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The influence of fly ash characteristics and reaction conditions on strength and structure of geopolymers
Construction and Building Materials 94 (2015) 361–370
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
The influence of fly ash characteristics and reaction conditions on strength and structure of geopolymers Violeta Nikolic´ a,⇑, Miroslav Komljenovic´ a, Zvezdana Bašcˇarevic´ a, Nataša Marjanovic´ a, Zoran Miladinovic´ b, Rada Petrovic´ c a b c
Institute for Multidisciplinary Research, University of Belgrade, Serbia Institute of General and Physical Chemistry, Belgrade, Serbia Faculty of Technology and Metallurgy, University of Belgrade, Serbia
h i g h l i g h t s Effects of FA characteristics and reaction conditions on geopolymer strength were studied. Development of FA-geopolymer structure was studied using SEM/EDS and NMR analyses. The correlation of FA characteristics, structure and strength of geopolymer was given. Strength of geopolymers was correlated with a fraction of Al-rich structural units. Rapid test for assessment of FA reactivity was evaluated.
a r t i c l e
i n f o
Article history: Received 17 March 2015 Received in revised form 27 May 2015 Accepted 8 July 2015
Keywords: Fly ash Reactivity Geopolymer Compressive strength
a b s t r a c t Reactivity of Class F fly ash (FA), development of strength, and structure of FA-based geopolymers, depending on the reaction conditions, were examined in this paper. The results of SEM/EDS and NMR analyses revealed that the composition of aluminosilicate gel changed during reaction, i.e. Si/Al atomic ratio decreased with the reaction time. Higher strength of geopolymers was associated with a higher fraction of aluminum rich structural units, higher crosslinking, and more compact structure. A rapid test for the assessment of FA reactivity and thus the applicability of FA for geopolymer synthesis was proposed. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Geopolymers represent an alternative to Portland cement due to similar or even better binding properties [1,2]. Geopolymers are formed by alkali activation of solid aluminosilicate materials, usually fly ash (FA), or metakaolin, while alkali hydroxides and/or alkali silicates are generally used as alkali activators [3,4]. The main product of the reaction is a highly cross-linked network structure in the form of an amorphous aluminosilicate gel [5–7]. Since FA represents industrial waste that can be found all over the world, it is particularly attractive for the synthesis of geopolymers. Despite the fact that research in this area is intense and there are a large number of publications that suggest a wide range of ⇑ Corresponding author. E-mail address: [email protected] (V. Nikolic´). http://dx.doi.org/10.1016/j.conbuildmat.2015.07.014 0950-0618/Ó 2015 Elsevier Ltd. All rights reserved.
applications of these materials, FA-based geopolymers are still far from practical applications on a large scale. One of the main reasons is the variability of FA composition, which differs from source to source, and even within the same source. On the other hand, a large number of research results, which analyze different synthesis conditions of geopolymers, make it difficult to clearly identify the key factors that determine reactivity of FA, as well as the structure and characteristics of geopolymers. There are many factors that influence FA reactivity, and thus characteristics of FA-based geopolymers [8–12]. The most important factors are characteristics of the initial material (particle size distribution, content of glassy phase, reactive silicon, and aluminum, presence of iron, calcium, and inert particles), nature and concentration of the activator, as well as the reaction conditions [1–4,7]. Compressive strength of geopolymers is commonly used as a quantitative indicator of FA reactivity and characteristics of FA-based geopolymers [13,14].
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Particle size distribution is the key physical factor in the process of geopolymer synthesis [15,16]. FA reactivity increases with increasing FA fineness regardless of the nature and concentration of alkali activator [17]. Classification (extraction of fine fraction), milling, or mechanical activation of FA significantly increases its reactivity, resulting in geopolymers with improved properties [18–20]. Content of glassy phase of the initial material is an important parameter, since only a glassy phase represents reactive material that is converted during geopolymerization into the compacted binder [5,21,22]. Accordingly, FA reactivity or FA solubility in an alkaline solution depends on the composition and content of FA glassy phase [23]. Generally, higher glass content is associated with lower Si/Al ratio in the glassy phase [24], which means that higher amount of glassy phase contains higher amount of soluble aluminum. The relative amount of soluble silicon and aluminum present in an initial FA defines the Si/Al ratio in aluminosilicate gel, and thus the mechanical properties of resulting geopolymers [21]. However, as the term ‘‘reactivity’’ of FA does not only refer to the glassy phase, but to the FA as a whole, it means that the content of FA glassy phase has limited significance. Therefore, an assessment of FA reactivity is usually based on the FA dissolution rate in an alkaline solution, by measuring FA mass change or concentration of dissolved FA components over time [25–27]. Nature and concentration of alkali activator have dominant influence on the structure and properties of geopolymers [17]. If activation of FA is performed by alkali hydroxide, a gel rich in aluminum is first formed, followed by a gel rich in silicon [28,29]. However, in systems where the activation of FA is performed by solution containing high-ordered silicate species, a gel rich in silicon is formed immediately, without the previous formation of an aluminum-rich gel [30]. Properties of geopolymers are also determined by reaction conditions (temperature, time, and relative humidity), whereby an elevated reaction temperature leads to faster development of geopolymer strength [29]. There are opposing views on the optimal temperature for FA-based geopolymers synthesis. Some authors consider that optimal temperature of synthesis is 75–80 °C, since a significant occurrence of microcracks [31] or even strength loss above this temperature was observed [32–34]. On the other hand, very high strength (over 100 MPa) of geopolymers is achieved by alkali activation of FA at 95 °C [35]. Longer reaction time is generally beneficial for geopolymer strength increase [29]; however, after 48 h of reaction at elevated temperature the increase of strength is usually not significant [36]. Some authors consider that in order to achieve optimal properties of geopolymers at elevated temperature (95 °C), a shorter reaction time (6 h compared to 24 h) is more beneficial [37]. At last, but certainly not least, high relative humidity (>90%) is imperative for achieving optimal composition and structure of reaction products, as well as mechanical characteristics of geopolymers [35,38,39]. Since the FA presents industrial waste with variables characteristics, one of the main obstacles toward practical application of FA-based geopolymers is a lack of standards and evaluation criteria for FA reactivity in the process of geopolymerization. Besides, the factors that control phase development and their influence on the geopolymers characteristics are not yet fully understood. Given that there are a large number of influencing factors, it is clear that there is no simple methodology for assessing FA reactivity in the process of geopolymerization. The main aim of the present work was to evaluate rapid tests for assessment of FA reactivity and suitability of FA for geopolymer synthesis, by studying the development of strength and structure of geopolymers as the function of FA characteristics and geopolymerization reaction conditions (temperature and time). Rapid tests might be useful for practical application.
2. Materials and methods 2.1. Materials In this study, FA samples from three thermal power plants (TPP) from Serbia were used: 1. FA Morava, TPP Morava, Svilajnac. 2. FA Kolubara, TPP Kolubara, Veliki Crljani. 3. FA Kostolac B1, TPP Kostolac B1, Kostolac. Sodium silicate solution was used as alkali activator (‘‘Galenika-Magmasil’’, 8.72% Na2O, 26.5% SiO2, 64.78% H2O). Starting sodium silicate modulus n = SiO2/Na2O (mass ratio) of 3.04 was further adjusted by adding NaOH (p.a. (min.99%), VWR). 2.2. Characterization of the initial FA samples It should be noted that in this study FA was used in its original form, as received from electrostatic precipitators, without classification (extraction of fine fractions) or removal of large, less reactive particles (predominantly quartz and char). 2.2.1. Chemical, physical, and mineralogical characterization of FA Chemical composition of FA samples was determined by classic chemical analysis – alkali melting. FA fineness was determined by sieving through meshes of 63 lm and 43 lm, according to Serbian standard SRPS B.C1.018, which refers to pozzolanic materials – constituents for cement production – classification, technical conditions and test methods. Mineral composition of FA samples was determined by means of X-ray diffraction (XRD) analysis. Powder diffractometer ‘‘Phillips PW 1710’’ (Cu Ka = 1.54178 Å) was used. The diffraction patterns were recorded within 5–50° 2h range, with a step of 0.02° and holding time of 1 s per step. For the identification of the crystalline phases, the software PCPDFWIN was used. 2.2.2. Glassy phase of FA Glassy phase content was determined by dissolving FA in 1% HF acid – Arjuan’s method [40]. 1% HF dissolves the glassy phase of FA, while crystalline phases (usually quartz, mullite, hematite, and magnetite) remain intact [5,21,25]. This method involves the treatment 1 g of FA with 100 ml of 1% HF for six hours with constant stirring. After the treatment with HF acid, FA samples were dried at 105 °C to constant mass. The content of glassy phase was determined by subtracting the residual mass from the initial one. 2.2.3. Solubility of FA in a highly alkaline medium FA solubility in a highly alkaline medium was determined using concentrated sodium hydroxide (NaOH) solution, whereby two different methods were used: 1. FA dissolution at room temperature (20 °C, 5 h in 10 M NaOH) [27]. 2. FA dissolution at elevated temperature (95 °C, 0.5 h in 7 M NaOH). It is well known that the dissolution process accelerates as the temperature increases. The experiment at elevated temperature (95 °C) was chosen not only to accelerate endothermic reaction of dissolution, but also because the temperature of 95 °C represented the maximum temperature of reaction during this investigation. Prior to dissolution the FA was dried at 105 °C for 1 h. The liquid/solid (L/S) mass ratio was 40. The dissolution process was performed with constant stirring. Given that the high dissolutions are necessary to avoid undesirable precipitation [41], the resulting solution of FA dissolution in NaOH solution was further diluted 50 times and acidified with 37% HCl to a pH value of 7. Optical emission spectrometry with inductively coupled plasma (ICP-OES, Spectro Analytical Instruments GmbH) was used to determine the concentration of dissolved elements. 2.3. Synthesis of geopolymers The synthesis of geopolymers was performed according to previously optimized procedure [17]. Sodium silicate solution with modulus 1.5 was used as alkali activator, whereby modulus was adjusted by adding (10 M) NaOH solution to the starting sodium silicate solution. Concentration of the activator was 10% Na2O with respect to the FA mass. Geopolymer mortars were prepared by mixing FA with alkali activator and water, and then with standard sand (EN 196-1). Water was added in the amount required to obtain equal consistency (mortar flow measured on a flow table was 125 ± 5 mm). Water/binder ratios (water represents the total amount of water in the system, including water from the activator, while binder represents the total mass of FA and a solid part of activator) in the case of mortar based on FA Morava, Kolubara, and Kostolac B1 were 0.46, 0.61, and 0.67
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respectively. FA/sand ratio was 1/3. Casting of mortar into molds (prisms 40 40 160 mm) was carried out on a vibrating table. Molds with mortar prisms, wrapped in a plastic foil to prevent moisture loss, were exposed to an elevated temperature (55 °C, 80 °C, and 95 °C) in an oven, for 4 h, 8 h, 16 h, and 24 h. Geopolymer pastes, prepared by mixing FA, activator, and water, were exposed to the same conditions as mortars. After the specified period of reaction, paste samples were crushed and pulverized in isopropyl alcohol for 1 h to stop further reaction. After pulverization, the samples were filtered, rinsed with acetone, and dried at 50 °C to constant mass.
2.4. Characterization of geopolymers Determination of compressive strength of geopolymers was performed on mortar samples, while SEM/EDS and NMR analyses were performed on paste samples. Compressive strength was investigated according to SRPS EN 196-1 standard using the CONTROLS ADVANTEST 9 device. Microstructural analysis was done by scanning electron microscopy (SEM, VEGA TS 5130 MM, Tescan). Energy dispersive X-ray spectroscopy (EDS) was performed by INCAPentaFET-x3 (OXFORD Instruments). Samples for EDS analysis were prepared by grinding and polishing (MTI Corporation) of epoxy impregnated samples. Grinding was done using SiC grinding papers (300, 600, 1200, and 2000 grit, 3 min each) with acetone as lubricant. Final polishing of the samples was done using polishing cloths and diamond pastes (0.5 lm and 0.25 lm, MTI Corp., 3 min each). Prior to SEM/EDS analysis samples were Au-Pd coated. 29 Si MAS NMR spectra of geopolymers were obtained at Larmor frequency of 79.49 MHz using Bruker MSL 400 system, Apollo console upgraded (Tecmag). Chemical shifts d(29Si) were externally referenced to the 2,2-dimethyl-2-silapenta ne-5-sulfonate (DSS) standard. Gaussian peak deconvolution of the obtained spectra was performed using DMFIT application [42], by employing a common routine for peak distribution analysis. Prior to 29Si MAS NMR analysis, iron content of geopolymers was minimized by exposing samples to a strong magnetic field.
3. Results and discussion 3.1. Characterization of the initial FA 3.1.1. Chemical, physical, and mineralogical characterization of FA Chemical composition of the initial FA samples, fineness, and glassy phase content are shown in Table 1. All FA samples had a low content of CaO (<10%) and unburned material (<4%). Since the sum of oxides SiO2, Al2O3 and Fe2O3 exceeded 70%, all three FA can be classified as the Class F (according to ASTM C 618-12 standard). The highest content of glassy phase and particles smaller than 43 lm was observed in FA Morava. On the other hand, in FA Kostolac B1 the lowest content of glassy phase and the highest content of iron oxide were observed.
Table 1 FA chemical composition, fineness, and glassy phase content. Component
Morava
Kolubara
Kostolac B1
SiO2 (%) Al2O3 (%) Fe2O3 (%) CaO (%) MgO (%) SO3 (%) Na2O (%) K2O (%) Loss on ignition at 1000 °C (%) Total
55.23 21.43 7.42 7.94 2.61 0.81 0.64 1.35 1.66 99.09
62.13 17.20 5.95 5.67 2.00 0.67 0.58 1.04 2.88 98.12
46.85 23.20 12.14 8.26 2.77 1.48 0.40 0.81 3.44 99.35
Classification according to ASTM C 618-12 standard
Class F
Class F
Class F
Particle size > 63 lm (%) Particle size 43–63 lm (%) Particle size < 43 lm (%)
23.30 8.05 68.65
42.65 5.30 52.05
56.50 6.00 37.50
Glassy phase content (%)
48
38
34
Fig. 1. X-ray diffraction analysis of the initial FA samples.
Smaller fly ash particles (<20 lm) are more likely to have a highly glassy composition, as small particles quench faster than large particles [43]. Van Riessen and Chen-Tan [18] noticed that the amorphous material is the most abundant in the fine fraction of FA particles. Higher reactivity of aluminosilicate glass induced by the increase of particles fineness was confirmed by other authors [44]. The results of this work also indicated that the content of FA glassy phase increased with the increase of particles fineness. The results of X-ray diffraction analysis of the initial FA samples are presented in Fig. 1. These results showed the presence of crystalline phases typical for FA such as: quartz (PDF# 46-1045), feldspar (PDF# 89-1477), anhydrite (PDF# 06-0226), hematite (PDF# 88-2359), mullite (PDF# 85-1460), and amorphous phase (indicated by diffuse halo extending from approximately 20° to 35° 2h). It is evident that there were differences in the mineral composition of the initial FA samples. FA Kolubara had the highest content of quartz, while FA Kostolac B1 had somewhat higher content of hematite than other two FAs. These XRD findings are in accordance with the results of FA chemical analysis. 3.1.2. Solubility of FA in sodium hydroxide solution Testing results of FA solubility in concentrated NaOH solution under different reaction conditions are shown in Table 2. Concentrations of soluble Si and Al were several times higher at elevated temperature (95 °C) in relation to room temperature, despite lower molarity of the alkali activator (7 M NaOH compared to 10 M NaOH), and shorter dissolution time (0.5 h compared to 5 h). Therefore, the solubility of FA in alkaline solution was more pronounced at elevated temperature. The highest concentration of soluble Si and Al was observed in
Table 2 Solubility of FA in NaOH solution. Sample FA
Conditions of dissolution 20 °C (5 h in 10 M NaOH)
Morava Kolubara Kostolac B1
95 °C (0.5 h in 7 M NaOH)
[Si] (mmol/l)
[Al] (mmol/l)
[Si] (mmol/l)
[Al] (mmol/l)
15.2 14.4 13.9
7.3 6.9 7.3
73.6 61.1 59.1
33.3 32.1 28.9
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Fig. 2. Compressive strength of geopolymers versus temperature and time of reaction.
FA Morava, which contained the highest amount of glassy phase and finer particles. Elevated temperature increased the dissolution rate of FA glassy phase components in the alkaline solution as observed previously [26]. Presented results clearly indicate the importance of elevated temperature in the process of FA dissolution, and thus in the entire geopolymerization process. 3.2. Characterization of geopolymers 3.2.1. Compressive strength of geopolymers The results of compressive strength of geopolymer mortars (Fig. 2) indicated the increase of geopolymer strengths with the increase of temperature from 55 °C to 95 °C, regardless of the FA characteristics. Strength also increased with the increase of reaction time, which was more pronounced in the initial period. Generally, the geopolymers exposed for 8 h to temperature of 80 °C or 95 °C, reached an equal or higher compressive strength compared to the strength of geopolymers exposed for 24 h to 55 °C. At higher temperature considerably shorter time was necessary to achieve high strength, which is consistent with the results of other authors [29]. Although some authors [32–34] consider that the optimal temperature for the geopolymers synthesis is 75–80 °C, since higher temperatures lead to a strength loss, the results of this study showed that it is not universal trend, i.e. with the increase of temperature reaction up to 95 °C the strength loss did not occur. Quite contrary, the strength of geopolymers synthesized at 95 °C in all
Table 3 Relative strength of geopolymers. FA
Temp. (°C)
Relative strength (%) in relation to strength after 24 h at 95 oC 4h
8h
16 h
24 h
Morava Kolubara Kostolac B1
55
17 16 15
52 46 50
75 71 74
82 79 75
Morava Kolubara Kostolac B1
80
59 47 46
83 79 71
93 85 82
93 94 92
Morava Kolubara Kostolac B1
95
74 57 53
91 81 77
96 93 85
100 100 100
investigated cases was higher than the strength of geopolymers synthesized at 80 °C, regardless of the initial FA characteristics. The reasons for this discrepancy probably lie in different characteristics of the investigated fly ashes and/or different experimental conditions. Considering the rapid development of geopolymer strength at elevated temperature (after 4 h of reaction at 95 °C relative strength exceeded 50% of strength achieved after 24 h of reaction at the same temperature, while after 8 h of reaction relative strength amounted above 75%; Table 3), reaction at 95 °C in a relatively short time (4–8 h) provides an opportunity for rapid assessment of FA reactivity in reaction of geopolymerization.
Fig. 3. Compressive strength of geopolymers (after 24 h at different temperatures) versus FA solubility in 7 M NaOH at 95 °C: (a) Si and (b) Al.
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3.2.2. Correlation of geopolymer strength with the initial FA characteristics Higher content of glassy phase does not necessarily lead to higher reactivity [25], although some literature data suggested that, higher content of glassy phase in FA leads to higher FA reactivity and better mechanical properties of FA-based geopolymers [5,18]. Compared to the content of FA glassy phase, more reliable
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measure of FA reactivity is the content of soluble silicon and aluminum [26]. Dependence of geopolymers compressive strength (after 24 h at different temperatures) on the concentration of soluble Si and Al, resulting from the FA dissolution in 7 M NaOH at 95 °C, is given in Fig. 3. The highest geopolymer compressive strength under all experimental conditions was obtained based on FA Morava, which had the highest content of particles smaller
Fig. 4. Microstructure of geopolymers: (a) FA Morava – 4 h at 55 °C, (b) FA Morava – 24 h at 55 °C, (c) FA Morava – 4 h at 95 °C, (d) FA Morava – 24 h at 95 °C, (e) FA Kolubara – 4 h at 95 °C and (f) FA Kostolac B1 – 4 h at 95 °C.
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than 43 lm, the highest content of glassy phase, and the highest content of soluble Si and Al. On the other hand, geopolymers based on FA Kostolac B1, which had the lowest content of particles smaller than 43 lm, glassy phase, and soluble Si and Al, showed the lowest compressive strength in all cases. Since alkali activation of FA was carried out by sodium silicate, an excess of soluble Si was initially present in the solution. In that case, the major differences in chemical reactivity of FA derived from the soluble Al, i.e. soluble Al represented the measure of FA reactivity [41]. A certain amount of soluble aluminum is required to initiate a geopolymerization reaction [7]. In addition, aluminum is a component that determines important characteristics of geopolymers, such as setting time and strength [28,45,46]. Therefore, from the viewpoint of chemical characteristics, the content of soluble aluminum is the dominant factor that determines the process of geopolymerization. Due to the established correlation, the process of FA dissolution in 7 M NaOH at 95 °C for 0.5 h could be used for rapid assessment of FA reactivity and thus the applicability of FA for the synthesis of geopolymers.
method should not be regarded as absolute values, but rather as values that indicate a certain trend. Presented results indicate that the composition of aluminosilicate gel changed during geopolymerization reaction, i.e. the Si/Al atomic ratio decreased with the reaction time, regardless of the temperature. Considering that the alkali activation of FA was performed by sodium silicate, an excess of soluble silica was initially introduced in the system, which resulted in a very high Si/Al atomic ratio (4.4–4.5) in the initial period of reaction (after 4 h). As the reaction proceeded, further dissolution of FA (in which Si/Al ratio was lower) led to reduction of Si/Al atomic ratio in aluminosilicate gel, again independently of reaction temperature. 3.2.5. NMR analysis 29 Si MAS NMR spectra of FA Morava and geopolymer pastes (G) obtained by alkali activation of the same FA and exposed to different reaction conditions are shown in Fig. 6. Spectrum of the
3.2.3. SEM analysis of geopolymers Microstructure of geopolymers based on different FAs and synthesized under different reaction conditions is shown in Fig. 4. Products of geopolymerization reaction represented a heterogeneous material, i.e. the matrix comprising aluminosilicate gel and incorporated unreacted FA particles. After a short time (4 h) hardened matrix of geopolymers based on FA Morava, contained a significant proportion of unreacted FA particles (Fig. 4a and c), while after 24 h (Fig. 4b and d) considerably less unreacted FA particles were noticed. Less compact matrix, compared to the geopolymers based on FA Morava, was observed in the case of the geopolymers based on FA Kolubara (Fig. 4e) and FA Kostolac B1 (Fig. 4f). It is well known that high strength is related to the compact microstructure [13]. Different microstructure of geopolymers was the result of differences in the reactivity of FA particles, which was consistent with the results of compressive strength testing. 3.2.4. EDS analysis of geopolymers Influence of the reaction conditions (temperature and time) on the composition of aluminosilicate gel and structure of geopolymers was examined on geopolymers based on FA Morava that showed the highest strength. Atomic ratios of major elements in aluminosilicate gel depended on the temperature and time of reaction, whereby Si/Al atomic ratio ranged from 3.2 to 4.5 (Fig. 5). Phase inhomogeneities showing regions of high silicon concentration (Si/Al 5) were also observed by other authors [47]. Therefore, displayed values of Si/Al atomic ratios obtained by graphical
Fig. 6. 29Si NMR analysis of FA Morava (a) and geopolymers based on FA Morava (b).
Fig. 5. Atomic ratios of major elements of aluminosilicate gel versus temperature and time of reaction (geopolymers based on FA Morava): (a) 4 h and (b) 24 h.
V. Nikolic´ et al. / Construction and Building Materials 94 (2015) 361–370
initial FA (Fig. 6a) was very wide and pronouncedly asymmetric indicating heterogeneous distribution of silicon structural units. Resonance ranged between 80 and 108 ppm is mainly associated to the initial glassy phase material, while most of the resonances appearing around and above 108 ppm are assigned to different crystalline phases of silica (Q4(0Al) signals) [15,48]. Contrary to the spectrum of the initial FA, spectra of the geopolymer samples (Fig. 6b) showed broad and considerably symmetric resonance lines, which were centered around 93 ppm indicating poorly ordered aluminosilicate structures. The 29Si MAS NMR spectra in this work were deconvoluted assuming Gaussian line shapes, constant line widths, and same number of peaks. The results of geopolymer 29Si MAS NMR spectra deconvolution are shown in Fig. 7. All of these spectra comprise of overlapping resonances attributed to the silicon sites present in aluminosilicate gel, un-reacted glassy phases, and crystalline phases originating from the initial FA [49,50]. It is generally accepted that 29Si MAS NMR spectra of geopolymers consist of all five Q4(mAl) silicon species, with Q4(4Al), Q4(3Al), Q4(2Al), Q4(1Al) and Q4(0Al) resonating at approximately 84, 89, 94, 99 and 107 ppm, respectively [49–52]. In addition to the Q4(mAl) silicon peaks, small resonances at approximately 77, 107 and 115 ppm (Fig. 7a) were observed in all of the 29Si MAS NMR spectra. Small peak at 77 ppm is usually ascribed to less condensed silicon species, Q1 and Q2 [48]. Sharp peak at 107 ppm indicates an ordered structure, which can be attributed to quartz originating from the initial FA [49,50]. Peak at 115 ppm can be attributed to different silica polymorphs [5,15] or Q4(0Al) units in the products of alkali activation reaction [49,50,53]. The deconvolution 29Si MAS NMR spectra showed that the most common structural units in aluminosilicate gel were Q4(2Al) and
Fig. 7. Deconvolution of
29
367
Q4(3Al), regardless of the reaction temperature and time. The most pronounced change over time at 55 °C was the reduction of fraction of Q4(0Al) structural units (Table 3). In contrast, the fraction of Q4(3Al), Q4(2Al) and Q4(1Al) structural units increased, while the fraction of Q4(4Al) unit did not change significantly. On the other hand, at 95 °C the most pronounced change over time was the increase of the fraction of Q4(4Al) unit, while the fraction of other structural units Q4(mAl), m = 0, 1, 2, or 3, slightly changed. The increase of fraction of Q4(4Al) unit over time during reaction at 95 °C was a consequence of higher solubility and higher availability of aluminum at higher temperature. The increase of fraction of Q4(4Al) unit was also the most significant difference in the structural organization of aluminosilicate gel at 95 °C in respect to the reaction at 55 °C. The deconvolution of 29Si MAS NMR spectra also indicated the presence of less condensed structural units of silicon (Q1 and Q2), probably from unreacted activator. Other authors also attributed these signals to the residual and less condensed species, monomer or dimer units with silanol groups [28,48]. Fraction of Q1 and Q2 did not change significantly over time at 95 °C, probably due to the faster reaction at higher temperature and consequently hindered diffusion, i.e. these units remained in the structure of the initially formed gel. However, at 55 °C the reaction was slower, diffusion through hardened matrix was faster and the fraction of these units decreased over time. Despite observed significant differences in geopolymer strength, the results of 29Si MAS NMR spectra deconvolution indicate that the increase of reaction temperature and time led to small changes of geopolymer Si/Al atomic ratio. Similar phenomenon was also noticed by other authors [28,48,54]. Discrepancy in the values of Si/Al atomic ratios obtained by NMR spectra deconvolution and EDS analysis (Fig. 5 and Table 4)
Si MAS NMR spectra of geopolymers based on FA Morava: (a), (b) at 55 °C and (c), (d) at 95 °C.
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Table 4 Si MAS NMR spectra deconvolution of geopolymers based on FA Morava (Fig. 7).
29
Reaction time and temperature
Q1,2a
Q4(4Al)
Q4(3Al)
Q4(2Al)
Q4(1Al)
Q4(0Al)
Quartzb
Q4b
Si/Alc
4 h 55 °C
d (ppm) Width (ppm) Area (%)
77.60 7.0 3.94
83.22 7.0 12.15
88.45 7.0 25.87
94.07 7.0 25.52
100.09 7.0 16.37
107.36 7.0 13.25
107.24 1.4 0.95
115.07 7.0 1.95
1.92
d (ppm) Width (ppm) Area (%)
77.00 7.0 2.26
83.30 7.0 12.09
88.64 7.0 27.63
94.20 7.0 26.93
100.05 7.0 16.87
107.25 7.0 10.62
107.16 1.32 1.16
115.07 7.0 2.44
1.86
d (ppm) Width (ppm) Area (%)
77.20 7.0 4.05
84.10 7.0 13.57
89.23 7.0 25.78
94.60 7.0 26.37
100.52 7.0 16.30
107.66 7.0 10.25
107.29 1.36 1.12
116.18 7.0 2.55
1.84
d (ppm) Width (ppm) Area (%)
77.00 7.0 4.67
83.86 7.0 15.28
89.28 7.0 26.45
94.55 7.0 25.76
100.28 7.0 16.14
107.34 7.0 9.42
107.16 1.32 1.04
115.61 7.0 1.22
1.79
24 h 55 °C
4 h 95 °C
24 h 95 °C
a b c
Signals associated with less condensed silicon species, Q1 and Q2 [48–50]. Signals associated with quartz ( 107 ppm) and other Q4 silicon units [5,15,49,50,53]. Si/Al ratio calculated from the deconvolution results [52].
was also noticed by Fernandez-Jimenez et al. [28]. Lodeiro et al. suggested that this discrepancy is associated with deconvolution procedure and an assumption that the principal components of the 29Si MAS NMR spectrum are based on Q4(mAl) structural units [55]. 3.2.6. Correlation of strength of geopolymers with structural organization of the aluminosilicate gel In general, during alkali activation of FA with sodium silicate solution, the fraction of aluminum rich structural units [Q4(3Al) and Q4(4Al)] increased in respect to the fraction of silicon rich structural units [Q4(0Al), Q4(1Al) and Q4(2Al)], regardless of the reaction temperature (55 °C or 95 °C). On the other hand, the strength of geopolymers increased over time, also regardless of the reaction temperature (Fig. 2). Therefore, it is clear that the geopolymer strength depended on the fraction of Q4(mAl) structural units. The relationship between compressive strength of geopolymers based on FA Morava and fraction of Q4(mAl) units in the 29Si MAS NMR spectra is given in Fig. 8. The increase of the fraction of aluminum rich structural units indicates higher degree of substitution of Si–O–Si bond by Si–O–Al bonds and reorganization of the three-dimensional aluminosilicate network during geopolymerization, which resulted in the strength increase. Although this phenomenon was more
Fig. 8. Compressive strength of geopolymers based on FA Morava versus fraction of 29 Si MAS NMR Q4(mAl) units.
pronounced at lower temperature (55 °C), it was also clearly present at higher temperature (95 °C). As the compressive strength of geopolymers is an indication of the degree of polymerization of newly formed aluminosilicate gel [56], the differences in compressive strength of geopolymers could be explained by different degree of gel polymerization at various temperatures. Therefore, higher strength of geopolymers was the result of a higher fraction of aluminum rich structural units, higher crosslinking, and more compact structure. 4. Conclusion This paper examined the development of compressive strength and structure of geopolymers depending on characteristics of Class F fly ash (FA) and conditions of geopolymerization reaction. Geopolymers were synthesized by alkali activation of FA using sodium silicate solution with modulus 1.5 at elevated temperature (55, 80, and 95 °C) in the period up to 24 h. There was a clear relationship between FA characteristics, reaction conditions, and strength of geopolymers. The different reactivity of FA was a consequence of their different characteristics (fineness, content of glassy phase, soluble silicon, and aluminum in a highly alkaline medium). Maximum strengths of geopolymers were obtained after 24 h at 95 °C, regardless of the FA characteristics. The geopolymer with the highest compressive strength, regardless of the reaction temperature and time, was synthesized based on FA Morava, which had the highest content of particles smaller than 43 lm, the highest content of glassy phase, and the highest content of soluble aluminum. Given that alkali activation of FA was performed by sodium silicate solution, content of soluble aluminum was the dominant factor that determined the process of geopolymerization. Elevated temperature accelerated geopolymerization reaction and led to the rapid formation of aluminosilicate gel as the main reaction product. At lower temperature the reaction time became an important factor. The composition of aluminosilicate gel changed during geopolymerization reaction, i.e. the Si/Al atomic ratio decreased over time (SEM/EDS analysis). The results of 29Si MAS NMR analysis confirmed that the increase of reaction time led to the increase of fraction of aluminum rich structural units [Q4(3Al) and Q4(4Al)] compared to silicon rich units [Q4(0Al), Q4(1Al) and Q4(2Al)]. Higher strength of geopolymers was the result of higher fraction of aluminum rich structural units, higher crosslinking, and more compact structure.
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Due to the established correlation it was concluded that FA solubility in 7 M NaOH (0.5 h at 95 °C) might serve as a test for rapid assessment of FA reactivity. The highest increase of strength took place during the first 4–8 h of the reaction, which represents rational reaction time from a technological point of view. Compressive strength of geopolymer after 4 h of reaction at 95 °C also might serve as a test for rapid assessment of FA reactivity and thus the applicability of FA for synthesis of geopolymers. Acknowledgements The results presented in this paper are part of the research project TR 34026, financed by the Ministry of Education, Science and Technological Development, Republic of Serbia and COST Action TU 1301. The authors would like to thank Ljiljana Milicˇic´ from Institute for testing materials and Prof. Dr. Miroslav Nikolic´ from Institute for Multidisciplinary Research, University of Belgrade, for their valuable help during the preparation of this paper. References [1] C. Shi, A. Fernandez-Jimenez, A. Palomo, New cements for the 21st century: the pursuit of an alternative to Portland cement, Cem. Concr. Res. 41 (2011) 750– 763. [2] F. Pacheco-Torgal, J. Castro-Gomes, S. Jalali, Alkali-activated binders: a review. Part 2. About materials and binders manufacture, Constr. Build. Mater. 22 (2008) 1315–1322. [3] D. Khale, R. Chaudhary, Mechanism of geopolymerization and factors influencing its development: a review, J. Mater. Sci. 42 (2007) 729–746. [4] M. Komljenovic´, Mechanical strength and Young’s modulus of alkali-activated cement-based binders, in: F. Pacheco-Torgal, J.A. Labrincha, C. Leonelli, A. Palomo, P. Chindaprasirt (Eds.), Handbook of Alkali-activated Cements, Mortars, and Concretes, Woodhead Publishing Series in Civil and Structural Engineering, 2015, pp. 171–217 (Number 54). [5] A. Fernandez-Jimenez, A.G. de la Torre, A. Palomo, G. Lopez-Olmo, M.M. Alonso, M.A.G. Aranda, Quantitative determination of phases in the alkali activation of fly ash. Part I. Potential ash reactivity, Fuel 85 (2006) 625–634. [6] J.L. Provis, C.Z. Yong, P. Duxon, J.S.J. van Deventer, Correlating mechanical and thermal properties of sodium silicate-fly ash geopolymers, Colloids Surf. A 336 (2009) 57–63. [7] P. Duxon, A. Fernandez-Jimenez, J.L. Provis, G.C. Lukey, A. Palomo, J.S.J. van Deventer, Geopolymer technology: the current state of the art, J. Mater. Sci. 42 (2007) 2917–2933. [8] A. Fernandez-Jimenez, A. Palomo, M. Criado, Microstructure development of alkali-activated fly ash cement: a descriptive model, Cem. Concr. Res. 35 (2005) 1204–1209. [9] J.S.J. van Deventer, J.L. Provis, P. Duxon, G.C. Lukey, Reaction mechanisms in the geopolymeric conversion of inorganic waste to useful products, J. Hazard. Mater. A139 (2007) 506–513. [10] C.A. Rees, J.L. Provis, G.C. Lukey, J.S.J. van Deventer, The mechanism of geopolymer gel formation investigated through seeded nucleation, Colloids Surf. A (2008) 97–105. [11] A. Hajimohammadi, J.L. Provis, J.S.J. van Deventer, Effect of alumina release rate on the mechanism of geopolymer gel formation, Chem. Mater. 22 (2010) 5199–5208. [12] A. Hajimohammadi, J.L. Provis, J.S.J. van Deventer, The effect of silica availability on the mechanism of geopolymerisation, Cem. Concr. Res. 41 (2011) 210–216. [13] M. Steveson, K.S. Crenstil, Relationships between composition, structure and strength of inorganic polymers. Part 2. Fly ash-derived inorganic polymers, J. Mater. Sci. 40 (2005) 4247–4259. [14] J.E. Oh, Y. Jun, Y. Jeong, P.J.M. Monteiro, The importance of the networkmodifying element content in fly ash as a simple measure to predict its strength potential for alkali-activation, Cem. Concr. Compos. 57 (2015) 44–54. [15] A. Fernandez-Jimenez, A. Palomo, Characterisation of fly ashes. Potential reactivity as alkaline cements, Fuel 82 (2003) 2259–2265. [16] J.G.S. van Jaarsveld, J.S.J. van Deventer, G.C. Lukey, The characterisation of source materials in fly ash-based geopolymers, Mater. Lett. 57 (2003) 1272– 1280. [17] M. Komljenovic´, Z. Bašcˇarevic´, V. Bradic´, Mechanical and microstructural properties of alkali-activated fly ash geopolymers, J. Hazard. Mater. 181 (2010) 35–42. [18] A. van Riessen, N. Chen-Tan, Beneficiation of Collie fly ash for synthesis of geopolymer: Part 1 – Beneficiation, Fuel 106 (2013) 569–575. [19] N. Marjanovic´, M. Komljenovic´, Z. Bašcˇarevic´, V. Nikolic´, R. Petrovic´, Improving reactivity of fly ash and properties of ensuing geopolymers through mechanical activation, Constr. Build. Mater. 57 (2014) 151–162.
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Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
The influence of fly ash characteristics and reaction conditions on strength and structure of geopolymers Violeta Nikolic´ a,⇑, Miroslav Komljenovic´ a, Zvezdana Bašcˇarevic´ a, Nataša Marjanovic´ a, Zoran Miladinovic´ b, Rada Petrovic´ c a b c
Institute for Multidisciplinary Research, University of Belgrade, Serbia Institute of General and Physical Chemistry, Belgrade, Serbia Faculty of Technology and Metallurgy, University of Belgrade, Serbia
h i g h l i g h t s Effects of FA characteristics and reaction conditions on geopolymer strength were studied. Development of FA-geopolymer structure was studied using SEM/EDS and NMR analyses. The correlation of FA characteristics, structure and strength of geopolymer was given. Strength of geopolymers was correlated with a fraction of Al-rich structural units. Rapid test for assessment of FA reactivity was evaluated.
a r t i c l e
i n f o
Article history: Received 17 March 2015 Received in revised form 27 May 2015 Accepted 8 July 2015
Keywords: Fly ash Reactivity Geopolymer Compressive strength
a b s t r a c t Reactivity of Class F fly ash (FA), development of strength, and structure of FA-based geopolymers, depending on the reaction conditions, were examined in this paper. The results of SEM/EDS and NMR analyses revealed that the composition of aluminosilicate gel changed during reaction, i.e. Si/Al atomic ratio decreased with the reaction time. Higher strength of geopolymers was associated with a higher fraction of aluminum rich structural units, higher crosslinking, and more compact structure. A rapid test for the assessment of FA reactivity and thus the applicability of FA for geopolymer synthesis was proposed. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Geopolymers represent an alternative to Portland cement due to similar or even better binding properties [1,2]. Geopolymers are formed by alkali activation of solid aluminosilicate materials, usually fly ash (FA), or metakaolin, while alkali hydroxides and/or alkali silicates are generally used as alkali activators [3,4]. The main product of the reaction is a highly cross-linked network structure in the form of an amorphous aluminosilicate gel [5–7]. Since FA represents industrial waste that can be found all over the world, it is particularly attractive for the synthesis of geopolymers. Despite the fact that research in this area is intense and there are a large number of publications that suggest a wide range of ⇑ Corresponding author. E-mail address: [email protected] (V. Nikolic´). http://dx.doi.org/10.1016/j.conbuildmat.2015.07.014 0950-0618/Ó 2015 Elsevier Ltd. All rights reserved.
applications of these materials, FA-based geopolymers are still far from practical applications on a large scale. One of the main reasons is the variability of FA composition, which differs from source to source, and even within the same source. On the other hand, a large number of research results, which analyze different synthesis conditions of geopolymers, make it difficult to clearly identify the key factors that determine reactivity of FA, as well as the structure and characteristics of geopolymers. There are many factors that influence FA reactivity, and thus characteristics of FA-based geopolymers [8–12]. The most important factors are characteristics of the initial material (particle size distribution, content of glassy phase, reactive silicon, and aluminum, presence of iron, calcium, and inert particles), nature and concentration of the activator, as well as the reaction conditions [1–4,7]. Compressive strength of geopolymers is commonly used as a quantitative indicator of FA reactivity and characteristics of FA-based geopolymers [13,14].
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Particle size distribution is the key physical factor in the process of geopolymer synthesis [15,16]. FA reactivity increases with increasing FA fineness regardless of the nature and concentration of alkali activator [17]. Classification (extraction of fine fraction), milling, or mechanical activation of FA significantly increases its reactivity, resulting in geopolymers with improved properties [18–20]. Content of glassy phase of the initial material is an important parameter, since only a glassy phase represents reactive material that is converted during geopolymerization into the compacted binder [5,21,22]. Accordingly, FA reactivity or FA solubility in an alkaline solution depends on the composition and content of FA glassy phase [23]. Generally, higher glass content is associated with lower Si/Al ratio in the glassy phase [24], which means that higher amount of glassy phase contains higher amount of soluble aluminum. The relative amount of soluble silicon and aluminum present in an initial FA defines the Si/Al ratio in aluminosilicate gel, and thus the mechanical properties of resulting geopolymers [21]. However, as the term ‘‘reactivity’’ of FA does not only refer to the glassy phase, but to the FA as a whole, it means that the content of FA glassy phase has limited significance. Therefore, an assessment of FA reactivity is usually based on the FA dissolution rate in an alkaline solution, by measuring FA mass change or concentration of dissolved FA components over time [25–27]. Nature and concentration of alkali activator have dominant influence on the structure and properties of geopolymers [17]. If activation of FA is performed by alkali hydroxide, a gel rich in aluminum is first formed, followed by a gel rich in silicon [28,29]. However, in systems where the activation of FA is performed by solution containing high-ordered silicate species, a gel rich in silicon is formed immediately, without the previous formation of an aluminum-rich gel [30]. Properties of geopolymers are also determined by reaction conditions (temperature, time, and relative humidity), whereby an elevated reaction temperature leads to faster development of geopolymer strength [29]. There are opposing views on the optimal temperature for FA-based geopolymers synthesis. Some authors consider that optimal temperature of synthesis is 75–80 °C, since a significant occurrence of microcracks [31] or even strength loss above this temperature was observed [32–34]. On the other hand, very high strength (over 100 MPa) of geopolymers is achieved by alkali activation of FA at 95 °C [35]. Longer reaction time is generally beneficial for geopolymer strength increase [29]; however, after 48 h of reaction at elevated temperature the increase of strength is usually not significant [36]. Some authors consider that in order to achieve optimal properties of geopolymers at elevated temperature (95 °C), a shorter reaction time (6 h compared to 24 h) is more beneficial [37]. At last, but certainly not least, high relative humidity (>90%) is imperative for achieving optimal composition and structure of reaction products, as well as mechanical characteristics of geopolymers [35,38,39]. Since the FA presents industrial waste with variables characteristics, one of the main obstacles toward practical application of FA-based geopolymers is a lack of standards and evaluation criteria for FA reactivity in the process of geopolymerization. Besides, the factors that control phase development and their influence on the geopolymers characteristics are not yet fully understood. Given that there are a large number of influencing factors, it is clear that there is no simple methodology for assessing FA reactivity in the process of geopolymerization. The main aim of the present work was to evaluate rapid tests for assessment of FA reactivity and suitability of FA for geopolymer synthesis, by studying the development of strength and structure of geopolymers as the function of FA characteristics and geopolymerization reaction conditions (temperature and time). Rapid tests might be useful for practical application.
2. Materials and methods 2.1. Materials In this study, FA samples from three thermal power plants (TPP) from Serbia were used: 1. FA Morava, TPP Morava, Svilajnac. 2. FA Kolubara, TPP Kolubara, Veliki Crljani. 3. FA Kostolac B1, TPP Kostolac B1, Kostolac. Sodium silicate solution was used as alkali activator (‘‘Galenika-Magmasil’’, 8.72% Na2O, 26.5% SiO2, 64.78% H2O). Starting sodium silicate modulus n = SiO2/Na2O (mass ratio) of 3.04 was further adjusted by adding NaOH (p.a. (min.99%), VWR). 2.2. Characterization of the initial FA samples It should be noted that in this study FA was used in its original form, as received from electrostatic precipitators, without classification (extraction of fine fractions) or removal of large, less reactive particles (predominantly quartz and char). 2.2.1. Chemical, physical, and mineralogical characterization of FA Chemical composition of FA samples was determined by classic chemical analysis – alkali melting. FA fineness was determined by sieving through meshes of 63 lm and 43 lm, according to Serbian standard SRPS B.C1.018, which refers to pozzolanic materials – constituents for cement production – classification, technical conditions and test methods. Mineral composition of FA samples was determined by means of X-ray diffraction (XRD) analysis. Powder diffractometer ‘‘Phillips PW 1710’’ (Cu Ka = 1.54178 Å) was used. The diffraction patterns were recorded within 5–50° 2h range, with a step of 0.02° and holding time of 1 s per step. For the identification of the crystalline phases, the software PCPDFWIN was used. 2.2.2. Glassy phase of FA Glassy phase content was determined by dissolving FA in 1% HF acid – Arjuan’s method [40]. 1% HF dissolves the glassy phase of FA, while crystalline phases (usually quartz, mullite, hematite, and magnetite) remain intact [5,21,25]. This method involves the treatment 1 g of FA with 100 ml of 1% HF for six hours with constant stirring. After the treatment with HF acid, FA samples were dried at 105 °C to constant mass. The content of glassy phase was determined by subtracting the residual mass from the initial one. 2.2.3. Solubility of FA in a highly alkaline medium FA solubility in a highly alkaline medium was determined using concentrated sodium hydroxide (NaOH) solution, whereby two different methods were used: 1. FA dissolution at room temperature (20 °C, 5 h in 10 M NaOH) [27]. 2. FA dissolution at elevated temperature (95 °C, 0.5 h in 7 M NaOH). It is well known that the dissolution process accelerates as the temperature increases. The experiment at elevated temperature (95 °C) was chosen not only to accelerate endothermic reaction of dissolution, but also because the temperature of 95 °C represented the maximum temperature of reaction during this investigation. Prior to dissolution the FA was dried at 105 °C for 1 h. The liquid/solid (L/S) mass ratio was 40. The dissolution process was performed with constant stirring. Given that the high dissolutions are necessary to avoid undesirable precipitation [41], the resulting solution of FA dissolution in NaOH solution was further diluted 50 times and acidified with 37% HCl to a pH value of 7. Optical emission spectrometry with inductively coupled plasma (ICP-OES, Spectro Analytical Instruments GmbH) was used to determine the concentration of dissolved elements. 2.3. Synthesis of geopolymers The synthesis of geopolymers was performed according to previously optimized procedure [17]. Sodium silicate solution with modulus 1.5 was used as alkali activator, whereby modulus was adjusted by adding (10 M) NaOH solution to the starting sodium silicate solution. Concentration of the activator was 10% Na2O with respect to the FA mass. Geopolymer mortars were prepared by mixing FA with alkali activator and water, and then with standard sand (EN 196-1). Water was added in the amount required to obtain equal consistency (mortar flow measured on a flow table was 125 ± 5 mm). Water/binder ratios (water represents the total amount of water in the system, including water from the activator, while binder represents the total mass of FA and a solid part of activator) in the case of mortar based on FA Morava, Kolubara, and Kostolac B1 were 0.46, 0.61, and 0.67
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respectively. FA/sand ratio was 1/3. Casting of mortar into molds (prisms 40 40 160 mm) was carried out on a vibrating table. Molds with mortar prisms, wrapped in a plastic foil to prevent moisture loss, were exposed to an elevated temperature (55 °C, 80 °C, and 95 °C) in an oven, for 4 h, 8 h, 16 h, and 24 h. Geopolymer pastes, prepared by mixing FA, activator, and water, were exposed to the same conditions as mortars. After the specified period of reaction, paste samples were crushed and pulverized in isopropyl alcohol for 1 h to stop further reaction. After pulverization, the samples were filtered, rinsed with acetone, and dried at 50 °C to constant mass.
2.4. Characterization of geopolymers Determination of compressive strength of geopolymers was performed on mortar samples, while SEM/EDS and NMR analyses were performed on paste samples. Compressive strength was investigated according to SRPS EN 196-1 standard using the CONTROLS ADVANTEST 9 device. Microstructural analysis was done by scanning electron microscopy (SEM, VEGA TS 5130 MM, Tescan). Energy dispersive X-ray spectroscopy (EDS) was performed by INCAPentaFET-x3 (OXFORD Instruments). Samples for EDS analysis were prepared by grinding and polishing (MTI Corporation) of epoxy impregnated samples. Grinding was done using SiC grinding papers (300, 600, 1200, and 2000 grit, 3 min each) with acetone as lubricant. Final polishing of the samples was done using polishing cloths and diamond pastes (0.5 lm and 0.25 lm, MTI Corp., 3 min each). Prior to SEM/EDS analysis samples were Au-Pd coated. 29 Si MAS NMR spectra of geopolymers were obtained at Larmor frequency of 79.49 MHz using Bruker MSL 400 system, Apollo console upgraded (Tecmag). Chemical shifts d(29Si) were externally referenced to the 2,2-dimethyl-2-silapenta ne-5-sulfonate (DSS) standard. Gaussian peak deconvolution of the obtained spectra was performed using DMFIT application [42], by employing a common routine for peak distribution analysis. Prior to 29Si MAS NMR analysis, iron content of geopolymers was minimized by exposing samples to a strong magnetic field.
3. Results and discussion 3.1. Characterization of the initial FA 3.1.1. Chemical, physical, and mineralogical characterization of FA Chemical composition of the initial FA samples, fineness, and glassy phase content are shown in Table 1. All FA samples had a low content of CaO (<10%) and unburned material (<4%). Since the sum of oxides SiO2, Al2O3 and Fe2O3 exceeded 70%, all three FA can be classified as the Class F (according to ASTM C 618-12 standard). The highest content of glassy phase and particles smaller than 43 lm was observed in FA Morava. On the other hand, in FA Kostolac B1 the lowest content of glassy phase and the highest content of iron oxide were observed.
Table 1 FA chemical composition, fineness, and glassy phase content. Component
Morava
Kolubara
Kostolac B1
SiO2 (%) Al2O3 (%) Fe2O3 (%) CaO (%) MgO (%) SO3 (%) Na2O (%) K2O (%) Loss on ignition at 1000 °C (%) Total
55.23 21.43 7.42 7.94 2.61 0.81 0.64 1.35 1.66 99.09
62.13 17.20 5.95 5.67 2.00 0.67 0.58 1.04 2.88 98.12
46.85 23.20 12.14 8.26 2.77 1.48 0.40 0.81 3.44 99.35
Classification according to ASTM C 618-12 standard
Class F
Class F
Class F
Particle size > 63 lm (%) Particle size 43–63 lm (%) Particle size < 43 lm (%)
23.30 8.05 68.65
42.65 5.30 52.05
56.50 6.00 37.50
Glassy phase content (%)
48
38
34
Fig. 1. X-ray diffraction analysis of the initial FA samples.
Smaller fly ash particles (<20 lm) are more likely to have a highly glassy composition, as small particles quench faster than large particles [43]. Van Riessen and Chen-Tan [18] noticed that the amorphous material is the most abundant in the fine fraction of FA particles. Higher reactivity of aluminosilicate glass induced by the increase of particles fineness was confirmed by other authors [44]. The results of this work also indicated that the content of FA glassy phase increased with the increase of particles fineness. The results of X-ray diffraction analysis of the initial FA samples are presented in Fig. 1. These results showed the presence of crystalline phases typical for FA such as: quartz (PDF# 46-1045), feldspar (PDF# 89-1477), anhydrite (PDF# 06-0226), hematite (PDF# 88-2359), mullite (PDF# 85-1460), and amorphous phase (indicated by diffuse halo extending from approximately 20° to 35° 2h). It is evident that there were differences in the mineral composition of the initial FA samples. FA Kolubara had the highest content of quartz, while FA Kostolac B1 had somewhat higher content of hematite than other two FAs. These XRD findings are in accordance with the results of FA chemical analysis. 3.1.2. Solubility of FA in sodium hydroxide solution Testing results of FA solubility in concentrated NaOH solution under different reaction conditions are shown in Table 2. Concentrations of soluble Si and Al were several times higher at elevated temperature (95 °C) in relation to room temperature, despite lower molarity of the alkali activator (7 M NaOH compared to 10 M NaOH), and shorter dissolution time (0.5 h compared to 5 h). Therefore, the solubility of FA in alkaline solution was more pronounced at elevated temperature. The highest concentration of soluble Si and Al was observed in
Table 2 Solubility of FA in NaOH solution. Sample FA
Conditions of dissolution 20 °C (5 h in 10 M NaOH)
Morava Kolubara Kostolac B1
95 °C (0.5 h in 7 M NaOH)
[Si] (mmol/l)
[Al] (mmol/l)
[Si] (mmol/l)
[Al] (mmol/l)
15.2 14.4 13.9
7.3 6.9 7.3
73.6 61.1 59.1
33.3 32.1 28.9
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364
Fig. 2. Compressive strength of geopolymers versus temperature and time of reaction.
FA Morava, which contained the highest amount of glassy phase and finer particles. Elevated temperature increased the dissolution rate of FA glassy phase components in the alkaline solution as observed previously [26]. Presented results clearly indicate the importance of elevated temperature in the process of FA dissolution, and thus in the entire geopolymerization process. 3.2. Characterization of geopolymers 3.2.1. Compressive strength of geopolymers The results of compressive strength of geopolymer mortars (Fig. 2) indicated the increase of geopolymer strengths with the increase of temperature from 55 °C to 95 °C, regardless of the FA characteristics. Strength also increased with the increase of reaction time, which was more pronounced in the initial period. Generally, the geopolymers exposed for 8 h to temperature of 80 °C or 95 °C, reached an equal or higher compressive strength compared to the strength of geopolymers exposed for 24 h to 55 °C. At higher temperature considerably shorter time was necessary to achieve high strength, which is consistent with the results of other authors [29]. Although some authors [32–34] consider that the optimal temperature for the geopolymers synthesis is 75–80 °C, since higher temperatures lead to a strength loss, the results of this study showed that it is not universal trend, i.e. with the increase of temperature reaction up to 95 °C the strength loss did not occur. Quite contrary, the strength of geopolymers synthesized at 95 °C in all
Table 3 Relative strength of geopolymers. FA
Temp. (°C)
Relative strength (%) in relation to strength after 24 h at 95 oC 4h
8h
16 h
24 h
Morava Kolubara Kostolac B1
55
17 16 15
52 46 50
75 71 74
82 79 75
Morava Kolubara Kostolac B1
80
59 47 46
83 79 71
93 85 82
93 94 92
Morava Kolubara Kostolac B1
95
74 57 53
91 81 77
96 93 85
100 100 100
investigated cases was higher than the strength of geopolymers synthesized at 80 °C, regardless of the initial FA characteristics. The reasons for this discrepancy probably lie in different characteristics of the investigated fly ashes and/or different experimental conditions. Considering the rapid development of geopolymer strength at elevated temperature (after 4 h of reaction at 95 °C relative strength exceeded 50% of strength achieved after 24 h of reaction at the same temperature, while after 8 h of reaction relative strength amounted above 75%; Table 3), reaction at 95 °C in a relatively short time (4–8 h) provides an opportunity for rapid assessment of FA reactivity in reaction of geopolymerization.
Fig. 3. Compressive strength of geopolymers (after 24 h at different temperatures) versus FA solubility in 7 M NaOH at 95 °C: (a) Si and (b) Al.
V. Nikolic´ et al. / Construction and Building Materials 94 (2015) 361–370
3.2.2. Correlation of geopolymer strength with the initial FA characteristics Higher content of glassy phase does not necessarily lead to higher reactivity [25], although some literature data suggested that, higher content of glassy phase in FA leads to higher FA reactivity and better mechanical properties of FA-based geopolymers [5,18]. Compared to the content of FA glassy phase, more reliable
365
measure of FA reactivity is the content of soluble silicon and aluminum [26]. Dependence of geopolymers compressive strength (after 24 h at different temperatures) on the concentration of soluble Si and Al, resulting from the FA dissolution in 7 M NaOH at 95 °C, is given in Fig. 3. The highest geopolymer compressive strength under all experimental conditions was obtained based on FA Morava, which had the highest content of particles smaller
Fig. 4. Microstructure of geopolymers: (a) FA Morava – 4 h at 55 °C, (b) FA Morava – 24 h at 55 °C, (c) FA Morava – 4 h at 95 °C, (d) FA Morava – 24 h at 95 °C, (e) FA Kolubara – 4 h at 95 °C and (f) FA Kostolac B1 – 4 h at 95 °C.
366
V. Nikolic´ et al. / Construction and Building Materials 94 (2015) 361–370
than 43 lm, the highest content of glassy phase, and the highest content of soluble Si and Al. On the other hand, geopolymers based on FA Kostolac B1, which had the lowest content of particles smaller than 43 lm, glassy phase, and soluble Si and Al, showed the lowest compressive strength in all cases. Since alkali activation of FA was carried out by sodium silicate, an excess of soluble Si was initially present in the solution. In that case, the major differences in chemical reactivity of FA derived from the soluble Al, i.e. soluble Al represented the measure of FA reactivity [41]. A certain amount of soluble aluminum is required to initiate a geopolymerization reaction [7]. In addition, aluminum is a component that determines important characteristics of geopolymers, such as setting time and strength [28,45,46]. Therefore, from the viewpoint of chemical characteristics, the content of soluble aluminum is the dominant factor that determines the process of geopolymerization. Due to the established correlation, the process of FA dissolution in 7 M NaOH at 95 °C for 0.5 h could be used for rapid assessment of FA reactivity and thus the applicability of FA for the synthesis of geopolymers.
method should not be regarded as absolute values, but rather as values that indicate a certain trend. Presented results indicate that the composition of aluminosilicate gel changed during geopolymerization reaction, i.e. the Si/Al atomic ratio decreased with the reaction time, regardless of the temperature. Considering that the alkali activation of FA was performed by sodium silicate, an excess of soluble silica was initially introduced in the system, which resulted in a very high Si/Al atomic ratio (4.4–4.5) in the initial period of reaction (after 4 h). As the reaction proceeded, further dissolution of FA (in which Si/Al ratio was lower) led to reduction of Si/Al atomic ratio in aluminosilicate gel, again independently of reaction temperature. 3.2.5. NMR analysis 29 Si MAS NMR spectra of FA Morava and geopolymer pastes (G) obtained by alkali activation of the same FA and exposed to different reaction conditions are shown in Fig. 6. Spectrum of the
3.2.3. SEM analysis of geopolymers Microstructure of geopolymers based on different FAs and synthesized under different reaction conditions is shown in Fig. 4. Products of geopolymerization reaction represented a heterogeneous material, i.e. the matrix comprising aluminosilicate gel and incorporated unreacted FA particles. After a short time (4 h) hardened matrix of geopolymers based on FA Morava, contained a significant proportion of unreacted FA particles (Fig. 4a and c), while after 24 h (Fig. 4b and d) considerably less unreacted FA particles were noticed. Less compact matrix, compared to the geopolymers based on FA Morava, was observed in the case of the geopolymers based on FA Kolubara (Fig. 4e) and FA Kostolac B1 (Fig. 4f). It is well known that high strength is related to the compact microstructure [13]. Different microstructure of geopolymers was the result of differences in the reactivity of FA particles, which was consistent with the results of compressive strength testing. 3.2.4. EDS analysis of geopolymers Influence of the reaction conditions (temperature and time) on the composition of aluminosilicate gel and structure of geopolymers was examined on geopolymers based on FA Morava that showed the highest strength. Atomic ratios of major elements in aluminosilicate gel depended on the temperature and time of reaction, whereby Si/Al atomic ratio ranged from 3.2 to 4.5 (Fig. 5). Phase inhomogeneities showing regions of high silicon concentration (Si/Al 5) were also observed by other authors [47]. Therefore, displayed values of Si/Al atomic ratios obtained by graphical
Fig. 6. 29Si NMR analysis of FA Morava (a) and geopolymers based on FA Morava (b).
Fig. 5. Atomic ratios of major elements of aluminosilicate gel versus temperature and time of reaction (geopolymers based on FA Morava): (a) 4 h and (b) 24 h.
V. Nikolic´ et al. / Construction and Building Materials 94 (2015) 361–370
initial FA (Fig. 6a) was very wide and pronouncedly asymmetric indicating heterogeneous distribution of silicon structural units. Resonance ranged between 80 and 108 ppm is mainly associated to the initial glassy phase material, while most of the resonances appearing around and above 108 ppm are assigned to different crystalline phases of silica (Q4(0Al) signals) [15,48]. Contrary to the spectrum of the initial FA, spectra of the geopolymer samples (Fig. 6b) showed broad and considerably symmetric resonance lines, which were centered around 93 ppm indicating poorly ordered aluminosilicate structures. The 29Si MAS NMR spectra in this work were deconvoluted assuming Gaussian line shapes, constant line widths, and same number of peaks. The results of geopolymer 29Si MAS NMR spectra deconvolution are shown in Fig. 7. All of these spectra comprise of overlapping resonances attributed to the silicon sites present in aluminosilicate gel, un-reacted glassy phases, and crystalline phases originating from the initial FA [49,50]. It is generally accepted that 29Si MAS NMR spectra of geopolymers consist of all five Q4(mAl) silicon species, with Q4(4Al), Q4(3Al), Q4(2Al), Q4(1Al) and Q4(0Al) resonating at approximately 84, 89, 94, 99 and 107 ppm, respectively [49–52]. In addition to the Q4(mAl) silicon peaks, small resonances at approximately 77, 107 and 115 ppm (Fig. 7a) were observed in all of the 29Si MAS NMR spectra. Small peak at 77 ppm is usually ascribed to less condensed silicon species, Q1 and Q2 [48]. Sharp peak at 107 ppm indicates an ordered structure, which can be attributed to quartz originating from the initial FA [49,50]. Peak at 115 ppm can be attributed to different silica polymorphs [5,15] or Q4(0Al) units in the products of alkali activation reaction [49,50,53]. The deconvolution 29Si MAS NMR spectra showed that the most common structural units in aluminosilicate gel were Q4(2Al) and
Fig. 7. Deconvolution of
29
367
Q4(3Al), regardless of the reaction temperature and time. The most pronounced change over time at 55 °C was the reduction of fraction of Q4(0Al) structural units (Table 3). In contrast, the fraction of Q4(3Al), Q4(2Al) and Q4(1Al) structural units increased, while the fraction of Q4(4Al) unit did not change significantly. On the other hand, at 95 °C the most pronounced change over time was the increase of the fraction of Q4(4Al) unit, while the fraction of other structural units Q4(mAl), m = 0, 1, 2, or 3, slightly changed. The increase of fraction of Q4(4Al) unit over time during reaction at 95 °C was a consequence of higher solubility and higher availability of aluminum at higher temperature. The increase of fraction of Q4(4Al) unit was also the most significant difference in the structural organization of aluminosilicate gel at 95 °C in respect to the reaction at 55 °C. The deconvolution of 29Si MAS NMR spectra also indicated the presence of less condensed structural units of silicon (Q1 and Q2), probably from unreacted activator. Other authors also attributed these signals to the residual and less condensed species, monomer or dimer units with silanol groups [28,48]. Fraction of Q1 and Q2 did not change significantly over time at 95 °C, probably due to the faster reaction at higher temperature and consequently hindered diffusion, i.e. these units remained in the structure of the initially formed gel. However, at 55 °C the reaction was slower, diffusion through hardened matrix was faster and the fraction of these units decreased over time. Despite observed significant differences in geopolymer strength, the results of 29Si MAS NMR spectra deconvolution indicate that the increase of reaction temperature and time led to small changes of geopolymer Si/Al atomic ratio. Similar phenomenon was also noticed by other authors [28,48,54]. Discrepancy in the values of Si/Al atomic ratios obtained by NMR spectra deconvolution and EDS analysis (Fig. 5 and Table 4)
Si MAS NMR spectra of geopolymers based on FA Morava: (a), (b) at 55 °C and (c), (d) at 95 °C.
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368
Table 4 Si MAS NMR spectra deconvolution of geopolymers based on FA Morava (Fig. 7).
29
Reaction time and temperature
Q1,2a
Q4(4Al)
Q4(3Al)
Q4(2Al)
Q4(1Al)
Q4(0Al)
Quartzb
Q4b
Si/Alc
4 h 55 °C
d (ppm) Width (ppm) Area (%)
77.60 7.0 3.94
83.22 7.0 12.15
88.45 7.0 25.87
94.07 7.0 25.52
100.09 7.0 16.37
107.36 7.0 13.25
107.24 1.4 0.95
115.07 7.0 1.95
1.92
d (ppm) Width (ppm) Area (%)
77.00 7.0 2.26
83.30 7.0 12.09
88.64 7.0 27.63
94.20 7.0 26.93
100.05 7.0 16.87
107.25 7.0 10.62
107.16 1.32 1.16
115.07 7.0 2.44
1.86
d (ppm) Width (ppm) Area (%)
77.20 7.0 4.05
84.10 7.0 13.57
89.23 7.0 25.78
94.60 7.0 26.37
100.52 7.0 16.30
107.66 7.0 10.25
107.29 1.36 1.12
116.18 7.0 2.55
1.84
d (ppm) Width (ppm) Area (%)
77.00 7.0 4.67
83.86 7.0 15.28
89.28 7.0 26.45
94.55 7.0 25.76
100.28 7.0 16.14
107.34 7.0 9.42
107.16 1.32 1.04
115.61 7.0 1.22
1.79
24 h 55 °C
4 h 95 °C
24 h 95 °C
a b c
Signals associated with less condensed silicon species, Q1 and Q2 [48–50]. Signals associated with quartz ( 107 ppm) and other Q4 silicon units [5,15,49,50,53]. Si/Al ratio calculated from the deconvolution results [52].
was also noticed by Fernandez-Jimenez et al. [28]. Lodeiro et al. suggested that this discrepancy is associated with deconvolution procedure and an assumption that the principal components of the 29Si MAS NMR spectrum are based on Q4(mAl) structural units [55]. 3.2.6. Correlation of strength of geopolymers with structural organization of the aluminosilicate gel In general, during alkali activation of FA with sodium silicate solution, the fraction of aluminum rich structural units [Q4(3Al) and Q4(4Al)] increased in respect to the fraction of silicon rich structural units [Q4(0Al), Q4(1Al) and Q4(2Al)], regardless of the reaction temperature (55 °C or 95 °C). On the other hand, the strength of geopolymers increased over time, also regardless of the reaction temperature (Fig. 2). Therefore, it is clear that the geopolymer strength depended on the fraction of Q4(mAl) structural units. The relationship between compressive strength of geopolymers based on FA Morava and fraction of Q4(mAl) units in the 29Si MAS NMR spectra is given in Fig. 8. The increase of the fraction of aluminum rich structural units indicates higher degree of substitution of Si–O–Si bond by Si–O–Al bonds and reorganization of the three-dimensional aluminosilicate network during geopolymerization, which resulted in the strength increase. Although this phenomenon was more
Fig. 8. Compressive strength of geopolymers based on FA Morava versus fraction of 29 Si MAS NMR Q4(mAl) units.
pronounced at lower temperature (55 °C), it was also clearly present at higher temperature (95 °C). As the compressive strength of geopolymers is an indication of the degree of polymerization of newly formed aluminosilicate gel [56], the differences in compressive strength of geopolymers could be explained by different degree of gel polymerization at various temperatures. Therefore, higher strength of geopolymers was the result of a higher fraction of aluminum rich structural units, higher crosslinking, and more compact structure. 4. Conclusion This paper examined the development of compressive strength and structure of geopolymers depending on characteristics of Class F fly ash (FA) and conditions of geopolymerization reaction. Geopolymers were synthesized by alkali activation of FA using sodium silicate solution with modulus 1.5 at elevated temperature (55, 80, and 95 °C) in the period up to 24 h. There was a clear relationship between FA characteristics, reaction conditions, and strength of geopolymers. The different reactivity of FA was a consequence of their different characteristics (fineness, content of glassy phase, soluble silicon, and aluminum in a highly alkaline medium). Maximum strengths of geopolymers were obtained after 24 h at 95 °C, regardless of the FA characteristics. The geopolymer with the highest compressive strength, regardless of the reaction temperature and time, was synthesized based on FA Morava, which had the highest content of particles smaller than 43 lm, the highest content of glassy phase, and the highest content of soluble aluminum. Given that alkali activation of FA was performed by sodium silicate solution, content of soluble aluminum was the dominant factor that determined the process of geopolymerization. Elevated temperature accelerated geopolymerization reaction and led to the rapid formation of aluminosilicate gel as the main reaction product. At lower temperature the reaction time became an important factor. The composition of aluminosilicate gel changed during geopolymerization reaction, i.e. the Si/Al atomic ratio decreased over time (SEM/EDS analysis). The results of 29Si MAS NMR analysis confirmed that the increase of reaction time led to the increase of fraction of aluminum rich structural units [Q4(3Al) and Q4(4Al)] compared to silicon rich units [Q4(0Al), Q4(1Al) and Q4(2Al)]. Higher strength of geopolymers was the result of higher fraction of aluminum rich structural units, higher crosslinking, and more compact structure.
V. Nikolic´ et al. / Construction and Building Materials 94 (2015) 361–370
Due to the established correlation it was concluded that FA solubility in 7 M NaOH (0.5 h at 95 °C) might serve as a test for rapid assessment of FA reactivity. The highest increase of strength took place during the first 4–8 h of the reaction, which represents rational reaction time from a technological point of view. Compressive strength of geopolymer after 4 h of reaction at 95 °C also might serve as a test for rapid assessment of FA reactivity and thus the applicability of FA for synthesis of geopolymers. Acknowledgements The results presented in this paper are part of the research project TR 34026, financed by the Ministry of Education, Science and Technological Development, Republic of Serbia and COST Action TU 1301. The authors would like to thank Ljiljana Milicˇic´ from Institute for testing materials and Prof. Dr. Miroslav Nikolic´ from Institute for Multidisciplinary Research, University of Belgrade, for their valuable help during the preparation of this paper. References [1] C. Shi, A. Fernandez-Jimenez, A. Palomo, New cements for the 21st century: the pursuit of an alternative to Portland cement, Cem. Concr. Res. 41 (2011) 750– 763. [2] F. Pacheco-Torgal, J. Castro-Gomes, S. Jalali, Alkali-activated binders: a review. Part 2. About materials and binders manufacture, Constr. Build. Mater. 22 (2008) 1315–1322. [3] D. Khale, R. Chaudhary, Mechanism of geopolymerization and factors influencing its development: a review, J. Mater. Sci. 42 (2007) 729–746. [4] M. Komljenovic´, Mechanical strength and Young’s modulus of alkali-activated cement-based binders, in: F. Pacheco-Torgal, J.A. Labrincha, C. Leonelli, A. Palomo, P. Chindaprasirt (Eds.), Handbook of Alkali-activated Cements, Mortars, and Concretes, Woodhead Publishing Series in Civil and Structural Engineering, 2015, pp. 171–217 (Number 54). [5] A. Fernandez-Jimenez, A.G. de la Torre, A. Palomo, G. Lopez-Olmo, M.M. Alonso, M.A.G. Aranda, Quantitative determination of phases in the alkali activation of fly ash. Part I. Potential ash reactivity, Fuel 85 (2006) 625–634. [6] J.L. Provis, C.Z. Yong, P. Duxon, J.S.J. van Deventer, Correlating mechanical and thermal properties of sodium silicate-fly ash geopolymers, Colloids Surf. A 336 (2009) 57–63. [7] P. Duxon, A. Fernandez-Jimenez, J.L. Provis, G.C. Lukey, A. Palomo, J.S.J. van Deventer, Geopolymer technology: the current state of the art, J. Mater. Sci. 42 (2007) 2917–2933. [8] A. Fernandez-Jimenez, A. Palomo, M. Criado, Microstructure development of alkali-activated fly ash cement: a descriptive model, Cem. Concr. Res. 35 (2005) 1204–1209. [9] J.S.J. van Deventer, J.L. Provis, P. Duxon, G.C. Lukey, Reaction mechanisms in the geopolymeric conversion of inorganic waste to useful products, J. Hazard. Mater. A139 (2007) 506–513. [10] C.A. Rees, J.L. Provis, G.C. Lukey, J.S.J. van Deventer, The mechanism of geopolymer gel formation investigated through seeded nucleation, Colloids Surf. A (2008) 97–105. [11] A. Hajimohammadi, J.L. Provis, J.S.J. van Deventer, Effect of alumina release rate on the mechanism of geopolymer gel formation, Chem. Mater. 22 (2010) 5199–5208. [12] A. Hajimohammadi, J.L. Provis, J.S.J. van Deventer, The effect of silica availability on the mechanism of geopolymerisation, Cem. Concr. Res. 41 (2011) 210–216. [13] M. Steveson, K.S. Crenstil, Relationships between composition, structure and strength of inorganic polymers. Part 2. Fly ash-derived inorganic polymers, J. Mater. Sci. 40 (2005) 4247–4259. [14] J.E. Oh, Y. Jun, Y. Jeong, P.J.M. Monteiro, The importance of the networkmodifying element content in fly ash as a simple measure to predict its strength potential for alkali-activation, Cem. Concr. Compos. 57 (2015) 44–54. [15] A. Fernandez-Jimenez, A. Palomo, Characterisation of fly ashes. Potential reactivity as alkaline cements, Fuel 82 (2003) 2259–2265. [16] J.G.S. van Jaarsveld, J.S.J. van Deventer, G.C. Lukey, The characterisation of source materials in fly ash-based geopolymers, Mater. Lett. 57 (2003) 1272– 1280. [17] M. Komljenovic´, Z. Bašcˇarevic´, V. Bradic´, Mechanical and microstructural properties of alkali-activated fly ash geopolymers, J. Hazard. Mater. 181 (2010) 35–42. [18] A. van Riessen, N. Chen-Tan, Beneficiation of Collie fly ash for synthesis of geopolymer: Part 1 – Beneficiation, Fuel 106 (2013) 569–575. [19] N. Marjanovic´, M. Komljenovic´, Z. Bašcˇarevic´, V. Nikolic´, R. Petrovic´, Improving reactivity of fly ash and properties of ensuing geopolymers through mechanical activation, Constr. Build. Mater. 57 (2014) 151–162.
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