Characterization of mechanical and microstructural properties of palm oil fuel ash geopolymer cement paste

Construction and Building Materials 65 (2014) 592–603

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Characterization of mechanical and microstructural properties of palm oil fuel ash geopolymer cement paste Moslih Amer Salih a,b,⇑, Abang Abdullah Abang Ali a, Nima Farzadnia a a b

Housing Research Center, Department of Civil Engineering, Faculty of Engineering, Universiti Putra Malaysia UPM, 43400 Serdang, Selangor, Malaysia Foundation of Technical Education, Ministry of Higher Education and Scientific Research, Baghdad, Iraq

h i g h l i g h t s  POFA was geopolymerized effectively by alkali activators.  Qualitative observations confirmed POFA viability to be used as a geopolymer binder.  Compressive strength achieved by POFA geopolymerization is comparable to OPC paste.

a r t i c l e

i n f o

Article history: Received 14 April 2014 Received in revised form 4 May 2014 Accepted 8 May 2014

Keywords: Palm oil fuel ash POFA Alkali activation Geopolymer binder

a b s t r a c t This study delineates activation of palm oil fuel ash (POFA) by a combination of sodium silicate and sodium hydroxide at 60 °C to be used as a geopolymer binder. Qualitative observations as well as compressive strength were recorded to assess the viability of POFA utilization. Also, XRD, SEM/EDX, DSC, FTIR tests were conducted to investigate underlying mechanisms of geopolymerization. The post-test observations revealed that activation of POFA is applicable and compressive strength of up to 32.48 MPa at the age of 28 days was achieved. Chemical tests indicated that formation of calcium silicate hydrate was the dominant cause of geopolymerization. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction One potential replacement for cement is alkali activated materials (geopolymer binders) which seem to yield similar mechanical properties as Portland cement [1] although these materials are still at the beginning stages of development [2]. According to a recent rigorous and useful definition by Provis [3] ‘‘Alkali activated materials are produced through the reaction of an aluminosilicate— normally supplied in powder form as an industrial by-product or other inexpensive material—with an alkaline activator, which is usually a concentrated aqueous solution of alkali hydroxide, silicate, carbonate or sulfate.’’ Five groups for alkali-activated cements are categorized by Shi et al. [4]: Alkali-activated slag-based cements, alkaliactivated pozzolan cements, alkali-activated lime-pozzolan/slag cements, akali-activated calcium aluminate blended cement, and alkali-activated Portland blended cement (hybrid cements).

⇑ Corresponding author at: Housing Research Center, Department of Civil Engineering, Faculty of Engineering, Universiti Putra Malaysia UPM, 43400 Serdang, Selangor, Malaysia. Tel.: +60 1139087451; fax: +60 389467869. E-mail address: [email protected] (M.A. Salih). http://dx.doi.org/10.1016/j.conbuildmat.2014.05.031 0950-0618/Ó 2014 Elsevier Ltd. All rights reserved.

So far, different types of aluminosilicate materials such as Metakaolin [5–9], fly ash [10–12], and slag [13–15] were applied as alkali-activated cements. Palm oil fuel ash, an agro-waste produced in massive amounts in Malaysia, is another potential alkali-activated cement which is categorized as a pozzolanic material [16–19]. POFA has been lately used in binary mixes with other aluminosilicate materials such as ground granulated blast furnace slag (GGBS), rice husk ash (RHA) and fly ash in order to produce geopolymer concrete [20,21]. In general, alkali activation process was first introduced by Kuhl in 1908 and later the geopolymer terminology was proposed by Davidovits [3]. It was reported that poly-condensation of hydrolyzed aluminate and silicate was the main reason for hardening of the geopolymer binder in the form of zeolitic crystalline structure [22,23]. However, co-existence of geopolymeric gel and calcium silicate hydrate was also reported in previous works [24,25]. Formation of C–S–H was mainly regarded to dissolved calcium from surface of the source with presence of available silicate species in alkali ambient [24]. It was also proposed that the simultaneous formation of geopolymeric gel and C–S–H may help to bridge the gaps in the matrix and hence increase the compressive strength especially at early ages [24].

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One of governing factors on properties of the gel and its formation is type and dosage of alkali activators [26]. So far, liquid sodium silicate (water glass) (Na2SiO3) and liquid sodium hydroxide (NaOH) were the most used activating solutions in geopolymers [27]. According to Komnitsas and Zaharaki [28] alkali hydroxide is required for dissolution of aluminosilicate sources while water glass solution acts as a binder, alkali activator and dispersant or plasticizer. However, because of the soluble silicate available in the liquid water glass, it is a preferred activating solution, which tends to increase the rate of the polymerization reaction [29]. Alkaline solutions induce a certain amount of Si and Al atoms to dissolve the aluminosilicate sources, forming monomers in solutions, and then poly-condense to form a rigid framework [30]. Previous works have shown that solid to liquid (S/L) ratio and Na2SiO3 to NaOH ratio (SS/SH) have tremendous effect on mechanical properties of geopolymer binders [31]. Xu and Van Deventer [32] reported that the solid to liquid ratio by mass should be around 3.0 for activation of fly ash in order to allow the geopolymerization process to take place. Nonetheless, the shape of particles has an effect on the required quantity of the alkali activator to result in advanced dissolution. Fly ash has better workability than Metakaolin in alkali activation process because of the spherical shape of its particle which reduces the demand for liquid as compared to the plate-like structure of Metakaolin particles. As such, Kong et al. [33] suggested a very low ratio of solid to liquid (0.8) by mass in order to activate the Metakaolin with optimum strength. In a few studies the effect of sodium silicate to sodium hydroxide ratio was also investigated. Hadjito and Rangan [34] reported that the combination of the alkaline activator ratio had a clear effect on the compressive strength of low-calcium fly ash based in geopolymer concrete. The findings of their research showed an increase in the compressive strength of the geopolymer concrete by 28% with an increase in the ratio of sodium silicate to sodium hydroxide. The recommended ratio by mass was 2.5, while Wang et al. [35] reported a lower ratio of 0.24 for Metakaolin based geopolymer. One of the available aluminosilicate materials which can be widely found in Malaysia is palm oil fuel ash (POFA). Palm oil fuel ash is a by-product from the palm oil industry produced in massive amounts (approximately 4 million tons per year) [36]. So far, POFA is used as a partial replacement for ordinary Portland cement in conventional concrete for the purpose to enhance strength and durability of concrete [37–43]. Recently, some works have utilized POFA as a supplementary material in mixes with other aluminosilicate materials to make geopolymer cement paste or mortars. For example, low calcium fly ash blended with POFA [20] was used in order to produce geopolymer cement and so compressive strength of up to 28 MPa was obtained. Also, a geopolymer binder was fabricated from a ternary mix of slag, palm oil fuel ash and rice husk ash; however, a low content of POFA was used in the binder [21]. In another study by Mijrash et al. [44], treated POFA was activated to produce the geopolymer binder. Supplementary materials such as silica fume, calcium hydroxide and alumina hydroxide in addition to the alkaline activator were also used to increase the efficiency of treated POFA in the production of geopolymer products [44]. In other studies [45–47], a combination of ultrafine palm oil fuel ash and ground blast furnace slag was used to investigate the compressive strength and microstructure of geopolymer binder. Strength development for a geopolymeric binder from ground granulated blast furnace slag and palm oil fuel ash was also investigated by Islam et al. [48]. The study revealed that the binder with the binary mix of low content of POFA and GGBS achieved the highest compressive strength. In aforementioned studies sodium silicate and sodium hydroxide were used as alkali activators. Still, no study investigated the activation mechanism and the microstructure of the resulted geopolymer from the activation of

POFA as an only aluminosilicate material source. The overarching purpose of this study is to activate POFA albeit its very low content of aluminum. The objectives include studying the process of alkali activation of POFA to produce a geopolymer binder, identifying the best ratio of solid to liquid, and sodium silicate to sodium hydroxide. The findings of this research will also explain the mechanism by which aluminosilicates with low aluminum content are involved in the geopolymerization. This study may encourage and promote further research on the use of POFA in geopolymer technology in mortar and concrete as well as the use of other aluminosilicate materials with low aluminum content which will ultimately lead to development of more environmentally friendly products with low energy consumption and very low CO2 emissions. 2. Experimental method 2.1. Materials 2.1.1. Palm oil fuel ash The palm oil fuel ash (POFA), obtained from burning of palm oil shells, husk and fibers, was collected from a mill at Johor State, south of Malaysia. The raw palm oil fuel ash was oven dried at 110 ± 5 °C for 24 h , sieved with a 300 lm sieve to remove large unwanted particles and incompletely combusted materials [49,50], and then it was ground by a modified Los Angeles machine [42]. The specific surface area after grinding was 0.915 m2/g. The chemical composition of the POFA by XRF test is shown in Table 1. As can be seen, major components are SiO2 and CaO with concentrations of 47.37% and 11.83%, respectively with a low amount of Al2O3 (3.53%). The relatively high amount of CaO available in the POFA is most likely from lime and fertilizer [41]. It is worth to mention that POFA is a not toxic waste material in terms of heavy metals leachability [51]. The XRD patterns of ground POFA are shown in Fig. 1. As can be seen, major phases of alpha quartz (SiO2) and cristobalite (SiO2) were traced [49,50]. The location of the highest hump was also detected in the XRD profile from 20° to 40° (2 theta), representing an amorphous phase [50]. The particle morphology of the raw POFA was investigated by scanning electron microscope SEM (Fig. 2). Fig. 2 indicates that raw POFA consisted of very irregular shaped particles with porous cellular surfaces [37,40]. The grinding process was effective to turn the POFA to a smaller sized and more homogeneous powder. The shape of particles was in crushed form and spherical with rough surface as can be seen from Fig. 3 [52]. 2.1.2. Alkaline activators Sodium hydroxide and sodium silicate were chosen as alkali activators in this investigation. The sodium hydroxide was in industrial-grade with minimum 99% purity. Industrial grade water glass (Na2SiO3) solution was chosen with a chemical composition of 15.33% Na2O, 31.28% SiO2, and 53% H2O. The alkaline activator selection was based on the recommendations in [34,53–55]. The alkali activation solution was prepared by mixing Na2SiO3 with NaOH within ratios ranged between 0.5 and 3.0 [34]. 2.2. Preparation of POFA geopolymer paste Preliminary experiments were conducted to study the alkali activation of POFA [56]. It revealed that raw POFA cannot be used without sieving and grinding because of the low compressive strength results which is related to the porous structure and high demands for the alkaline activator. Experimental program was designed in order to investigate the ability of POFA to be incorporated in geopolymer technology. NaOH solution was mixed with Na2SiO3 to produce six ratios (0.5, 1.0, 1.5, 2.0, 2.5, and 3) to prepare alkaline activator solution 24 h prior to use. Ground POFA and the alkaline activator were then mixed with two solid-to-liquid ratios (1.0, 1.32) as in [34,53,57,58]. Two groups of twelve mixes were prepared based on the solid to liquid ratio as shown in Table 2. Mix one with sodium silicate to sodium hydroxide ratio (0.5) was unable to be used; the geopolymer paste had such low workability that could not be cast in molds. The palm oil fuel ash was mixed directly with the alkaline activator. First the ash was discharged to the mixer pan, and then the alkaline was added and mixed for 1.0 min at a normal speed rate (gear one). Then, the mixer stopped for 10–20 s in order to scrap the un-mixed ash on the sides of the paddle and the

Table 1 Chemical composition of POFA by XRF test. Oxide

SiO2

Al2O3

CaO

Fe2O3

MgO

P2O5

SO3

L.O.I

Concentration %

47.37

3.53

11.83

6.19

4.19

3.31

1.22

1.84

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Fig. 1. XRD patterns of raw POFA.

Table 2 Mix proportions of materials for the preparation of pastes. Na2SiO3/NaOH

w/b

POFA (kg/m3)

Alkaline (kg/m3)

1.0 Group one

0.5 1.0 1.5 2.0 2.5 3.0

0.53 0.53 0.53 0.53 0.53 0.53

1133.3 1133.3 1133.3 1133.3 1133.3 1133.3

1133.3 1133.3 1133.3 1133.3 1133.3 1133.3

1.32 Group two

0.5 1.0 1.5 2.0 2.5 3.0

0.4 0.4 0.4 0.4 0.4 0.4

1133.3 1133.3 1133.3 1133.3 1133.3 1133.3

858.6 858.6 858.6 858.6 858.6 858.6

Solid to liquid 1 2 3 4 5 6 7 8 9 10 11 12

2.3. Testing procedure Fig. 2. Raw POFA before grinding. Changes in the physical properties of the POFA were evaluated to ensure efficiency of the treatment process as well as uniformity of the geopolymerized POFA. Specific gravity and particle size were assessed using a laser diffraction particle size analyzer. X-ray fluorescence spectrometry scanning test was conducted by Bruker S8 Tiger to find out the chemical composition of POFA. Compressive strength and flowability tests were measured in accordance with ASTM C109 [60] and ASTM C1437 [61], respectively. A minimum of three specimens were tested to evaluate the compressive strength at 3 ages of 7, 14, and 28 days. SEM/EDX test was conducted using a Hitachi S-3400N to reveal the microstructure and various degrees of reaction at different Solid/Liquid and Na2SiO3/NaOH ratios. The XRD diffraction was also performed using XRD-6000, Shimadzu X-ray diffractometer. Fourier Transform InfraRed (FTIR) spectra of geopolymer samples were recorded on Spectrum 100 FT-IR Spectrometer (PerkinElmer precisely). The DSC thermograms were also measured by Differential Scanning Calorimetry instrument (Mettler Toledo, DSC 832e).

3. Results and discussion 3.1. Qualitative observations

Fig. 3. POFA after grinding.

pan, then the mixing was continued with a medium speed (gear two) for another 1.0 min in accordance to ASTM C305 [59]. All the geopolymer paste mixes were blended and produced with a small blender (HOBART Mixer). The fresh geopolymer paste was then cast into 50  50  50 mm iron molds in two layers immediately after mixing. To compact the specimen, each layer was vibrated by a vibrating table for 25–30 s. This procedure was used to remove the bubbles from the paste. After casting and vibrating, oven curing was applied directly at 60 °C for 2 h. The molds were then kept in the laboratory to reach ambient temperature. The specimens were kept in a plastic bags after de-molding to prevent any moisture loss and were stored to the date of testing [55].

3.1.1. Soundness in water In order to observe the soundness of POFA geopolymer paste in water, samples from all mixes were soaked in water up to 180 days. It was observed that POFA geopolymer paste did not disintegrate or collapse in water and no cracks were observed by which the stability of the proposed geopolymer paste in water was confirmed. Davidovits [62] related the soundness to stability of three dimensional network structures in water; however, this behavior was investigated in limited researches. Previously, disintegration of geopolymer with kaolin clay was reported at 7 days [57]. 3.1.2. Efflorescence Efflorescence was observed for the two groups of solid to liquid ratios. In general, efflorescence for group (1) (solid to liquid ratio 1.0)

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was higher in comparison with group (2) (solid to liquid ratio 1.32) which is attributable to a higher content of the activator in group one. For POFA geopolymer paste with solid to liquid ratio 1.0, visual percentage of efflorescence decreased with the decrease of NaOH percentage of the total quantity of the activator. The decrease was represented by increasing the sodium silicate to sodium hydroxide ratio from 1.0 to 3.0 (Fig. 4). As can be seen from Fig. 4, mix one (Na2SiO3/NaOH = 1.0) and mix two (Na2SiO3/NaOH = 1.5) gave the highest visual percentage of efflorescence, while in mix 3, 4, and 5, efflorescence did not appear with the decrease of the sodium hydroxide percentage especially at the age of 28 days. This behavior may correspond to the alkaline leaching out on the surface of the specimens which

can be described as sweating. According to Škvára et al. [63], Na ions are bound only weakly in the nanostructure of the geopolymer gel and are leachable almost completely without compromising the compressive strength. This causes alkali-activated materials to be prone to efflorescence with excessive amounts of alkalis in the system. Na ions may diffuse to the surface where it reacts with atmospheric CO2 while forming visible salts such as Na2CO3
nH2O, NaHCO3, K2CO3, and KHCO3. Thus, decreasing the NaOH in the mixing ratio may have resulted in reduction of the sodium ion and hence decreased the efflorescence on the surface. Fig. 5 illustrates the efflorescence in specimens in group two at 28 days. As can be seen from the figure, in POFA geopolymer pastes with solid to liquid ratio of 1.32, less visual percentage of the white

Fig. 4. Visual percentage of efflorescence in group one with SS/SH ratio of (a) 1, (b) 1.5, (c), (d) 2.5, and (e) 3 at 28 days.

Fig. 5. Visual percentage of efflorescence in group two with SS/SH ratio of (a) 1, (b) 1.5, (c), (d) 2.5, and (e) 3 at 28 days.

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deposit was observed on the surface of specimens at all ages. However, efflorescence was the highest in samples with SS/SH = 1. The visual percentage lowered with the increase of sodium silicate to sodium hydroxide ratio in the rest of mixes. Efflorescence reduction may be attributed to the low availability of sodium ion in the alkaline liquid which reduces sodium ability to leache out and react with atmospheric CO2. 3.1.3. Flowability The flowability of fresh POFA geopolymer paste with solid to liquid ratios of 1 and 1.32 is shown in Figs. 6 and 7, respectively. As can be seen from the figures, the flow of POFA geopolymer paste for group one with solid to liquid ratio 1.0 was higher than that of group two with solid to liquid ratio 1.32. This can be mainly attributed to a higher content of water in group one comparing that of group two by 24%. The same observation was mentioned by Li et al. [64] for a geopolymer paste prepared using fly ash as a source material. The flow was increased from 145 mm to 173 mm by decreasing the water to the source material ratio. From another angle, the flow decreased with an increase of the sodium silicate to sodium hydroxide ratio [65] from 1.0 to 3.0 in both groups. It may be safe to state that the flowability depends on the ratio of sodium silicate to sodium hydroxide by mass. It can be explained by nature of sodium silicate as a suspension liquid [65].

3.1.4. Bulk density The bulk densities of POFA geopolymer paste at solid to liquid ratio 1.00 and 1.32 with different sodium silicate to sodium hydroxide ratios are illustrated in Table 3. In general, the resulted geopolymer pastes showed comparable density with ordinary Portland cement paste for group one and two. The results showed that a higher bulk density was obtained in samples in group two with solid to liquid ratio 1.32. This may be related to a higher geopolymer condensation rate with the presence of lower content of activator. In the present study, POFA geopolymer pastes showed bulk densities up to 1800 kg/m3 at 28 days which is similar to the ordinary Portland cement paste [66]. The obtained POFA geopolymer paste density in this study was higher than that of fly ash geopolymer which is registered to be between 1500 and 1600 kg/m3 [67]. However, it can be seen that there is a decreasing trend in density of some series from 7 days to 28 days. In previous studies on fly ash and Metakaolin the same trend was observed which was reported to be mainly proportional to the curing temperature [9,67]. The drawn conclusion was that increasing the curing temperature made the geopolymer structure less dense and less compact.

Fig. 6. Flow of POFA geopolymer paste (group one, solid to liquid ratio 1.0) for different sodium silicate to sodium hydroxide ratios.

Fig. 7. Flow of POFA geopolymer paste (group two, solid to liquid ratio 1.32) for different sodium silicate to sodium hydroxide ratios.

Table 3 Bulk density for POFA geopolymer paste (group one and two) at different sodium silicate to sodium hydroxide ratios. Mix

Curing temperature (°C)

Bulk density (kg/m3) 7 days

14 days

28 days

Group one S/L = 1.0 Mix 1 Mix 2 Mix 3 Mix 4 Mix 5

SS/SH 1.0 1.5 2.0 2.5 3.0

60 60 60 60 60

1760.0 1760.0 1776.0 1794.4 1769.0

1762.0 1744.0 1752.0 1808.0 1816.3

1754.0 1784.0 1722.4 1792.0 1784.0

Group two S/L = 1.32 Mix 1 Mix 2 Mix 3 Mix 4 Mix 5

SS/SH 1.0 1.5 2.0 2.5 3.0

60 60 60 60 60

1782.0 1785.4 1797.0 1800.0 1824.0

1788.0 1783.0 1832.3 1789.0 1829.0

1752.4 1784.0 1798.0 1796.0 1813.0

3.2. Compressive strength Figs. 8 and 9 show the compressive strength of POFA geopolymer cement paste at two solid to liquid ratios. The maximum strength of 32 MPa was obtained at 28 days which may be an evidence that POFA can be described as high reactive material contributing effectively to the high rate of reaction development. The activity of alkali activated POFA may correspond to the ability of the ash to transition from a paste state to a hard mass when mixed with activator. This is compatible with other geopolymer binders using fly ash, GGBS, Metakaolin, etc. [54,68]. Previously, in a study by [48], the application of geopolymer with 100% POFA

Fig. 8. Compressive strengths of group one (SS/SH = 1) with solid to liquid ratio of 1 at 7, 14, and 28 days.

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Fig. 9. Compressive strengths of group two (SS/SH = 1.32) with solid to liquid ratio of 1 at 7, 14, and 28 days.

was not a great success in mortar as the maximum strength achieved at 28 days was 18 MPa with 65 °C for 24 h oven curing and SS/SH ratio of 2.49. The lower strength achieved comparing the present study may be due to a longer oven curing process. In the present study, oven curing of more than 2 h resulted in appearance of cracks which lowered the compressive strength [55]. The long oven curing time had also a negative effect on POFA geopolymerization in a study by Yusuf et al. [45], although sieving, grinding and incineration processes were applied on POFA. However, it was reported in [45,48] that POFA was blended with 70% and 20%

597

GGBS, respectively to reach higher strength. The binary mix of fly ash and low content POFA (30%) was also used to increase the compressive strength of geopolymer concrete [20]. Also, application of ternary mixes in geopolymer with the presence of POFA was reported in a work by Karim et al. [21]. They obtained strength up to 40 MPa at 28 days with ternary mixes mortars with POFA such as slag, and rice husk ash with the presence of NaOH as an activator. In this study, the results showed that there was a direct proportion between the compressive strength and SS/SH ratio. The compressive strength was increased by increasing the sodium silicate to sodium hydroxide ratio. This behavior may be attributed to the joint effect of sodium silicate and sodium hydroxide. The increment in the SS/SH ratio would result in increasing the sodium silicate and hence an increase in geopolymerization [34]. The augmented content of water glass as a liquid may act as a binder, alkali activator and dispersant or plasticizer which may advantageously affect the compressive strength [28]. As can be seen, the maximum strength was achieved at SS/SH ratio of 2.5 for both groups. However, at SS/SH ratio of 3.0 there was a slight decrease in compressive strength which may be attributed to the high amount of activating solution hindering the geopolymerization process. Previously, it was reported that the excess of sodium silicate is understood to inhibit the geopolymerization reaction through Al–Si phase precipitation which prevented the contact between the reacting material and the activator solution and decrease activator concentration [69,70]. A

Fig. 10. XRD patterns of group 1 (S/L) = 1 with SS/SH of (a) 1, (b) 1.5, (c) 2, (d) 2.5, and (e) 3.

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conclusion may be drawn that SS/SH ratio of 2.5 represents the optimum ratio for the applied activators. The same activator ratio was used in [20,48]. In a study by Hardjito and Rangan [34], SS/SH ratio of 2.5 also resulted in a higher compressive strength for activation of a low calcium fly ash to produce fly ash based geopolymer binder. However, the optimum amount of SS/SH ratio was reported to be 1.5 in geopolymerizing natural zeolite in a study by Villa et al. [70] where ratios of 0.4, 1.5, 5, and 10 were used. From another angle, it was observed that the compressive strength was higher in samples with a higher ratio of solid to liquid. In the first group of mixes with solid to liquid ratio (1.00), incorporation of POFA resulted in the strength of 24.48 MPa and 23.83 MPa for sodium silicate to sodium hydroxide ratios of 2.5 and 3.0, respectively, while solid to liquid ratio of 1.32 led to strength of 32.84 and 31.72 MPa at the same sodium silicate to sodium hydroxide ratio at 28 days. The low strength in group 1 firstly can be attributed to the high content liquid in the activator. This may affect the volume of voids and porosity in the pastes which directly influences the strength of the samples [33]. More air bubbles embedded in the structure of samples after hardening were also seen in this study which is in agreement with the previous works [65]. It was also reported that with the presence of higher content of the activator, the excess OH concentration left in the system weakens the structure of paste formed [58]. This may increase the efflorescence effect as was seen in Fig. 5.

3.3. Chemical and microstructural analysis 3.3.1. X-ray diffraction (XRD) X-ray diffraction patterns of ground POFA is illustrated in Fig. 1. The XRD pattern of ground POFA powder represents amorphous material characterized with low intensity diffused halo peak. This halo peak is centered from 15 to 40 two theta (°). The XRD pattern is also showing crystalline phase consists of strong dominant quartz (SiO2) at 26.64° and weak crystal at 20.88° and 29.38° (2 theta). The geopolymerization process showed two effects on the XRD patterns (Figs. 10 and 11). The first effect was changing the hump shape by shifting the diffused halo peak in the center towards larger angles. The change in the shape and the shift of the pattern may introduce an evidence to formation of a new amorphous material [71]; moreover, the new positioning of the hump was reported as a characteristic of a geopolymeric gel at 30° (2 theta) [72]. Secondly, it is clear that the lower crystalline peaks in XRD pattern of ground POFA disappeared after the alkali activation process; however, the main peak showed a decrease in its intensity rather than disappearing, and this behavior was detected for both groups of mixes. The same observation was reported in previous works [70,73,74]. This may correspond to the increase in the reaction with the increase of activator ratio. However, some researchers related the strength enhancement in the final geopolymer product to the presence of some of aforementioned crystalline phases [75,76]. Somna et al. [71] reported some crystalline phases

Fig. 11. XRD patterns of group 2 (S/L) = 1.32 with SS/SH of (a) 1, (b) 1.5, (c) 2, (d) 2.5, and (e) 3.

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Fig. 12. SEM image of geopolymerized POFA.

which are described as predominant quartz (SiO2) in addition to the main amorphous phase. As can be seen from Fig. 10, in group one with solid to liquid ratio of 1.0, there was a decrease in total intensity of the pattern to be more amorphous, while the SiO2 main peaks can still be strongly observed with a reduction of intensity 19.9%, 64%, 35%, 55%, and 51% for alkaline activator ratio 1.0, 1.5, 2.0, 2.5, and 3.0, respectively. In group two, Fig. 11, with solid to liquid ratio of 1.32, the reduction in lower pattern intensity was also observed, while reduction in the SiO2 main peaks was 31%, 31.4%, 28.83%, 0%, and 51% for alkaline activator ratios 1.0, 1.5, 2.0, 2.5, and 3.0 respectively. 3.3.2. Scanning electron microscope (SEM) In this study, the SEM images were used to illustrate the morphological features of POFA geopolymer pastes with the most effective ratio of SS/SH (2.5) and solid to liquid ratio of 1.32 for which the highest compressive strength was obtained. To better illustrate the formation of geopolymer gel, SEM images of ground and unground POFA were also provided and compared with geopolymerized POFA (Figs. 2 and 3). Fig. 2 shows the raw POFA before the grinding process while Fig. 3 displays the ground particles of POFA with median size of 15 lm. As can be seen, the size of particles are annotated. It can be seen that a discrete matrix of fine particles was formed after grinding. Grinding was reported to increase the reactivity of particles by geometry deformation from sponge like particles with very irregular shape and porous cellular surface [37,40] to crushed spherical particles with smaller size, rough surface and more homogenous distribution [52]. The geopolymerized POFA is shown in Fig. 12 and the main microstructural features; un-reacted matrix, partially reacted particles and dense gel phase are identified. As can be seen, a large portion of the raw material was activated and turned into a dense matrix with a well-connected structure consisting of glassy phase with no definite boundary for the dissolved particles which may well explain the registered compressive strength results. The glassy phase shown in the SEM image is in agreement with the results from XRD test. The gel formation was also reported by previous studies using GGBS and fly ash [74,77]. 3.3.3. Energy-dispersive X-ray spectroscopy analysis (EDX) To trace geopolymerization, EDX was also conducted on samples with solid to liquid ratio of 1.32 and SS/SH ratio of 2.5. Fig. 13 shows different points at which mapping was carried out. As can be seen, 7 points with different morphologies were selected and investigated. Fig. 14 shows the EDX spectrums for each point. As can be seen, major elements present are O, Si, Na, Ca, Fe, K, low Al and Mg [15]. The identified elements by EDX test are compatible with the

chemical composition of the raw material (POFA) which consisted of low content of alumina, high content of silicate, and calcium content of 11% (Table 1). Except for the available elements in POFA, sodium was also detected in the binding phase as it is the major element of the activator solution from sodium hydroxide and sodium silicate [78]. The high content of Si and Ca in the EDX spectroscope and a very low content of alumina may indicate that POFA geopolymer gel was mostly a result of formation of C–S–H rather than sodium-alumino-silicate hydrates (N–A–S–H) as reported in previous studies [20,44]. However, there was no evidence of calcium hydroxide precipitation in POFA geopolymer paste (Figs. 10 and 11). Also, it can be stated that the formed gel has a low Ca/Si ratio as can be seen from Fig. 15 which may play a pivotal role in strength gain in geopolymer binder [79]. As illustrated in the figure, Ca/Si ratio can be categorized in 3 classes: less than 0.1, 0.1–0.55 and 0.55–0.82 which may refer to low, medium and high levels of geopolymerization. The contribution of Ca to formation of C–S–H gel as mentioned above can be identified by two mechanisms [24,80]. The first mechanism states that Ca+ act as a charge-balancing agent and is integrated into the geopolymeric N–A–S–H network. The second mechanism of Ca+ contributes to the formation of C–S–H gel which can coexist with the geopolymeric gel. In this case, the geopolymeric and C–S–H gel act as independent phases [24]. However, in this study, the second mechanism seems to be dominant where

Fig. 13. EDX of POFA geopolymer paste with solid to liquid ratio 1.32 and SS/SH of 2.5 at different points.

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Fig. 14. EDX spectrums of points 1–7 (pt1–pt7).

formation of N–A–S–H was not remarkable. In a recent work by Provis [3], it is well discussed that products of the reaction between higher calcium binders and alkali metal silicates or hydroxide solutions are generally dominated by an aluminum substituted calcium silicate hydrate gel (C–A–S–H) which is tobermorite like in structure and broadly comparable to the gel structure resulting from Portland cement hydration. Substitution of Al in the C–S–H structure leads to a higher degree of polymerization and degree of crosslinking between tobermorite chains. However, in POFA the amount of Al is low and hence the dominant gel was C–S–H. The formation of C–S–H may contribute to soundness of the paste as discussed earlier in Section 3.1.1. Furthermore, the available Na from the alkali activators as seen from Fig. 15 may act as charge balancing sites or sorbed onto the gel itself and be

written as C–(N)–A–S–H [3]. In case of POFA with low content of Al, the gel structure may be addressed as C–(N)–S–H. Nonetheless, Škvára et al. [63] suggested that Na is weakly bound in the nanostructure of the gel and does not have a participation in the gel strength. 3.3.4. Fourier Transform Infra-Red (FTIR) The spectra of raw POFA and POFA geopolymer paste (S/ L = 1.32) with different sodium silicate to sodium hydroxide ratios were distinguished with six groups of bands in the regions of 750– 850 cm 1, 900–1200 cm 1, 1200–1300 cm 1, 1300–1600 cm 1, 1600–1700 cm 1, and 2850–3700 cm 1, (Fig. 16). The first peak was observed at wave number of 750–850 cm 1 [81] centered at 783 cm 1 which may refer to symmetric

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Fig. 17. DSC diagrams of raw POFA and alkali activated POFA (S/L = 1.32). Fig. 15. Ca/Si ratio in different points (pt1–pt7) from EDX test.

Fig. 16. FTIR spectra of raw POFA and geopolymerized POFA of group 2 (S/L = 1.32).

stretching vibration of Si–O–Si . It can be seen that transition was lower as alkali activators were added which may be attributable to a higher absorption rate of infrared by geopolymerized framework. The second peak may present asymmetric stretching vibration band of Si–O–T (T = Al, Si) which can be described as the strongest band registered in the region of 950–1222 cm 1. In this study it is assumed that this peak represents the C–S–H structure [21,72] since a very little amount of Al was traced in EDX test (Fig. 14). The third peak at wave number range of 1200–1300 cm 1 formed only in geopolymerized POFA which again may refer to stretching vibration of Si–O. This may be another evidence of geopolymerization process when alkali activators were added to the raw POFA. The other peak ranged from 1300 to 1600 cm 1 which may represent the stretching vibration of O–C–O [81]. This peak is attributable to the carbonation reaction. As can be seen, this reaction is more highlighted in geopolymerized POFA than that of raw POFA as there was an excessive amount of Na available in alkali activator which reacts with CO2 [77]. The other broad bands was almost centered around 3450 cm 1 (peak location between 3700 and 2850 cm 1) and centered at 1730 (1811–1684) cm 1 which may be due to the stretching vibration of O–H and bending vibration of H–O–H respectively [74]. These peaks may characterize the spectrum of stretching and deformation vibration of O–H and H–O–H groups from weakly bound water molecules which were adsorbed on the surface or trapped in large cavities between the rings of C–S–H [71,81]. In general, from the wide difference between the FTIR spectra of the raw POFA and the POFA activated with different sodium silicate to sodium hydroxide ratios, it can be concluded that an effective alkaline activation process of raw POFA (geopolymerization) occurred. 3.3.5. Differential Scanning Calorimetry (DSC) The Differential Scanning Calorimetry (DSC) thermograms of POFA geopolymer paste with solid to liquid ratio 1.32 and five

sodium silicate to sodium hydroxide ratios at the age of 28 days are illustrated in Fig. 17. As can be seen from the figure, one major endothermic peak was detected. This major peak centered at 117.55 °C, 114.49 °C, 119.40 °C, 121.42 °C, and 120.02 °C for the pastes with sodium silicate to sodium hydroxide ratio 1.0, 1.5, 2.0, 2.5, and 3.0, respectively. The peaks may correspond to release of water from partial dehydration of C–S–H clusters [82]. The endothermic results were 3.88803, 3.43106, 3.84209, 5.14029, and 7.41029 W g 1 for sodium silicate to sodium hydroxide ratios 1.0, 1.5, 2.0, 2.5, and 3.0, respectively. These results show that as alkaline ratio increased from 1 to 3 more energy was required to evaporate water from the C–S–H network which may indicate a higher rate of geopolymerization and a more stable geopolymerized network. The results from DSC are in agreement with the post-test observation from FTIR at wave number range of 2850–3700 cm 1 at which a broad bending vibration of H–O–H was observed with the presence of activator comparing that of raw POFA. A higher strength development was also observed at SS/SH ratio of 2.5 and 3 which may also show the enhanced geopolymerization process at aforementioned activator rates. The similar observations were reported in previous works [34]. 4. Conclusions This study focused on utilization of POFA as a geopolymer binder in paste. Qualitative observations and compressive strength of samples were recorded to assess the applicability of POFA as a single source of aluminosilicate in geopolymerization. The following conclusions may be drawn:  POFA geopolymer paste did not disintegrate or collapse in water and no cracks were observed by which the stability of the proposed geopolymer paste in water was confirmed.  Efflorescence was observed for all samples with higher concentration in mixes with solid to liquid ratio 1.0. Visual percentage of efflorescence decreased with the decrease of NaOH percentage of the total quantity of the activator.  The flowability of POFA geopolymer paste for group one with solid to liquid ratio 1.0 was higher than that of group two with solid to liquid ratio 1.32.  In general, the resulted geopolymer pastes showed comparable density with ordinary Portland cement paste for group one and two. The results showed that higher bulk density was obtained in samples in group two with solid to liquid ratio 1.32.  The maximum strength of 32 MPa was obtained at 28 days which may be an evidence that POFA can be described as high reactive material contributing effectively to the high rate of reaction development. The results showed that there was a direct proportion between the compressive strength and SS/

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SH ratio and the compressive strength was increased by increasing the sodium silicate to sodium hydroxide ratio. The maximum strength was achieved at SS/SH ratio of 2.5 for both groups. It was also observed that the compressive strength was higher in samples with higher ratio of solid to liquid. In the first group of mixes with solid to liquid ratio of 1.00, incorporation of POFA resulted in the strength of 24.48 MPa and 23.83 MPa for sodium silicate to sodium hydroxide ratios of 2.5 and 3.0, respectively, while solid to liquid ratio of 1.32 led to strength of 32.84 and 31.72 MPa at the same sodium silicate to sodium hydroxide ratio at 28 days.  The geopolymerization process showed two effects on the XRD patterns. The first effect was changing the hump shape by shifting the diffused halo peak in the center towards larger angles. Secondly, lower crystalline peaks in XRD pattern of ground POFA was observed after the alkali activation process.  SEM images showed that a large portion of the raw material was activated and turned into a dense matrix with a well-connected structure consisting of glassy phase with no definite boundary for the dissolved particles. In general, from the wide difference between the FTIR spectra of the raw POFA and the POFA activated with different sodium silicate to sodium hydroxide ratios, it can be concluded that an effective alkaline activation process of raw POFA (geopolymerization) occurred. However, EDX spectroscope indicated that POFA geopolymer gel was mostly a result of formation of C–S–H rather than sodium-alumino-silicate hydrates. The results from DSC test also confirmed the existence of C–S–H related endothermic peaks.

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