Construction and Building Materials 264 (2020) 120268

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Construction and Building Materials

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The influence of activator type and quantity on the transport propertiesof class F fly ash geopolymer

https://doi.org/10.1016/j.conbuildmat.2020.1202680950-0618/� 2020 Elsevier Ltd. All rights reserved.

⇑ Corresponding author.E-mail address: ismailatabey@nevsehir.edu.tr (_I._I. Atabey).

_Ismail _Isa Atabey a,⇑, Okan Karahan b,c, Cahit Bilim d, Cengiz Duran Atis� b

aNevs�ehir Hacı Bektas� Veli University, Civil Engineering Department, Nevs�ehir, Turkeyb Erciyes University, Civil Engineering Department, Kayseri, TurkeycKayseri University, Civil Engineering Department, Kayseri, TurkeydMersin University, Civil Engineering Department, Mersin, Turkey

h i g h l i g h t s

� Transport properties of class F fly ash based geopolymer were investigated.� Influence of activator type and amount on transport properties of geopolymer were reported.� The best results were obtained at %15 Na ratios for all transport characteristics.� The findings could help for selection of Na ratio and activator type for geopolymers.

a r t i c l e i n f o

Article history:Received 1 April 2020Received in revised form 9 July 2020Accepted 13 July 2020

Keywords:GeopolymerClass F fly ashTransport propertiesActivator typeActivator ratios

a b s t r a c t

It is known that transport properties of porous materials were important in terms of its durability aspect.In this paper, the effect of activator type, Na amount on the transport properties of geopolymer mortarmade with class F fly ash were explored. Two different class F fly ash were employed in producinggeopolymer that activated alkaline solutions. As alkali activator, sodium hydroxide solution, and combi-nation of sodium silicate with sodium hydroxide were used. Activator contained 6%, 9%, 12% and 15% offly ash weight as Na amount. Heat curing regimes were imposed to specimens at 100 �C temperature for24 h duration. Tests were carried out on the geopolymer samples including water absorption, volume ofpermeable voids, sorptivity, depth of penetration of water under pressure, chloride ion penetration andaccelerated corrosion. It is found that the transport properties of geopolymer samples are improved byincreasing the Na amounts of mortar mixtures from 6% to 15%. In general, better results were obtainedwith 15% Na ratio. The mortars produced with only sodium hydroxide have shown better transport prop-erties than mortars made with combination of sodium hydroxide-sodium silicate mixture.

� 2020 Elsevier Ltd. All rights reserved.

1. Introduction

Cement is a common binder as construction materials. TheOrdinary Portland Cement (OPC) production needs limestone andclay as raw materials, and temperature reaches to 1450 �C in theproduction process. For each ton of production of OPC is responsi-ble for formation of 1 kg sulphur dioxide (SO2), about 810 kg of car-bon dioxide (CO2), and 10 kg waste powder [1]. In estimate, it isknown that the production of fly ash-based geopolymer needsfor nearly 60% less energy and has at least 80% less CO2 emissions[2] in comparison to production of OPC. There are lots of studiesand developments focused on the geopolymer concrete and mortar

in literature. Geopolymer is thought to be a sustainable buildingmaterial for the future [3]. The production of fly ash-basedgeopolymer has lower CO2 emission compared to that of OPC [4–6]. Fly ash-based geopolymer concrete completely moves awayfrom OPC and the high CO2 emission associated with OPC produc-tion [6–8].

A term geopolymer was first introduced by Davidovits in 1979to represent the inorganic polymers. The production of geopolymerrequires source materials that are rich in silica and alumina con-tent, such as metakaolin, slag, fly ash, etc. [9]. Geopolymers canprovide an alternative to OPC binders, not only for the environ-mental benefits, but also in terms of their strength and durability[10].

Fly ash based geopolymer mortar need heat curing for geopoly-meric reaction. This can be a drawback for production of geopoly-

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mer as the industrial material [10]. The properties of geopolymersare usually dependent on the characteristics of the alumina-silicatesource materials i.e. content and particle size. Fineness, the contentof glassy phase particle morphology, chemical property of thewaste materials have important effect on the activity of the alumi-nosilicate sources. The alkaline solutions such as sodium hydroxide(NaOH), sodium silicate (Na2SiO3), and potassium hydroxide (KOH)or potassium silicate are commonly used alkali materials in prepa-ration of geopolymers. In comparison, NaOH and KOH indicate agreater level of alkalinity. It has been found that NaOH possessesgreater capacity to release silicate and aluminate monomers[11,12]. The activator can also be prepared by mixing NaOH andnSiO2Na2O. The most important factor in the mixing of NaOHand nSiO2Na2O is adjustment of the SiO2/Na2O ratio called the sil-ica modulus (MS) [13]. Fly ash based geopolymers need heat curingfor 24–28 h, at temperature range from 60 to 100 �C. After heatcuring in oven, the geopolymer can be subjected to longer curingat ambient room temperature [1].

Transport properties of fly ash based geopolymer were studiedby water and ion transportation form. The most significant factorswhich influence water and ion transport are materials pore struc-ture, pore volume, pore size distribution, connectivity and shape ofthe pores [14,15]. The knowledge of water transport and its rela-tionship with microstructural properties is an important [16]. Inthe evaluation of the durability related properties of a geopolymeras an alternative binder, it is important to know its transport prop-erties and microstructural properties and their relationships.Water penetration and ion diffusion properties of materials arerelated with pore structure of porous materials. These durabilityrelated properties are closely related to durability problems, inparticular for reinforced concrete systems. Water is a carrier forCl�, SO4 or CO2. These ions are malicious to concrete and its rein-forcement since they can take place in the reaction products and/or reach the steel/paste interface. These ions cause corrosion ofsteel and lead to deterioration of the reinforcement. Therefore, asdurability related properties, water transport and permeabilityare essentially considered as important parameter [11,13].

For this purpose, there are several studies focused on the trans-port properties of fly ash based geopolymer mortar. The high sorp-tivity coefficient indicates the presence of a highly porousstructure or pore network [9].

It is reported that properties of the fly ash-based geopolymerdependant on activator amount and characteristics of fly ash used.Heat curing temperature and its duration are other important fac-tors [6,17].

Geopolymeric mortars presented higher durability relatedproperties than OPC mortars. Particularly, this is important whenthey were attacked by chlorides, sulphates, acids or alkali-silicareactions [18–20]. Attaching steel reinforced bars is an importantrisk facilitated by the proximity steel bars to the concrete surfaceand are very susceptible to corrosion [19]. Chloride ions penetrateconcrete and form HCl acids, reduce pH value of environment. Thelow pH value environment causes corrosion on the steel bar. Iron(Fe) conversion to Fe(OH)x, creating volume expansion that devel-ops cracks in concrete [21]. Pore structure of porous materials is animportant parameter that influences the transport of ions. Thegeopolymerization process depends on many parameters thatcan influence the transport properties and durability related prop-erties of a geopolymer composite [9]. The size distribution of flyash supplied from different thermal power plants has a significanteffect on demand of water. It also affects pore structure develop-ment of geopolymer [21–23].

In the literature, various studies on geopolymer are mainlyfocused on mechanical properties of geopolymer. Some studiespresented results on the transport properties of fly ash geopoly-

mer; however, there is not much study on chloride penetrationand steel corrosion for geopolymer.

Zhu et al. [21] investigated the durability related properties ofchloride penetration of fly ash geopolymer pastes and mortarsand compared them with OPC counterpart. They highlighted thesignificance of utilizing fly ash role with appropriate liquid/solidratio in the development of durable fly ash geopolymer.

Noushini and Castel [9] carried out a study to investigate thetransport properties of geopolymer produced using class F fly ashand at different medium. They concluded that heat cured fly ashgeopolymer at 75 and 90 �C up to 24 h, developed lower sorptivitycoefficient than that of samples cured at ambient conditionsthereby indicating lower porosity values.

Reddy et al. [24] focused on the durability properties ofgeopolymer made with class F fly ash. Samples were subjected toa corrosive marine environment. They were tested geopolymerconcrete beams with 13-mm rebar centrally reinforcement. Flyash geopolymer was produced with 8 M and 14 M alkali solutionsof NaOH and Na2SiO3. Accelerated corrosion test was carried out byexposing samples to wet and dry cycling periods in artificial sea-water and an induced current. They concluded that class F fly ashgeopolymer concrete showed higher resistance than that of controlcement concrete to corrosion cracking due to chloride attack.

Chindaprasirt and Chalee [25] were studied the influence ofsodium hydroxide concentrations on chloride penetration, steelcorrosion and strength of fly ash geopolymer under marine envi-ronment. They prepared geopolymer by activating fly ash withblend of Na2SiO3/NaOH mixture solution. Molar concentrations ofsodium hydroxide were changed from 8 to 16 with an incrementstep of 2. Silica to alumina rate was kept constant. They reportedthat increase in molar ratio of alkali activator result with lowerchloride penetration as well as corrosion of embedded steel.

Meesala et al. [26] in review paper, they expressed that struc-ture of raw material, alkaline concentration and its type, curingtemperature and technique of curing, time of heat curing, water/binder ratio and rest period influence the performance of geopoly-mers. Investigations on durability and long-term aspects of FAbased geopolymer composites are scarce. Hence, they emphasizedmore investigations need to be conducted in this area. Theyreported that the volume of permeable voids and water absorptionof geopolymer samples were less than 5% and 12% respectively,and these values stay under the classes of low.

Findings of Gorhan and Kurklu [27] indicated that increase inheat curing time caused decrease in water absorption values ofgeopolymer mortars produced with fly ash. They reported thatalkali solution concentration influenced the water absorption valueof samples. The increase in curing duration result with a reductionof geopolymer mortars porosity values whilst alkali solutions con-centrations affected both the porosity values and reaction process.

Ma et al. [14], concentrated and pointed on the influence of cur-ing temperature and curing duration on pore structure and waterpermeability. They reported that longer heat curing duration couldsignificantly reduce water permeability of fly ash based geopoly-mer. Moreover, they stated that, higher silica content remarkablycauses a reduction in water permeability at longer term, whereashigh alkali concentration showed more pronounced influence onwater permeability at long term.

Zhuang et al. [6] carried out an experimental studied on fly ashbased geopolymer. They stated that fly ash based geopolymer wasa good binder and can be utilized in production of geopolymer con-crete. Fly ash-based geopolymer binder represented better behav-ior than that of traditional cement. For instance, fly ash-basedgeopolymer concrete developed denser microstructure causinglower porosity. Therefore, it results with lower chloride diffusionin comparison to cement concrete.

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In the literature, several studies are dealing with the permeabil-ity properties of geopolymers. On the other hand, there is scanty ofstudy on the comparative transport properties of geopolymer com-positions. The effects of a number of major factors such as the useof chemical activator, particle size distribution and chemical prop-erties of powder materials and aggressive environment exposureon the transport properties of the geopolymer mortar are exhaus-tively deliberated. A series of durability property tests were carriedout in this study on geopolymer mortar specimens, to evaluate theeffect of permeable and capillary voids after exposure to 100 �Ctemperature. Four different Na ratio of NaOH and Na2SiO3-NaOHconcentration were used in order to determine the effect of activa-tor concentration and type on geopolymer mortars. Two different Ffly ashes were used in order to determine the effect of fly ash onphysical and chemical properties on geopolymer mortars.

Geopolymer mortars were produced from two different Class Ffly ashes obtained from two different Thermal Power Plants in Tur-key. NaOH and blend of Na2SiO3-NaOH were used as separate alka-line activators. Geopolymer mortars were produced at 6%, 9%, 12%and 15% Na amount of fly ash. They were subjected to 100 �C heatcuring for 24 h. Water absorption, volume of permeable voids(VPV), sorptivity, depth of penetration of water under pressure,chloride ion penetration, and accelerated corrosion tests weretaken place on the geopolymer samples produced. Thus, the effectof activator type, composition of fly ash and activator ratio ontransport properties were evaluated.

2. Materials and methods

2.1. Fly ash

Two different class F fly ashes [28] were used in the study. Thepozzolanic activity index of the Sugözü (SG) fly ash obtained fromthe Sugözü thermal power plant is 87%, its specific gravity is 2.29,and its residue on 45 mm sieve is 19%. On the other hand, the poz-zolanic activity index of the Çatalagzı (CA) fly ash obtained fromthe Çatalagzı thermal power plant is 83%, its specific gravity is2.13 and its residue on 45 mm sieve is 29%. Chemical propertiesof fly ashes used are given in Table 1.

2.2. Standard sand

Rilem Cembureau Standard Sand in accordance with TS EN 196-1 [29] was used for geopolymer sample production. The dry speci-fic gravity of the sand is 2.63 and the water absorption rate is0.57%. Sieve analysis result and standard limits are given in Table 2.

2.3. Activator and water

In the experimental study, two different alkaline solutions wereprepared, consisting of a mixture of sodium hydroxide (NaOH) anda blend of sodium silicate-sodium hydroxide (Na2SiO3-NaOH) toactivate the fly ash. In appearance, NaOH (NH) is in a white flakeform and Na2SiO3 (NS) is a transparent liquid. In addition, sodiumhydroxide in Na2SiO3-NaOH solution contributed to lowering theMS module (SiO2/Na2O) of the solution. The purity of NaOH is98.3%. The specific gravity of liquid sodium silicate is 1.4 g/cm3,MS module is 2.02. Drinkable tap water obtained from the city net-

Table 1Chemical properties of fly ashes.

Oxide SiO2 Al2O3 Fe2O3 CaO M

Sugözü (SG) 60.51 21.69 7.85 1.52 1Çatalagzı (CA) 54.68 25.94 6.81 3.12 1

work was used in the production of mortars. Its chemical and phys-ical properties were complied with TS EN 1008 [30].

3. Experimental program

3.1. Casting and curing of test specimens

CEN standard sand, class F fly ashes and two different activators(NH and NH-NS) were used in geopolymer mortar production.Sand / fly ash ratio was taken as 3 and water/fly ash ratio was takenas 0.3. Na dosage for the activator was chosen 6%, 9%, 12% and 15%of weight of fly ash. The most suitable mixture not showing rapidsetting was the mortar made with MS of 0.2 in for sodium silicate-sodium hydroxide solution.

As the activator solutions were hot, they were cooled in glassjars in a laboratory environment until they reached room temper-ature. Following that, a Hobart type mixer was used in the prepa-ration of the mixes. Then, fresh mortars were placed in molds.Geopolymer mixtures were given in Table 3 [29,31].

The mortar samples placed into the molds were cured in anoven at 100 �C temperature for 24 h. Three specimens for eachNa ratio were subjected to transport properties tests. Results wasobtained based on the average of values obtained from three sam-ples. After heat curing period, 16 groups of samples were removedfrom the mold after cooling to room temperature and they weresubjected to experiments.

3.2. Test procedures

The prism specimens with size of 40 � 40 � 160 mm3 wereused to determine water absorption, volume of permeable voidsand sorptivity coefficient in the mortars. While water absorptionand apparent volume of permeable voids tests were done as perASTM C642 [32], to determine sorptivity coefficient of the samples,the weight measurements were performed at 1, 5, 10, 20 and30 min, at 1, 2, 3, 4, 5 and 6 h, and at 1, 2, 3, 4, 5, 6, 7 and 8 daysaccording to ASTM C1585 [33].

150 � 150 � 150 mm3 cube samples were produced in accor-dance with the TS EN 12390-8 [34] for the penetration depth ofwater within 24 h under pressure.

Cylinder samples with Ø100 � 200 mm size were produced forchloride ion penetration test which was carried out according toASTM C1202 [35]. The chloride ion permeability value in Coulombwas determined depending on the resistance of the samples tochloride ion penetration.

For accelerated corrosion testing, the ribbed S420 steel having16 mm diameter was used. The pull-out samples measuredØ100 � 200 mm in cross section. Pull-out cylinders had a600 mm length rod 16 mm diameter, placed centrally in the cylin-der, with an embedment length of 175 mm. Corrosion resistance ofreinforced steel was measured by applying 12 V constant currentin NaCl solution. The bond strength of the reinforcements wasdetermined according to ASTM A944 [36].

gO SO3 Na2O Free CaO Cl- LOI

.65 0.53 0.92 0.183 0.010 2.42

.55 0.34 0.39 0.020 0.004 3.30

Table 2Sieve analysis of Rilem sand and limit values.

Property Sieve diameter (mm)

2.00 1.60 1.00 0.50 0.16 0.08

Remaining (%) 0.0 3.1 32.7 68.9 87.1 99.6Limits of Specification (%) 0 7 ± 5 33 ± 5 67 ± 5 87 ± 5 99 ± 1

Table 3Geopolymer mortar mixtures compositions.

Mix No Fly ash type Flyash, g Sand g Activator type Na concentration, % Total water, g NaOH, g Na2SiO3, g

1 SG/CA 450 1350 NaOH 6 135 46.96 –2 SG/CA 450 1350 NaOH 9 135 70.43 –3 SG/CA 450 1350 NaOH 12 135 93.91 –4 SG/CA 450 1350 NaOH 15 135 117.39 –5 SG/CA 450 1350 Na2SiO3-NaOH 6 135 42.16 28.276 SG/CA 450 1350 Na2SiO3-NaOH 9 135 63.25 42.407 SG/CA 450 1350 Na2SiO3-NaOH 12 135 84.33 56.538 SG/CA 450 1350 Na2SiO3-NaOH 15 135 105.41 70.66

Fig. 2. Volume of permeable voids results of samples.

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4. Experimental results

4.1. Absorption and volume of permeable voids results

The water absorption and volume of permeable voids (VPV) val-ues of mortars were given in Figs 1 and 2, respectively.

For SG fly ash geopolymers mortars activated with NH, waterabsorption values were determined between 1.4% and 5.2% whileVPV values were between 3.2% and 10.7%. For SG fly ash geopoly-mers mortars activated with NS-NH, water absorption values weredetermined between 2.8% and 5.3% while VPV values werebetween 6.0% and 11.1%. On the other hand, for CA fly ash geopoly-mers mortars activated with NH, water absorption values weredetermined between 2.9% and 6.4% while VPV values werebetween 6.0% and 12.8%. For CA fly ash geopolymers mortars acti-vated with NS-NH, water absorption values were determinedbetween 4.4% and 6.5% while VPV values were between 9.2% and13.2%. It can be seen from Figs 1 and 2 that the increase in theNa ratio result with a decrease in the water absorption and VPVvalues of the geopolymers produced regardless of ashes.

As seen in Figs 1 and 2, geopolymers produced with SG fly ashwere found to be better than that of geopolymers produced withCA fly ash in terms of water absorption and VPV values. Noushiniand Castel [9] reported that significant reductions occurred in voidvolume with increasing curing time from 8 h to 24 h as well as atemperature rising from 60 �C to 90 �C [9]. Zhu et al. [21] statedthat the hardened mortars were affected by the void structuredue to the grain shape and size of the fly ash supplied from differ-

Fig. 1. Water absorption results of samples.

ent sources. In addition, it could be said that water absorption andVPV values of NH activated mortars were better than those of mor-tars made with NS-NH as an activator in fly ash geopolymers. Sim-ilarly, Zhang et al. [1] demonstrated that mortars containing NHexhibited more positive properties than mortars prepared withNS-NH mixture [1].

4.2. Sorptivity results

In the study, the geopolymer samples were subjected to capil-lary water absorption test at room temperature. In this experimentwhere the test setup was shown in Fig. 3, the bottom surfaces ofthe samples were contacted with water. After that, the primarycapillary water absorption coefficient after 6 h and the secondary

Fig. 3. Sorptivity test setup.

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capillary water absorption coefficient at the end of the 8th daywere calculated [33]. The results obtained are given in Fig. 4.

As given in Fig. 4, initial (In) capillary water absorption coeffi-cients were calculated between 0.0116 and 0.0303 and secondary(Sec) capillary water absorption coefficients were calculatedbetween 0.0012 and 0.0014 for SG fly ash geopolymers activatedby NH. The primary capillary water absorption coefficients of SGfly ash geopolymers activated by NS-NH mixture were calculatedbetween 0.0067 and 0.0281 while the secondary capillary waterabsorption coefficients were calculated between 0.0013 and0.0022. The high Na dosage reduced primary capillary waterabsorption values as a result of the decrease in porosity. However,there was no significant change in secondary capillary waterabsorption values depending on Na dosage. The use of NS-NHinstead of NH as an activator for the SG fly ash group resulted inan increment in capillary water absorption.

On the other hand, primary capillary water absorption coeffi-cients ranged from 0.0208 to 0.0336 and secondary capillary waterabsorption coefficients ranged from 0.0015 to 0.0019 for CA fly ashgeopolymers activated by NH. The primary capillary water absorp-tion coefficients of CA fly ash geopolymers activated by NS-NHmixture ranged from 0.0235 to 0.0314 while the secondary capil-lary water absorption coefficients ranged from 0.0012 to 0.0015.As with SG fly ash, primary capillary water absorption decreasedwith the increase in Na ratio, but there was no decrease in sec-ondary capillary water absorption. The reason for this is thatgeopolymers are produced as a result of heat curing and gain theirsensitivity to water in primary sorptivity time.

In general, primary capillary water absorption values decreasedfrom 6% to 15% with the increase in Na ratio in activation of SG andCA fly ash with NH. In mortar specimens activated by both NH andNS-NH, the primary capillary water absorption coefficient is thelowest for 15% Na dosage. In geopolymers produced with SG andCA fly ash, the secondary capillary water absorption coefficienthas not changed much with the increase of Na and close valueshave been obtained. As the geopolymer samples were cured inthe oven, they lost the water in their structures. So, mortars tendedto absorb water in a short time for primary capillary water absorp-tion, this situation did not produce very large differences comparedto secondary capillary water absorption.

4.3. Depth of penetration of water under pressure results

The highest level of water penetration was measured after split-ting test of cube specimens. The penetration depths of water under

Fig. 4. Sorptivity result

pressure were given in Fig. 5 for geopolymer mortars, and the testsetup was shown in Fig. 6.

The penetration depths of water under pressure for the NH-activated SG fly ash geopolymers were measured between 9 and150 mm, while these values were between 18 and 150 mm inthe mixture with NS-NH. Also, the penetration depths of waterunder pressure for the NH-activated CA fly ash geopolymers weremeasured between 11 and 150 mm, while these values werebetween 21 and 150 mm in the mixture with NS-NH. The mosteffective activator concentration for low water penetration depthwas 15% Na dosage. So, these values for SG and CA fly ash werebetween 9 and 11 mm in activation with NaOH and between 18and 21 mm in activation with NS-NH mixture. The most effectiveactivator for low water penetration depth was NH at 15% Nadosage, which resulted in 9 mm depth for SG fly ash. The fact thatthe physical and chemical properties of SG fly ash are slightly bet-ter than CA fly ash also contributed to this situation. The low acti-vator dosage negatively affected the permeability. Water underpressure reached the top of the samples with SG and CA fly ashactivated by 6% Na for both activators. Water reached almost thetop surface (146 and 138 mm) in SG and CA fly ash samples acti-vated by NH at 9% Na. However, the sample activated with NS-NH at 9% Na let to pass water entirely. Water penetration depthunder pressure was measured at 18 and 27 mm in SG and CA flyash samples activated with NH at 12% Na, and 69 and 48 mm insamples activated with NS-NH mixture. Accordingly, those thatwere activated with NaOH were observed to give better results.The Na dosage increasing from 6% to 15% reduced the penetrationdepth of water under pressure in SG and CA fly ash samples acti-vated by both NH and NS-NH. Samples produced with SG fly ashhad better permeability values than CA fly ash for high Na ratio.The reason for this is that SG fly ash has a less value (18.47%) thanCA fly ash (29%) in terms of the residue on 45 mm sieve [37]. It isalso stated that fly ash improves the microstructure and positivelyaffects the capillarity properties in concrete depending on thereplacing with cement [38]. In addition, low pore structuregeopolymer mortar can be obtained with fly ash. Amran et al. sta-ted that 80% of total porosity has presented in pore diameters of10–50 nm in alkali-activated binder containing of 100% fly ash.In addition, 20% is between 10 and 100 nm [39]. As seen inFig. 7, at the same Na ratio and activator type and the physicalproperties of different fly ashes, depth of penetration of waterunder pressure is similar to the porosity. The decline in the depthof penetration of water pressure values with an increase in NH andNS-NH concentrations were noticeable at this stage. The increasein concentration of activator by reducing w/b (0.30) caused an

s of geopolymers.

Fig. 5. Depth of penetration of water under pressure results of geopolymers.

Fig. 6. Depth of penetration of water under pressure test setup (TS EN 12390-8).

Fig. 7. The relationship between depth of penetration of water under pressure –VPV.

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important acceleration of dissolution rate that it also reducedporosity [40].

The total depth of penetration in mortars with 6%-15% all NHconcentrations were determined 649 mm, but all NS concentra-tions were determined 756 mm. Depth of penetration of waterunder pressure values are generally correlated with VPV and waterabsorption values (Fig. 7). According to activator type, geopolymer-ization reactions increase at the optimum temperature and curingtime. 100 �C curing temperature, 24 h curing time and concentra-tion of NH were directly affected porosity and depth of water pen-etration. It was emphasized in the previous works: decrease in thewater absorption of the samples with increase in the NH concen-tration was also observed [27,40,41]. Additionally in literature,lower porosity and water absorption were determined whenhigher Na ratio content activators were used in the production ofgeopolymer samples [27,42,43]. A higher concentration of NHhad better ability to dissolve fly ash particles and resulted in bettergeopolymerization [44].

4.4. Chloride ion penetration results

The rapid chloride ion penetration results for geopolymer mor-tars were given in Fig. 8 and the test setup was shown in Fig. 9.

As given in Fig. 8, rapid chloride ion penetration results of SG flyash geopolymers mortars activated by NH were determinedbetween 391C and 7212C, while rapid chloride ion penetrationresults were determined between 376C and 6047C for SG fly ashgeopolymers mortars activated with NS-NH. For both NH andNS-NH, SG fly ash geopolymers activated with 6% and 9% Na werein the high chloride permeability class according to ASTM C1202[35] since they exceeded 4000C limit. Additionally, SG fly ashgeopolymer specimens activated by 12% Na were low class whilethose activated by 15% Na were very low class in terms of rapidchloride ion penetration as per this standard.

On the other hand, rapid chloride ion penetration results of CAfly ash geopolymers mortars activated by NH were determinedbetween 699C and 7254C, while rapid chloride ion penetrationresults were determined between 649C and 6561C for CA fly ashgeopolymers mortars activated with NS-NH. For both NH andNS-NH, CA fly ash geopolymers activated with 6% and 9% Na werein the high chloride permeability class according to ASTM C1202[35] since they exceeded 4000C limit. Additionally, CA fly ash

Fig. 8. Rapid chloride permeability resu

geopolymer specimens activated by 12% Na were low class whilethose activated by 15% Na were very low class in terms of rapidchloride ion penetration as per this standard.

Though high chloride penetration was observed in geopolymersamples at 6% and 9% Na dosages, rapid chloride ion penetration inalkali-activated samples decreased with Na dosage increasing from6% to 15%. So, for the SG and CA fly ash geopolymer mortars acti-vated by NH and NS-NH, low and very low chloride ion penetrationresults were obtained at 12% and 15% Na, respectively. Zhu et al.stated in their study that chloride penetration depended on poros-ity and liquid/solid ratio [21]. Since the liquid/solid ratio of thesamples produced in our study was a low value as 0.30, low chlo-ride penetration results were obtained at high Na ratios. Further-more, a relationship can be established between porosity andchloride penetration depending on the Na dosage increment.Namely, since a high NH concentration leads to more compactstructure solving more Si and Al from fly ash, the decrease inporosity reduces chloride permeability values of mortar specimensfor high Na dosages [6,45].

4.5. Accelerated corrosion testing results

The adherence test (pull-out) was performed to determine theadherence loss of the reinforcement located in the middle ofØ100 � 200 mm cylindrical samples subjected to accelerated cor-rosion test. The maximum force obtained was recorded and adher-ence strength was determined [46]. The test setup was shown inFig. 10 and accelerated corrosion testing results for geopolymermortars were given in Fig. 11.

The adherence strength of non-corroded geopolymer controlsamples with SG fly ash activated with NH is between 4.2 and13.4 MPa. After corrosion, it was determined between 3.6 and8.2 MPa. The adherence strength of non-corroded geopolymer con-trol samples with SG fly ash activated with NS-NH is between 4.2and 12.6 MPa. After corrosion, it was determined between 2.9 and8.7 MPa. In control samples containing SG fly ash, the highestadherence resistance was obtained with NH having 15% Na dosage,while the highest adherence resistance was obtained with NS-NHat 15% Na after corrosion. However, the values were very close toeach other.

The adherence strength of non-corroded geopolymer controlsamples with CA fly ash activated with NH is between 5.0 and

lts of geopolymers (ASTM C1202).

Fig. 9. Rapid chloride permeability test setup.

Fig. 10. Accelerated corrosion test.

Fig. 11. Accelerated corrosion testing results of geopolymers.

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12.7 MPa. After corrosion, it was determined between 4.7 and9.5 MPa. The adherence strength of non-corroded geopolymer con-trol samples with CA fly ash activated with NS-NH is between 4.7and 8.8 MPa. After corrosion, it was determined between 3.0 and4.6 MPa. Before and after corrosion for CA fly ash, the highestadherence resistance was obtained with NH having 15% Na dosage,while the lowest adherence resistance values were obtained fromthe geopolymer specimens activated by NS-NH. The fact that thephysical and chemical properties of CA fly ash are different fromSG fly ash and that the effective activator is NH is thought to bethe reason for this.

In general, mortars activated with alkali at high Na ratios havehigh corrosion resistance. In similar studies, the superiority ofgeopolymers has been revealed. Saraswathy et al. [47] accentuates

that the reinforcement corrosion resistance of the mixtures acti-vated with alkali is similar to the mortars obtained with OPC bin-ders, while Miranda et al. [48] emphasized that the pH level ofalkali-activated fly ash binders which will cause corrosion is betterthan OPC binder systems.

5. Conclusions

The conclusions described in the following can be summarizedbased on the experimental results conducted in this study:

� Water absorption and VPV values of geopolymer mortars acti-vated by alkali decreased with the Na ratio increasing from 6%to 15%. Additionally, SG fly ash had better values than samples

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produced with CA fly ash due to its physical properties, whilethose produced with NH gave better values than samples con-taining NS-NH.

� For both primary and secondary capillary water absorption val-ues, improvements were observed depending on the incrementin the Na ratio. Primary capillary water absorption amount werehigher than the secondary capillary. This is due to the loss ofwater in the pores after thermal curing.

� The penetration depth of water under pressure decreased withthe increase of Na ratio. However, all mixture groups at low Na(6%) ratio reached high permeability. This resulted from highporosity in these samples. So, it is thought that it would beappropriate to have Na value above 10% for decreasing thewater permeability under pressure.

� Rapid chloride permeability is related to the pore structure andion density in the voids. More Si and Al ions from fly ash in thehigher Na ratios accumulate in the voids, reducing the penetra-tion of chloride ion.

� Before and after corrosion, an improvement in adherencestrengths was generally seen with the increase of Na dosagein geopolymer mortars. However, CA fly ash was more affecteddue to its differences in physical and chemical propertiesdespite the decrease in strength of all groups after corrosion.

� The grain size distribution of fly ash directly affects the perme-ability properties. For this reason, geopolymers produced withSG fly ash having the low residue amount over 45 mm sieveexhibited better properties than those obtained with CA fly ash.

� The activator type showed difference properties on samples. Forexample, NH alone provided better permeability propertiesthan NS-NH mixture.

� Although the increment in Na dosage affected positively theproperties of samples, the similar results were obtained for 6%and 9% Na ratios. However, 12% and 15% were Na ratios whichgave better results for all permeability characteristics.

CRediT authorship contribution statement

_Ismail _Isa Atabey: Investigation, Resources, Writing - originaldraft, Visualization. : . Okan Karahan: Resources, Writing - originaldraft, Project administration. Cahit Bilim: Writing - review & edit-ing, Visualization. Cengiz Duran Atis�: Methodology, Supervision.

Declaration of Competing Interest

The authors declare that they have no known competing finan-cial interests or personal relationships that could have appearedto influence the work reported in this paper.

Acknowledgment

This research was supported financially by The Scientific andTechnological Research Council of Turkey (TUBITAK under the Pro-ject No: 115M171) and Scientific Research Projects CoordinationUnit of Erciyes University (Project No: FDK-2014-5613). Authorsthank to TUBITAK and Erciyes University.

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