydroxide and sodium sil-icate were mixed to provide 4% Na2O by weight of binder and Ms value of 1.

2.4. Unground slag aggregate

Unground slag aggregate was obtained from the Iskenderun Iron-Steel Factoryin Turkey. The maximum size was 4 mm (Fig. 1). The density and water absorptionvalue of unground slag aggregate at saturated surface dry condition were2.49 g/cm3 and 8.7%, respectively. The grading of unground slag aggregate ispresented in Table 2.

Table 2Unground slag aggregate and waste PET aggregate grading.

Particle size range (mm) Unground slag aggregate (%) PET aggregate (%)

dmax dmin

4 2 14.4 13.85

S. Akçaözoglu, C. Ulu / Construction and Building Materials 58 (2014) 31–37 33

2.5. Waste PET aggregate

The waste PET bottles granules used as aggregate were supplied from SASA PETBottles Plant, in Adana, in Turkey. It was obtained by picking up waste PET bottlesand washing then crushing by granules in machines (Fig. 2). Maximum size anddensity of PET aggregate were 4 mm and 1.27 g/cm3, respectively. The grading ofwaste PET aggregate is presented in Table 2.

2 1 38.3 72.601 0.5 30.4 11.550.5 0.25 12.7 1.600.25 0.125 2.9 0.350.125 0.063 0.95 0.050.063 0 0.35 0

100 100

Fig. 2. Waste PET aggregate.

2.6. Experimental program

The mortar mixture proportions were 1:2.75:0.5 by weight of binder, aggregateand water, respectively. Ground granulated blast furnace slag (GGBS) and metaka-olin (MK) were used as binder in the mixtures. MK was used by replacing withGGBFS about 10% and 20% by weight. Unground slag aggregate and waste PETaggregate used in the mixtures. Waste PET aggregate was used at 0%, 20%, 40%,60%, 80% and 100% replacement by volume of unground slag aggregate.

The SiO2/Na2O ratio (Ms) was chosen as 1 and Ms ratio was obtained by addingNaOH to liquid sodium silicate. Na2O concentration in the mixtures was determinedas 4%. The amount of water was adjusted to obtain a 0.5 water/binder ratio for allthe mortar mixes and the water amount in the activator was taken into account.

The activators and water were mixed together prior to mixing to binder andaggregates. Initially, binders and aggregates were dry-mixed for a minute and thenthe activator solution mixture was gradually added during mixing. The mixtureprocedure continued for about 3 min. Prepared fresh mixtures were cast into pris-matic steel molds (40 � 40 � 160 mm). The specimens were kept in laboratory con-ditions (20 �C) for 24 h. Afterwards the specimens were demoulded and placed in ahumidity cabinet at 98% relative humidity and 60 ± 0.5 �C temperature. Summary ofexperimental program is presented in Table 3.

The twelve prismatic specimens were prepared for each mixture for using in thetests. The unit weight, ultrasonic wave velocity and the strength measurementswere performed on three prismatic specimens for each day. The flexural tensilestrength and compressive strength values of mortar specimens were measuredaccording to TS EN 1015-11 [51]. The compressive strength test was carried outat uniaxial compression instrument with a capacity of 200 kN.

The ultrasonic wave velocity measurements of the specimens were conductedat 28 days of age. The ultrasonic non-destructive digital tester with a precision of0.1 ls was employed for the measurements. A transducer was used with a vibrationfrequency of 55 kHz. Sound transit times (t, ls) of specimens were measured withthrough transmission technique according to ASTM C 597-09 [52]. Average of tworeadings was taken for each specimen and ultrasonic wave velocity values (Vs, km/s)were calculated.

The water absorption and porosity ratios of the specimens were measured at28 days according to TS 3624 [53]. Three specimens were used for the measure-ments and the test results were interpreted by comparing to each other.

3. Results and discussion

3.1. Unit weight

The fresh and 28-day dry unit weights of the alkali-activatedmixtures are presented in Table 4. Measured unit weights of freshmixtures were in the range of between 1260 and 2390 kg/m3. The28-day dry unit weight values of the mixtures were between 1120and 2310 kg/m3. The dry unit weights of all alkali-activated mix-tures decreased in course of time due to the evaporation of free

Fig. 1. Unground slag aggregate.

water. The dry unit weight values of the AAS-4, AAS-5 and AAS-6mixtures (containing 60%, 80% and 100% waste PET aggregate,respectively) were lower than 1850 kg/m3; in other words, theywere within the limits of unit weight values specified at ACI213R [54] for structural lightweight concrete.

Since the density of MK used in this study was lower than GBFS,the unit weights of AAS–MK mixtures were lower than AAS mix-tures. In addition as mentioned in the literature that, high Blainefineness of metakaolin caused to workability more difficult inAAS–MK mixtures [20]. This situation led the mixtures have moreporous structure. Therefore the unit weights of the alkali-activatedmixtures decreased depending on increasing metakaolin amount.

The fresh and dry unit weights of all alkali-activated mixturesdecreased by depending on increasing waste PET aggregateamount in the mixture. The reduction of the unit weights can beexplained by the low density of PET aggregate (1.27 g/cm3) com-pared to the unground slag aggregate (2.49 g/cm3).

3.2. Compressive strength

The compressive strength values of the alkali-activated mortarsare presented in Table 4. The 28-day compressive strength of theAAS-1 mixture (containing slag aggregate completely) was70 MPa. This value was quite satisfactory. Wu et al. [55] indicatedthat, AAS mortars have equivalent strengths at one-day to OPCmortars, which also have 28- and 90-day strengths exceedingthose of the OPC mortars. Collins and Sanjayan [56] concluded thatequivalent one-day compressive strength to OPC is achievable with100% alkali-activated slag.

It was observed from Table 4 that the compressive strength ofspecimens decreased by increasing the amount of waste PET aggre-gate replacing the slag aggregate. The ratio of 28-day compressivestrengths of AAS-2, AAS-3, AAS-4, AAS-5 and AAS-6 based on con-trol mixture (AAS-1) were 58.6%, 49.9%, 32.3%, 24.6% and 18.6%,

Table 3Summary of experimental program.

Mix no GGBFS (%) MK (%) Unground slag aggregate (vol.%) PET aggregate (vol.%) w/b Activator type Ms (SiO2/Na2O) Na2O (%)

AAS-1 100 – 100 – 0.50 Na2SiO3 NaOH 1 4AAS-2 100 – 80 20 0.50 Na2SiO3 NaOH 1 4AAS-3 100 – 60 40 0.50 Na2SiO3 NaOH 1 4AAS-4 100 – 40 60 0.50 Na2SiO3 NaOH 1 4AAS-5 100 – 20 80 0.50 Na2SiO3 NaOH 1 4AAS-6 100 – – 100 0.50 Na2SiO3 NaOH 1 4AAS10MK-1 90 10 100 – 0.50 Na2SiO3 NaOH 1 4AAS10MK-2 90 10 80 20 0.50 Na2SiO3 NaOH 1 4AAS10MK-3 90 10 60 40 0.50 Na2SiO3 NaOH 1 4AAS10MK-4 90 10 40 60 0.50 Na2SiO3 NaOH 1 4AAS10MK-5 90 10 20 80 0.50 Na2SiO3 NaOH 1 4AAS10MK-6 90 10 – 100 0.50 Na2SiO3 NaOH 1 4AAS20MK-1 80 20 100 – 0.50 Na2SiO3 NaOH 1 4AAS20MK-2 80 20 80 20 0.50 Na2SiO3 NaOH 1 4AAS20MK-3 80 20 60 40 0.50 Na2SiO3 NaOH 1 4AAS20MK-4 80 20 40 60 0.50 Na2SiO3 NaOH 1 4AAS20MK-5 80 20 20 80 0.50 Na2SiO3 NaOH 1 4AAS20MK-6 80 20 – 100 0.50 Na2SiO3 NaOH 1 4

Table 4The test results of alkali-activated mortars.

Mixture Unit weight (kg/m3) Compressive strength (MPa) Flexural tensile strength (MPa) Ultrasonic wave velocity (km/s)

Fresh Dry 3 days 7 days 28 days 28 days 28 days

AAS-1 2390 2310 53.9 66.9 70.0 6.7 3.32AAS-2 2160 2110 31.8 35.4 41.0 4.6 3.12AAS-3 1940 1930 29.6 33.7 34.9 4.3 2.74AAS-4 1710 1630 18.8 20.0 22.6 3.7 2.16AAS-5 1510 1490 16.3 16.9 17.2 3.4 1.88AAS-6 1340 1300 12.0 12.8 13.0 2.9 1.53AAS10MK-1 2330 2110 7.3 10.9 11.6 2.6 2.48AAS10MK-2 2090 1920 6.6 7.6 9.3 1.5 1.68AAS10MK-3 1840 1740 3.4 5.5 7.5 1.1 1.19AAS10MK-4 1760 1560 3.3 4.9 5.2 1.0 1.00AAS10MK-5 1600 1420 1.4 1.6 2.3 0.9 0.65AAS10MK-6 1300 1170 1.2 1.7 2.2 0.7 0.49AAS20MK-1 2210 2070 3.7 5.1 5.9 1.9 2.05AAS20MK-2 2080 1900 3.3 4.6 5.0 1.1 1.11AAS20MK-3 1920 1670 3.1 3.3 3.4 0.8 0.73AAS20MK-4 1730 1520 2.3 2.4 2.9 0.7 0.61AAS20MK-5 1380 1210 1.9 2.2 2.5 0.6 0.56AAS20MK-6 1260 1120 1.7 1.7 1.8 0.5 –

34 S. Akçaözoglu, C. Ulu / Construction and Building Materials 58 (2014) 31–37

respectively. The main reason for this situation is thought that theconnection between waste PET aggregate and paste did not asstrong as the bond between slag aggregate and paste [41–45]. Adense and strong interfacial zone with the slag aggregate is notedas a result of the slag aggregate participation in the reactions andthe rough surfaces of the slag aggregate [39]. It is also stated inthe literature that, the porous nature of the slag aggregate trappedwater that served as a reserve for further blast furnace slag reac-tions and strength gain [24]. In addition increasing PET aggregateamount in the mixtures caused to difficulty in workability, this sit-uation also caused more porous structure. In this case, increasingPET aggregate amount in the alkali-activated mixtures caused todecrease on the compressive strength values of the mixtures sim-ilar to literature [41].

The compressive strength values at 28-day of AAS mixtureswere higher than structural lightweight concrete limit value of17 MPa [54]. However, only AAS-4 and AAS-5 mixtures (contain-ing 60% and 80% PET aggregate) were drop into structural light-weight concrete category. Because, the unit weight of AAS-1,AAS-2 and AAS-3 mixtures were higher than 1850 kg/m3. The28-day compressive strength value of AAS-6 mixture containing100% waste PET aggregate remained below the specifiedvalue.

The compressive strengths of all alkali-activated mixtures in-creased depending on the time. However, the compressivestrengths of the mixtures in this investigation developed rapidlyat early ages up to 7 days, after 7 days the speed of compressivestrength developments slowed down up to 28 days. This resultwas found to be different from strength development of OPCconcretes. It was stated in the literature that, AAS based concretesreach high mechanical strength at early ages [1,2,7,8,27,30,33,55–57]. High alkali activator content causes rapid strength gain after3 days. In addition AAS concretes show higher ultimate strengthsthan OPC. [21]. It was clearly seen at Table 4 that, in this investiga-tion, 7-days compressive strength values of the alkali-activatedmortars were above 74% of 28-day compressive strength values.As a result, early strength of AAS and AAS–MK mortars producedin this investigation was very high. This situation was similar forall alkali-activated mortars containing both slag aggregate andwaste PET aggregate.

In this investigation, produced AAS mixtures showed satisfac-tory strength values at the cure temperature about 60 ± 0.5 �C.However, the compressive strengths of AAS–MK mixtures werelower than AAS mortars at the same cure condition. It was thoughtthat, as the slag content decreased in the MK blended mixtures, thecompressive strength values of the produced alkali-activated

Fig. 3. The relationship between the 28-day compressive strength and flexuraltensile strength of the alkali-activated mixtures.

S. Akçaözoglu, C. Ulu / Construction and Building Materials 58 (2014) 31–37 35

mortars decreased [18]. Increasing MK content causes significantreductions in compressive strength, as a consequence of theincomplete reaction of the MK incorporated AAS mixtures, becauseof low activator concentration [1,19,58]. Also, MK addition had dif-ficulty on the workability of the mixtures. In this case the AAS–MKmixtures had more porous structure. It was another reason to thelower compressive strength of AAS–MK mixtures.

The other reason of this situation was the cure conditions thatadversely affected the reaction of MK. In this investigation, initiallyproduced fresh AAS and AAS–MK mixtures filled into the moldsand they were kept in laboratory conditions (20 �C) for 24 h. After24 h, the specimens were demoulded and cured in a humidity cab-inet at 98% relative humidity and 60 ± 0.5 �C temperature until thetest times. It is stated in the literature that high cure temperatureand high activator dosage are required to achieve high strength ofalkali-activated MK based systems [9]. Curing regime has great im-pact on mechanical strengths of alkali-activated slag–MK basedsystems. Alkali-activated slag–MK based mixtures under standardcuring regime have the lowest strength [59]. Aguilar et al. [17]indicated that, the curing of alkali-activated metakaolin mortarsat 75 �C for 24 h favored a rapid development of strength at1 day, but without a significant increase afterwards at 20 �C. Ra-shad [14] stated that, although it is stated that the optimum curingtemperature of alkali-activated metakaolin blend is around 60 �C,this curing temperature depends on many factors as MK fineness,activator type and dosage, etc. It was thought based on these re-sults that, if produced fresh AAS–MK mixtures were cured at high-er temperature (above 60 �C) immediately after casting into molds,they could show better compressive strength values. In additionusing higher dosage of activator could have positive effect on thecompressive strength values of AAS–MK mixtures.

3.3. Flexural tensile strength

The 28-day flexural tensile strength values of the AAS andAAS–MK mixtures are presented in Table 4. The flexural tensilestrengths of all alkali-activated mixtures decreased depending onincreasing PET aggregate amount. In addition the flexural tensilestrength values of AAS–MK mixtures were lower than AASmixtures. These results were similar to the compressive strengthresults of the mixtures in this study.

The relationship between the compressive strength and flexuraltensile strength values of alkali-activated mixtures is presented inFig. 3. The correlation coefficient of relationship was found to be0.91. It can be concluded from this result that there was a linearrelationship between compressive strength and flexural tensilestrength of the AAS and AAS–MK mixtures.

Fig. 4. The relationship between the compressive strength and ultrasonic wavevelocity values of the alkali-activated mixtures.

3.4. Ultrasonic wave velocity

Ultrasonic wave propagation speed in a material depends onthe porosity of that material; therefore it depends on the densityand elastic properties. The effects of waste PET aggregate andmetakaolin addition on the ultrasonic wave propagation of theAAS mixtures were evaluated by ultrasound measurements carriedout in this investigation. Ultrasonic wave velocities of AAS andAAS–MK mixtures at 28-days are given in Table 4. It was seen inTable 4 that, the ultrasonic wave velocity values of the mixturesdecreased as the amount of PET aggregate increased in mixture.This is attributed to the decrease in unit weight with the increaseof amount of PET aggregate replacement in mixture. Because, whilethe compactness and density of concrete decreases, the ultrasonicwave velocity and strength of concrete decrease together [60,61].In addition increasing MK content in the mixtures caused a de-crease at the ultrasonic wave velocity values.

The relationship between compressive strength and ultrasonicwave velocity of AAS and AAS–MK mixtures at 28 days is presentedin Fig. 4. A roughly linear relationship was observed between com-pressive strengths and ultrasonic wave velocities of mixtures andthe correlation coefficient of relationship was found as 0.79. Whilethe compressive strength values of the mixtures decreased, theultrasonic wave velocity of values decreased in similar manner.Since increasing PET aggregate amount caused difficulty in work-ability, specimens had more porous structure. This situationcaused in a reduction in the compressive strength and ultrasonicwave velocity of specimens. Demirboga et al. [62] reported that,there is a good relationship between ultrasonic wave velocityand compressive strength. Based on these results, it was consid-ered that there was potential of using ultrasonic wave velocitiesof specimens to predict the compressive strength of alkali-acti-vated mortars containing waste PET aggregate.

3.5. Water absorption and porosity ratio

The water absorption and porosity ratios of the AAS andAAS–MK mixtures at 28 days are given in Figs. 5 and 6, respectively.The water absorption ratio of the AAS mixture containing only slagaggregate (AAS-1) was 8.3%. This value increased depending onincreasing PET aggregate amount in the mixtures and the waterabsorption ratio of AAS-6 mixture (containing 100% PET aggregate)increased to 20.2%. In addition, increasing PET aggregate amountcaused more porous structure at all mixtures. The porosity ratio ofAAS-1 mixture increased from 18.1% to 30.9%, when the PETaggregate amount was 100% (AAS-6). Increasing porosity ratios of

Fig. 5. Water absorption ratios of alkali-activated mixtures.

Fig. 6. Porosity ratios of alkali-activated mixtures.

36 S. Akçaözoglu, C. Ulu / Construction and Building Materials 58 (2014) 31–37

the mixtures also caused to increase at water absorption ratios ofthe mixtures. The water absorption and porosity ratios of AAS–MKmixtures were higher than AAS mixtures. The more porous structureof AAS–MK mixtures caused more water absorption values. How-ever, the water absorption values of the all alkali-activated mixtureswere found to be under the water absorption limits of lightweightconcrete [63].

4. Conclusions

All alkali-activated slag (AAS) mortars produced in this investi-gation reached high mechanical strength at early ages. The AASmortars including 60% and 80% waste PET aggregate were drop intostructural lightweight concrete category. It can be concluded thatthe use of waste PET aggregate in AAS mortars up to 80% replace-ment of slag aggregate can be suitable to produce structural light-weight concrete. On the other hand, the compressive strengthvalues of all AAS–MK mortars were under the structural light-weight concrete category.

The unit weight, compressive strength, flexural tensile strengthand ultrasonic wave velocity values of alkali-activated mortars de-creased by depending on increasing waste PET aggregate amountin the mixtures. In addition, waste PET aggregate addition causedto increase on the water absorption and porosity ratios of themixtures.

The linear relationship between the compressive strength, flex-ural tensile strength and ultrasonic wave velocity values of all alka-li-activated mortars observed in this study. There was potential ofusing ultrasonic wave velocity which is a non-destructive testmethod to predict the compressive strength of alkali-activatedmortars.

Conclusively, it is thought that alkali-activated slag mortarscontaining waste PET aggregate may be a good alternative material

for recycled construction applications. In addition, the use ofwastes as aggregate and binder for concrete production will alsobe helpful to safe of natural resources and prevention of environ-mental pollution.

Acknowledgement

The authors would like thank Nigde University ScientificResearch Projects Unit that supported the present work (ProjectNumber: FEB 2012/31).

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