Composites: Part B 58 (2014) 371–377

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Composites: Part B

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The influence of the NaOH solution on the properties of the fly ash-basedgeopolymer mortar cured at different temperatures

1359-8368/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.compositesb.2013.10.082

⇑ Corresponding author. Tel.: +90 272 2281423; fax: +90 272 2281422.E-mail addresses: ggorhan@aku.edu.tr (G. Görhan), kurklu@aku.edu.tr (G.

Kürklü).

Gökhan Görhan ⇑, Gökhan KürklüAfyon Kocatepe University, Faculty of Engineering, Department of Civil Engineering, Afyonkarahisar, Turkey

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

Article history:Received 6 June 2013Received in revised form 27 September 2013Accepted 27 October 2013Available online 9 November 2013

Keywords:B. Physical propertiesB. PorosityD. Mechanical testingE. CureGeopolymer

In this study, geopolymer mortar was produced using Class F fly ash from the thermal power plant inKütahya Seyitömer (Turkey). The changes caused by the geopolymerization on the properties of the finalproduct were investigated by applying curing on geopolymer mortars in different NaOH concentrationsat different temperatures and for different curing times. The purpose of this process was to determine therelationship between alkali solution concentration, curing temperature and curing time. In order todetermine the effect of NaOH concentration on geopolymer mortars, three different molarities of NaOHconcentrations (3 M, 6 M and 9 M) were used together with sodium silicate (water glass) solution. Thesamples were cured at two different temperatures (65 and 85 �C). Physical properties such as porosity,bulk density, apparent density and water absorption, and mechanical properties such as flexural strengthand compressive strength were determined from the 7-day geopolymer mortar samples after the curingprocess. As a result, this study determined that curing temperature and curing time had an effect on thephysical properties of the geopolymer mortars. It was observed that NaOH concentration had a cleareffect on the properties of the mortar cured at 85 �C. Compressive strength values of 21.3 MPa and22 MPa were obtained from the mortar of 6 M concentration cured at 65 �C for 24 h and from a sampleof the same mortar cured at 85 �C, respectively. Compressive strength values of the geopolymer mortarscured at 85 �C increased depending on the curing time and the increase in NaOH concentration. Given thestrength values obtained, the optimal thermal curing temperature and the optimal NaOH concentrationwere 85 �C and 6 M, respectively.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

A new form of materials with cement characteristics called geo-polymer has been developed in recent years [1]. Alkaline activa-tion, usually referred to as geopolymerization, is defined as achemical process which alters the glassy constituents in well-com-pacted and cement-featured composites. A strong alkaline mediumis necessary to increase the surface hydrolysis of the particles ofraw materials with a dissolution consisting of a certain amountof silica and alumina found in materials during polymerization.This medium can be obtained by single or combined alkaline solu-tions referred to as activators [2].

Alkali activator solutions play an important role in the dissolu-tion of Si and Al oxides. Hence, geopolymers are synthesized bymixing the most common alkaline activators that are strong alka-line solutions such as sodium hydroxide (NaOH), potassiumhydroxide (KOH), potassium silicate or sodium silicate with alumi-nosilicate reactive materials [3–10].

The concentration of the activator has a significant effect on thecompressive strengths of the geopolymers. The ideal concentrationof the activator increases the strength of the geopolymer. Apartfrom the ideal concentration, some losses may occur in themechanical properties of the material due to the free OH� ions inthe alkali-activated matrix which can change the geopolymerstructure of the material. Age and curing temperature of the geo-polymers are other variables which have an effect on the compres-sive strength of the samples. However, for these variables to beeffective, sufficient concentration of activator must be present inthe medium during geopolymerization because NaOH concentra-tion has a greater effect on the strength values than on the curingtemperature and curing time values [2,11,12].

Geopolymers are generally formed by fly ash activated at lowtemperatures. Fly ash is an industrial waste with pozzolanic prop-erties obtained from thermal power plants and used instead of ce-ment in concrete construction. SiO2 and Al2O3 in the fly ash reactwith calcium hydroxide formed as a result of hydration of Portlandcement and start a pozzolanic reaction. For this reason, fly ash canbe used instead of Portland cement [1].

Fly ash contains high levels of amorphous silica and aluminaand is often mixed with alkaline solution to dissolve them. Silicates

372 G. Görhan, G. Kürklü / Composites: Part B 58 (2014) 371–377

start to dissolve when the fly ash come into contact with alkalisolution. Fly ash can be used as a source material in the productionof geopolymers because geopolymerization is essentially based onthe alumina-silicate chain. However, the type and concentration ofthe alkali solution play an important role in the solution of the flyash. The solubility of Al3+ and Si4+ ions in sodium hydroxide solu-bility is higher than in potassium hydroxide solution [13,14].Therefore, NaOH has an important effect on the structure and thecompressive strength of the geopolymers [15]. Si, Al and otherminor ions begin to dissolve when the fly ash come into contactwith NaOH. The amount of dissolution depends on the NaOHconcentration and dissolution time [1]. The NaOH concentrationaffects the dissolution process as well as the binding of solid parti-cles in the geopolymeric system’s final structure of the aqueousphase [15]. The mixture of fly ash with 10 molarity (M) of NaOHis suitable for the geopolymer synthesis [1].

The prepared mixture can be subjected to curing at room tem-perature or at a given temperature. Aluminosilicate reactive mate-rials dissolve in strong alkaline solutions and free SiO4 and AlO4

tetrahedral structure forms. However, the reaction of the fly ashin the production of geopolymers is low at ambient temperatures[3]. Therefore, the geopolymerization process through which thematerial can achieve high compressive strength at given tempera-tures between 40 and 95 �C can develop [7,12,15,16]. The micro-structure of the fly ash geopolymers contains aluminosilicate gel,unreacted fly ash and other crystal phases [17].

Previous studies have concluded that Class F fly ash is a goodsource for geopolymers [18] and that NaOH is better than otheractivators for the activation of the fly ash in the production of geo-polymers. When NaOH is used together with water glass, the com-pressive strength of the geopolymer material is higher than whenonly NaOH is used. The reason behind this is the fact that in thegeopolymerization process, water glass increases the reactionproducts in which the Si content is higher, and provides moremechanical strength [19]. Other studies show that when usingthe water glass with alkaline activators, compressive and flexuralstrength increase [4]; however when water glass is used togetherwith NaOH, instead of using only NaOH, samples gain significantstrength even in one day [20,21]. However, water glass in powderform exhibits lower performance than water glass in liquid form[4].

Some studies in the literature have tested combinations of dif-ferent curing times and temperatures with NaOH concentrations.Among these studies, researchers found both geopolymers inwhich fly ash and bottom ash were used [22], as well as geopoly-mers which were synthesized with fly ash, kaolin, sodium silicatesolution and NaOH [7]. In addition, other studies in the literatureare produced light inorganic polymers by synthesizing Class F flyash, metakaolin and lightweight aggregates with NaOH sodium sil-icate solution [23] or produced polymers by using fly ash and so-dium silicate solution [24], preparation and characterization ofnew geopolymer-epoxy resin hybrid mortars [25] and some otherstudies synthesized 7-day fly ash-based geopolymers [26] and pro-duced 7-day fly ash-based geopolymer mortars [27]. The previousstudies generally focused on geopolymer paste. In these studies,geopolymer materials were subjected to long curing times, andlong periods of time were taken into account for the detection oftheir properties. However, in the current study, NaOH and sodiumsilicate solution (water glass) were used to activate the Class F flyash in order to determine the physical and mechanical propertiesof the synthesized geopolymer materials in a short period of time.The changes in the geopolymerization process and the propertiesof the final product were investigated by curing geopolymer mor-tars, which were prepared using alkali-activated fly ash andcrushed sand, with differing NaOH concentrations, temperatures,and lengths of time. Thus, the changes in the properties of the

geopolymer mortars in short periods of time were revealed bydetecting the relationships between the alkali solution concentra-tion, curing temperature and curing time.

2. Materials and methods

2.1. The characterization of the fly ash

In this study, fly ash materials obtained from the SeyitömerPower Station, in Kütahya, Turkey were used. Chemical analysisof fly ash was carried out with XRF (Rigaku ZSX Primus). The min-eralogical analysis was carried out on powdered samples by XRD(Shimadzu-6000, Cu Ka, 1.544 ÅA

0

). Particle size analyses were car-ried out using the laser diffraction method (Malvern Mastersizer-2000).

2.2. The preparation of the geopolymer mortar

Three different molarities (M) of NaOH concentration were usedin order to determine the effect of NaOH concentration on geopoly-mer mortars. Dissolved NaOH pellets were used in order to obtainconcentration. The materials and the mixture proportions used inthe preparation of the geopolymer mortar samples are shown inTable 1.

The mortars were shaped in 40 mm � 40 mm � 160 mm metalmolds using a vibration method. Afterwards, the samples wereplaced in a laboratory-type oven to cure thermally. The curing tem-peratures and curing times used in a study in the literature [5]were applied to the geopolymer mortars in this study. The samplesare shown in Table 2. The samples cured at 65 �C were coded as Aand those cured at 85 �C as B. When a prepared geopolymer mortarwas activated by 3 M NaOH and cured at 65 �C for 24 h, this set ofsamples was coded as 3A-24. Other mortars were also encodeddepending on NaOH concentration, curing temperature and curingtime (Table 2). Table 3 shows the initial molar ratios of SiO2, Al2O3,Na2O and H2O of the alkali-activated fly ash paste.

2.3. Mixing procedure

NaOH was used together with sodium silicate solution to dis-solve the silica and alumina of the fly ash particles [1]. The NaOHsolutions were prepared in the planned concentrations and al-lowed to stand at room temperature for 24 h. Next, the fly ashand NaOH solution were mixed for 3 min. Sodium silicate solutionwas added to the mixture and mixed for another minute. Lastly,sand was added to the mixture and mixed for 3 min. A solutionwith the module 3 (SiO2/Na2O), sodium silicate (water glass) wasadded to the mixtures. Table 4 shows the properties of the NaOHand sodium silicate solution. The specific gravity of the crushedsand used in the production of mortar was 2.27 g/cm3. The grainsize of the crushed sand used in the preparation of mortar was2 mm at the most. Fineness modulus of the aggregate was 1.25.

2.4. Physical and mechanical tests

After the curing process, the geopolymer mortars were stored atroom temperature until physical and mechanical tests were con-ducted. 7-day geopolymer mortar samples were used in the tests.The average of three samples from each sample group was takenfor the physical tests and flexural strength test while the resultsof the compressive strength test were determined by taking theaverage of six samples.

Some of the geopolymer mortars were placed in a water tank todetermine their physical properties. The principles of Archimedeswere used to determine the physical properties of these samples

Table 1Mix proportion of mortars.

Concentration of NaOH (M) Fly ash (g) Sand (g) Sodium silicate solution (ml) Solution of NaOH (ml) Solid/liquid ratioa (mass) Liquid/fly ash ratio (mass)

3 450 1350 290 145 3.92 1.046 450 1350 290 145 4.14 1.009 450 1350 290 145 4.37 0.97

a Solid included fly ash, sand and NaOH in pellet form.

Table 2The prepared samples and curing process.

Geopolymer Concentration of NaOH(M)

Curing time(h)

Curing temperature(�C)

Geopolymer Concentration of NaOH(M)

Curing time(h)

Curing temperature(�C)

3A-2 3 2 65 3B-2 3 2 853A-5 3 5 65 3B-5 3 5 853A-24 3 24 65 3B-24 3 24 856A-2 6 2 65 6B-2 6 2 856A-5 6 5 65 6B-5 6 5 856A-24 6 24 65 6B-24 6 24 859A-2 9 2 65 9B-2 9 2 859A-5 9 5 65 9B-5 9 5 859A-24 9 24 65 9B-24 9 24 85

Table 3The initial molar ratios of alkali-activated fly ash paste.

Concentration of NaOH SiO2/Al2O3 Na2O/Al2O3 H2O/Na2O

3 M 5.24 0.85 30.626 M 5.24 1.36 18.879 M 5.24 1.86 13.47

Table 4Chemical materials and their properties.

Sodium silicate solution (water glass) Sodium hydroxide (NaOH)

Na2O: 7.5–8.5 % M: 40 g/molSiO2: 25.5–28.5 % NaOH P 97.0Density (20 �C) 1.296–1.396 g/mlFe 6 0.005 %Heavy metals: (as Pb) 6 0.005 % [28]

G. Görhan, G. Kürklü / Composites: Part B 58 (2014) 371–377 373

including water absorption, porosity, bulk density and apparentdensity. TS EN 772-4 [29] was used to determine the apparentporosity, bulk density and apparent density values of the sampleswhile TS EN 771-1 [30] was used to determine the water absorp-tion values of the samples. TS EN 196-1 [31] was used to determinethe mechanical properties of the geopolymer mortars.

3. Results and discussion

3.1. Physical and chemical properties of the fly ash

According to the results of the XRF analysis of the fly ash, thetotal SiO2 + Al2O3 + Fe2O3 value of the fly ash was 80.09% whilethe value of CaO was 6.06%. The fly ash was a Class F fly ash accord-ing to ASTM C 618 [32]. The silica/alumina ratio of the fly ash bymass was (SiO2/Al2O3) 2.49 (Table 5). According to the X-ray

Table 5Chemical composition of fly ash.

Oxide SiO2 Al2O3 Fe2O3 MgO N

Weight,% 48.90 19.63 11.56 4.31 0

diffraction graph (XRD) of the fly ash obtained in a previous study[33], it contained quartz, magnetite, anhydrite, anorthite, hematite,and other minerals (Fig. 1). Table 6 shows d10, d50 and d90 values ofthe fly ash. The specific surface area was calculated as 0.366 m2/g.

3.2. Physical properties of the fly ash based geopolymer mortars

Increases in curing time led to a reduction in apparent porosityin the geopolymer mortars cured thermally at 85 �C. In Fig. 2b isshow that the highest value of porosity was obtained from 3B-2(Fig. 2b) mortars which had low NaOH concentration while thelowest porosity values were obtained from 3B-24 mortars. Withan increase in the curing temperature, the apparent porosity ofthe geopolymer mortars increases at 2 h of cure state onwards,while it decreases at 24 h of cure state. Out of the samples curedfor 5 h, the porosity of the samples with a concentration of 6 Mand 9 M NaOH decreased with an increase in the curing tempera-ture. In the samples cured for 2 h higher porosity was observed be-cause the reaction was very low and provided greater evaporationat 85 �C than at 65 �C, while in the samples of 5 and 24 h poreswere clogged by the formed gel geopolymeric reaction (Fig. 2).

In addition to the curing process, NaOH solutions of differentconcentration used in the preparation of the geopolymer mortarshad an effect on the values of porosity. The samples cured at65 �C, especially the samples with 6 M NaOH concentration (6A-2, 6A-5 and 6A-24), had a more porous structure (Fig. 2a), andthe apparent porosity ranged from 26.1% to 29.2%.

The increase in the curing times reduced the apparent porosityin the samples cured at 85 �C. These values ranged from 25.3% to29.8%. The porosity reduced in inverse proportion to the increasein NaOH concentration in the samples cured for shorter periodsof time (3B-2, 6B-2 and 9B-2). However, this relationship was re-versed with an increase in the curing time, and porosity increasedin direct proportion to NaOH concentration in the mortars curedfor 24 h (3B-24, 6B-24 and 9B-24). The effect of the NaOH

a2O K2O SO3 CaO LOI Total

.73 2.06 1.65 6.06 2.32 97.22

Fig. 1. X-ray diffractogram of fly ash.

Table 6d10, d50 and d90 values of fly ash.

Sample d10 (lm) d50 (lm) d90 (lm)

Fly ash 8.103 32.829 92.778

374 G. Görhan, G. Kürklü / Composites: Part B 58 (2014) 371–377

concentration on the geopolymer mortar samples cured for 5 h isnot clear (Fig. 2b).

Apparent porosity values are generally correlated with waterabsorption values. Therefore, the water absorption values of thesamples showed similarities with the porosity values (Fig. 3). Theabsorption values of the samples cured at 65 �C varied between16.5% and 18.5% while those of the samples cured at 85 �C varied

(a) (

Fig. 2. The apparent porosity

(a) (

Fig. 3. The water absorption

between 15.6% and 19.2%. When curing was carried out at 85 �C,the curing time and concentration of NaOH was directly affectedwhile geopolymers cured at 65 �C had no clear trend (Fig. 3b). De-crease in the water absorption of the samples with increase in theNaOH concentration was also observed in the previous studies[34,35]. It was stated in the previous studies that; lower waterabsorption and apparent porosity were observed when higher al-kali content (% Na2O) activators were used in the production ofgeopolymer mortars [36,37].

The bulk density values of the samples cured thermally at 65 �Cdid not vary greatly (Fig. 4a). In fact, similar results were obtainedfrom the geopolymer mortars cured for 2 and 5 h. It was deter-mined that the samples cured for 24 h showed a denser structure,however, NaOH concentration was not a very important factor. Thebulk density values of the samples cured at this temperature ran-ged from 1577.4 to 1614.6 kg/m3.

In the samples cured at 85 �C, the bulk density increased withan increase in the curing time. The bulk density of the samplescured at this temperature ranged from 1552.9 to 1622.7 kg/m3. Itshould be noted that 9B mortars showed a denser structure whenthey were cured for 2 h while they had a lower density than 3B and6B when they were cured for 24 h (Fig. 4b).

An increase in the curing temperature increased the bulk den-sity of the geopolymer mortars with a 3 M NaOH concentration,while the effect of the temperature was not pronounced in thesamples with a 6 M and 9 M NaOH concentration. However,regardless of the curing temperature, it was observed that the bulkdensity of the geopolymer mortars increased with an increase inthe curing times (except the 9A samples).

For the bulk density, all the voids of the material(open pore + closed pore) were omitted. Therefore, as can be seen

b)

of geopolymer mortars.

b)

of geopolymer mortars.

(a) (b)

Fig. 4. The bulk density of geopolymer mortars.

G. Görhan, G. Kürklü / Composites: Part B 58 (2014) 371–377 375

in Fig. 4b, the bulk density of 3B-2 mortars is higher than that of6B-2 mortars although the apparent porosity of the former is high-er than the latter. This is due to the fact that the total porosity of6B-2 sample is higher than that of 3B-2 sample and therefore alower level of apparent density value was observed compared to3B-2 sample. The apparent density of the geopolymer mortars isshown in Fig. 5. According to the findings, the effect of the NaOHconcentration on the apparent density values is not clear. However,the apparent density values of the mortars decrease with an in-crease in the NaOH concentration of the samples cured at 85 �Cfor 24 h. The apparent density values of the samples cured at 65and 85 �C range from 2154.8 to 2229.9 and from 2153.6 to2214.3 kg/m3, respectively.

3.3. Mechanical properties of the fly ash based geopolymer mortars

It is important to increase the curing temperature for the sam-ples to gain strength quickly when higher strengths are intended tobe achieved during a shorter period of time [2]. The thermal curingprocess applied for more than a few hours at high temperatureshad a positive effect on compressive strength of the material[38]. The strengths of the samples increased as a result of the cur-ing times and the reactions between silica and alumina in the alka-line ions [3].

Figs. 6 and 7 show the compressive and flexural strengths of thegeopolymer mortars, respectively. According to the data; low con-tent of Si and Al ions dissolved from the fly ash particles, and thisresulted in relatively low compressive strength values due to thelow alkaline medium of the samples cured thermally at 85 �Cand activated by 3 M NaOH (Fig. 6). It was reported that a weakchemical reaction occurs with the use of a low alkaline solution

(a) (

Fig. 5. The apparent density

[39]. The compressive strength of the samples is expected to in-crease with an increase in NaOH concentration due to the fact thatsilica and alumina are filtered more in a highly concentrated NaOHsolution [40]. The dissolution of the fly ash accelerates becauseOH� concentration is sufficiently high in high molarities (9 M);however it further hinders polycondensation in the structure[41]. Higher NaOH concentration is more effective in dissolvingfly ash particles and can result in a better geopolymerization [42].

It was observed that the samples activated with 6 M NaOH hadan ideal alkaline environment and the highest compressivestrength values compared to 9 M NaOH samples. The compressivestrength values of 21.3 MPa and 22 MPa were obtained from the6A-24 sample cured at 65 �C and from the 6B-24 sample cured at85 �C, respectively. The reductions in the strength values of thesamples activated with 9 M NaOH are thought to have resultedfrom an increase in the coagulation of silica [1].

The flexural strength of the geopolymer mortars revealed differ-ent trends depending on the curing temperature. In the samplescured at 65 �C, the flexural strengths of the mortars cured for 2and 5 h decreased with an increase in the NaOH concentration. Adifferent situation was observed in the mortars cured for 24 h. Inthe samples cured at 65 �C, flexural strengths varied between 4.9and 7 MPa. In the geopolymer mortars cured at 85 �C, the flexuralstrengths of the samples with 3 M and 9 M NaOH concentration in-creased with an increase in the curing time. In addition, in the geo-polymer mortar samples with 6 M NaOH concentration (cured at85 �C), lower flexural strength values were observed in the samplescured for 2 h while the flexural strength values of the samplescured for 5 and 24 h were the same. The flexural strength valuesof the samples with 6 M and 9 M NaOH concentration cured at85 �C reached higher levels than those of the samples cured at

b)

of geopolymer mortars.

(a) (b)

Fig. 6. The compressive strength of geopolymer mortars.

(a) (b)

Fig. 7. The flexural strength of geopolymer mortars.

376 G. Görhan, G. Kürklü / Composites: Part B 58 (2014) 371–377

65 �C. The flexural strength values of the samples cured at 85 �Cranged from 5.5 to 8.1 MPa.

When the flexural and compressive strength values were eval-uated together, it was determined that 3A-2 mortars cured at65 �C for 2 h had higher strength values than those of other sam-ples (6A-2 and 9A-2). The decline in the strength values with anincrease in NaOH concentrations was noticeable at this stage. Itwas determined that the compressive strength values of the mor-tars cured for 2 h varied depending on the curing temperature andNaOH concentration [5]. Considering the samples cured at 65 �C for24 h, the samples with a concentration of 6 M NaOH were found tohave the highest strength values (Figs. 6 and 7). In the samplescured at 85 �C for 2 and 5 h, the samples with a concentration of6 M NaOH had the highest strength values.

4. Conclusions

The increase in the curing times led to reductions in the poros-ity values of the geopolymer mortars (samples except 3A and 6A)while the concentrations of NaOH solutions used in the prepara-tion of the geopolymer mortars affected both the porosity valuesand curing process.. The bulk density values of the samples curedat 85 �C increased with an increase in the curing time, while itdid not change substantially at 65 �C.

The samples of the seven-day geopolymer mortars which wereactivated by 6 M NaOH were observed to have the optimum condi-tion and the highest compressive strength values. When strengthvalues were considered the samples of the seven-day geopolymermortars which were activated by 6 M NaOH were observed to have

the optimum condition and the highest compressive strengthvalues when cured at 85 �C.

It was determined that an increase in the curing temperatureincreased the compressive strength while it did not have a signif-icant effect on the physical properties.

Acknowledgements

The authors would like to thank KOLSAN Inc. for the supply offly ash and sand and for their support. They would also like tothank Afyon Kocatepe University Scientific Research Unit for pro-viding financial support for this research (AKU BAP: 12.MUH.05).

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