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Geopolymerisation behaviour of size fractioned fly ash
Advanced Powder Technology xxx (2014) xxx–xxx
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Original Research Paper
Geopolymerisation behaviour of size fractioned fly ash Sanjay Kumar a,⇑, Ferenc Kristály b, Gabor Mucsi b a b
CSIR-National Metallurgical Laboratory, Jamshedpur 831007, India University of Miskolc, Miskolc 3515, Hungary
a r t i c l e
i n f o
Article history: Received 1 April 2014 Received in revised form 22 July 2014 Accepted 31 July 2014 Available online xxxx Keywords: Fly ash Size fractioned Geopolymerisation Microstructural characterization Strength development
a b s t r a c t Fly ash is a fine powder residue resulted from combustion of pulverized coal in thermal power plants. Different size fractions of fly ash have different properties. Four size fractions (with characteristic particle diameter D50 of 40.37, 23.64, 10.33 and 2.98 lm respectively) collected from different fields of an electrostatic precipitator and representing the entire particle spectrum of fly ash has been selected for the study. These fractions have been characterized for their granulometry, chemistry, glass content and mineralogical phases. Geopolymerisation of size fractioned fly ash has been carried out at ambient (27 °C) and elevated (60 °C) temperature using isothermal conduction calorimetry (ICC) and the microstructure has been studied using X-ray diffractometry (XRD), scanning electron microscopy with X-ray microanalysis probe (SEM-EDS) and Fourier transform infrared spectroscopy (FTIR). Calorimetric studies showed that the heat flow curve during geopolymerisation has linear correlation with the glass content of fly ash. The compressive strength development at both ambient and elevated temperature was due to the combined effect of SiO2/Al2O3 ratio, particle size and glass content. SEM-EDS studies have shown more reaction product in finer fractions and unreacted particles in coarser fractions. Formation of more thermonatrite phase was due to poor reactivity of coarse size fraction resulting into free alkali which in presence of atmospheric carbon formed Na2CO3H2O. Ó 2014 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
1. Introduction Fly ash is a by-product of thermal power plants resulted from the combustion of pulverized coal. It is a granular material with particle size typically varying from submicron size to 250 lm [1]. Interestingly the different size fractions of fly ash vary considerably in terms of chemistry, mineralogy and reactivity [2]. Due to its pozzolanic property, small particle size, flow characteristic and combination of crystalline and amorphous phase, fly ash is preferred material for many applications such as production of cement, concrete and brick [3]. Due to its unique combination of properties, uses of fly ash not merely as filler material, but as reactive component are continuously being explored. One such potential application where fly ash actively participates in reaction is its use for geopolymer synthesis. Geopolymers are new class of inorganic polymer materials synthesized by reaction between alumino-silicate and alkali
⇑ Corresponding author at: Recycling & Waste Utilization, CSIR-National Metallurgical Laboratory, Jamshedpur 831007, India. Tel.: +91 657 2345049, mobile: +91 9939326346. E-mail address: [email protected] (S. Kumar).
compounds at ambient or near ambient temperature [4]. Due to easy process of synthesis, low environmental impact and versatility in properties, they are fast emerging materials of choice for a range of building materials, fire resistant ceramics, composites, matrix for immobilization of toxic wastes, and many others. Traditionally metakaolin has been used as raw material for geopolymer synthesis [5,6]. Recently, there has been a change in trend where emphasis on raw material has been shifted from pure and naturally occurring materials to waste and by-products, more specifically fly ash. The reason for increased attention on fly ash is associated with easy availability, technical superiority, environmental and economic benefits [7–10]. The mechanism of geopolymerisation of fly ash is considered similar to metakaolin based geopolymer, but the degree of reaction varies due to the reactive and non-reactive fractions of fly ash [11]. The property of resulting geopolymer is influenced by the chemistry and reactivity of fly ash. The effect on role of chemistry on the properties of fly ash based geopolymer is well documented [12–15]. The reactivity of the fly ash in geopolymer system is mainly governed by two factors, (a) presence of reactive amorphous phase, and (b) fine particle size. Geopolymerisation of different size fractions of fly ash and its influence on the properties has been studied by many researchers [16–19]. However, in the majority of the studies, the different sizes have
http://dx.doi.org/10.1016/j.apt.2014.09.001 0921-8831/Ó 2014 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
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been obtained either by air classification or by milling of fly ash [16–19]. In the present work, the different size fractions of fly ash have been collected from the different hoppers of an electrostatic precipitator (ESP). The rationale for collecting from hoppers is based on ESP structure and existing practice of fly ash collection. Typically ESP has many hoppers in the direction of gas flow for the collection of fly ash. The fly ash collected in different hoppers has different properties. For example, the particle size decreases and specific surface area increases as the collection hopper gets distance from the boiler [20–22]. In practice, the fly ash collected from different hoppers is blended together before storage or disposal. Thus the distinct advantage of the properties of different size fractions is lost. Again for getting those properties, either fly ash is classified or milled. Also the chemistry and particle size distribution of fly ash collected in hoppers of ESP is not similar to those of air classifiers as they operate on different principles. In ESP, the separation efficiency depends on particle size, conductivity and gas viscosity, whereas in air classifiers, it depends on air flow rate and density [23]. Thus the resulting fly ash is different in chemistry and reactivity, even if they are close in particle size distribution. We have carried out studies on suitability of air classified and mechanically activated fly ash for synthesis of geopolymers [16]. The objective of the present work was to develop geopolymers from size classified fly ash directly collected from hoppers of ESP. The focus of the study is to elucidate the effect of different size fractions on geopolymerisation reaction using isothermal conduction calorimeter. The microstructure obtained after reactions were studied using XRD, FTIR and SEM-EDS. Attempt has been made to correlate the size, reaction, structure and properties.
2. Experimental The different size fractions of fly ash were collected from different fields of ESP at Tata Power, Jamshedpur, India. Out of samples of various size fractions, four representative samples were selected with characteristic particle diameter D50 of 40.37, 23.64, 10.33 and 2.98 lm and labeled as FA1, FA2, FA3 and FA4 respectively. The chemical analysis of the fly ash was carried out using combination of X-ray fluorescence, atomic absorption spectrophotometer and classical analysis. The glass content of the fly ash was determined by counting the grains in a polarizing microscope. The particle size analysis of fly ash was carried out using a laser particle size analyzer (MASTERSIZER S, Malvern, U.K.). The rate of heat evolution during the reaction (dq/dt) was measured using an eight channel isothermal conduction calorimeter (TAM AIR, Thermometric AB, Jarafalla, Sweden). The process involved the preparation of the alkaline activator solution at least 24 h before use, mixing of an alkaline activator with the powder sample, and loading of the mix in the calorimeter. Analytical grade sodium hydroxide in flake form (98% purity) was used to prepare the alkaline activator solution. An alkaline activator of 6 M concentration was prepared in distilled water. The choice of 6 M concentration was based on our previous works [7,9] where it was found that higher concentration often leads to free alkalies which are more susceptible to carbonation. 7 g Solid sample and 3.5 ml of activator solution were used throughout the study. The samples were mixed and then loaded into the calorimeter. Calorimetric studies were carried out at 60 °C. The results obtained were presented in offset mode. Fourier Transform Infrared Spectroscopy (FT-IR-410 JASCO, U.S.A) was used for structural characterization of geopolymers. The samples were prepared by mixing the powder with KBr. XRD patterns were recorded on a SIEMENS X-ray diffractometer (Model D500), using Co Ka radiation with a Fe-filter. The scanning speed of 1 deg/min was used and the samples were scanned from
10° to 60° 2h angle. Morphological characterization of the fractured samples was done by a scanning electron microscope (JEOL SEM 840A) fitted with a Kevex Energy Dispersive Spectrometer (EDS) for X-ray micro-analysis after carbon coating on the fractured surface. The X-ray micro-analysis of areas of interest was determined from the average of minimum six analyses and used to calculate elemental ratios. For all the physical testing, liquid (alkali solution) to solid ratio was kept at 0.35. The samples were prepared at 27 ± 2 °C and under relative humidity of 65%. For compressive strength 70 70 70 mm cubic samples were prepared by vibro-casting of geopolymer paste. Compressive strength was tested on an Automatic Compression Testing Machine (AIMIL COMPTEST 2000, India) at age of 3, 7, 14 and 28 days after casting the samples at 27 °C followed by curing at 60 °C for 24 h. Averages of six samples were tested for each result. 3. Results and discussion 3.1. Characterization of size fractioned fly ash Fig. 1 shows the schematic diagram of a typical electrostatic precipitator. The coarser fraction of fly ash is collected close to entry side whereas the finer fraction is collected close to exit side of ESP. The fly ash used in the present study was collected from different fields of the hoppers. Table 1 shows chemical analysis of size fractioned fly ash. The glass content of fly ash has also been included in the table. It is interesting to note that SiO2 content decreased with the fineness whereas the Al2O3 content remained more or less constant. Fe2O3 concentration was also found to be higher towards coarser size fractions. The increase in LOI in coarser particles was mainly due to the presence of free unburned carbon. The composition of glass fraction was mainly silicate with little amount of alumino-silicates. No clear trend of glass content with size fraction has been observed. However, the finer fractions have shown higher and coarser fractions have shown lower glass content. Fig. 2 shows the particle size distribution of FA1, FA2, FA3 and FA4 samples. The characteristic particle diameters D10, D50 and D90 have been tabulated in Table 2. A wide variation in particle size was observed in the fly ash. In FA4, 90% particles were <20 lm size whereas in the FA2, the coarser size just exceeded 100 lm. The mineralogical composition of fly ash is given in Fig. 3. FA4 sample, which has the finest particle side distribution, has shown the low intensity of quartz peak. The featureless hump between 15° and 40° 2h was more prominent in FA3 and FA4, which was due to the presence of amorphous and poorly crystalline phase. In all the size fractions, similar crystalline phases but with different peak intensity were detected. The most common phases were quartz (PDF 46-1045), hematite (PDF 88-2359) and Mullite (PDF 851460). 3.2. Geopolymerisation reaction To study the effect of size fractions on geopolymerisation reaction, isothermal conduction calorimeter was used under the following two conditions: (a) at ambient temperature (27 °C), and
Fig. 1. Schematic diagram of a typical electrostatic precipitator.
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S. Kumar et al. / Advanced Powder Technology xxx (2014) xxx–xxx
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Table 1 Chemical analysis of size fractionated fly ash. Radicals
SiO2 Al2O3 Fe2O3 TiO2 CaO MgO Na2O K2O P MnO S LOI SiO2/Al2O3 ratio Glass content
Fly ash samples (wt%) FA4
FA3
FA2
FA1
59.42 28.6 2.83 1.62 0.59 0.04 0.36 1.06 0.21 0.03 0.02 1.17 2.07 60
58.36 28.23 3.18 1.64 0.76 0.42 0.36 1.09 0.25 0.03 0.03 1.30 2.06 62
51.42 28.52 4.99 1.72 0.95 0.70 0.05 1.45 0.44 0.03 0.19 1.75 1.80 41
50.98 28.06 4.92 1.75 0.99 0.74 0.06 1.45 0.52 0.04 0.17 2.37 1.81 42.5
Fig. 3. X-ray diffractogram of size fractioned fly ash showing crystalline and amorphous phases.
Fig. 2. Particle size distribution of FA1, FA2, FA3 and FA4 samples.
Table 2 Characteristic particle diameter of fly ash samples. Characteristic diameter
D10 D50 D90
Fly ash samples (lm) FA4
FA3
FA2
FA1
0.34 2.98 19.86
0.41 10.33 69.76
8.89 40.37 103.21
1.74 23.64 91.49
(b) elevated temperature (60 °C). The peak intensity at ambient temperature was almost negligible so only the result of 60 °C was included in the study (Fig. 4). The first peak (which appears more like a straight line) in the beginning corresponds to wetting and partial dissolution of glassy content followed by small induction period as a consequence of low reactivity [24]. Although the peak intensity varied in all the samples, the peak behaviour was the same in FA1 and FA2 samples, and FA3 and FA4 samples. In FA3 and FA4, the second exothermic peak after induction period has shown the presence of three sub peaks (P1, P2 and P3). These sub peaks possibly correspond to different reactions such as dissolution–precipitation, formation of NASH (N = Na2O, A = Al2O3, S = SiO2, H = H2O) gel and polycondensation. It is interesting to note that FA4, which has the finest particle size, have shown less intense peak than FA3, which has the more glass content. But the peak P3 appeared early in FA4 sample. In FA1 and FA2, only one peak was observed with a small shoulder at the position of P1. To understand the degree of reaction in fly ash of different size fraction, the cumulative area under peak was measured by
Fig. 4. Isothermal conduction calorimetric curve of size fractioned fly ash geopolymerised at 60 °C.
Fig. 5. Cumulative heat flow curve derived from calorimetric data obtained in Fig. 4 showing rate of geopolymerisation.
integrating the calorimetric curve and results are compiled in Fig. 5. The order of fly ash fraction reactivity was:
FA3 > FA4 > FA1 > FA2
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3.3. Physical properties of geopolymer The variation in compressive strength with curing time was measured for the following two sets of samples: (a) sample cured at ambient temperature for different periods of time, (b) sample cured at 60 °C for 24 h followed by curing at ambient temperature for different periods of time. Fig. 6 shows the variation of compressive strength of the samples cured at ambient temperature. The strength development trend in FA3 and FA4 was different from FA1 and FA2. The strength development accelerated significantly after 7 days in FA3 and FA4 samples and reached to maximum value to 44–45 MPa after 28 days. However, the strength development was not so significant in FA1 and FA2 and obtained the maximum strength of 18 and 20 MPa respectively. The trend of strength development was different in the samples cured at 60 °C followed by ambient temperature curing (Fig. 7). All the samples have achieved 70% of ultimate strength on the 1st day itself. FA3 and FA4 have shown significant strength from beginning. By comparing Figs. 6 and 7, it can be inferred that the early strength development was faster in the samples cured at elevated temperature but the final strength was similar in both ambient and elevated temperature cured samples.
Fig. 7. Variation of compressive strength of size fractioned fly ash based geopolymer cured at elevated temperature.
3.4. Characterization of geopolymer samples 3.4.1. FTIR studies Fig. 8 shows the FTIR spectra of geopolymer matrixes derived from different size fractions of fly ash after elevated temperature curing. The bands at 450 cm1 and 555 cm1 is related to AlAO/ SiAO in plane and bending modes and 1004 cm1 with asymmetric AlAO/SiAO stretching [25,26]. The change in intensity of spectra with size fraction is associated with the extent of structural reorganization or aluminium incorporation while lowering the energy of the band and can be assigned to either AlAO or SiAO bonds [3,27,28]. The maximum peak intensity at 1004 cm1 corresponds to FA4 which is finest fly ash. The broad band at 3458 cm1 and a weak peak at 1657 cm1 is due to the stretching vibrations of OAH bonds and HAOAH bending vibrations of interlayer adsorbed H2O molecule, respectively. The change in intensity of these peaks can be attributed to the presence of structural water in fly ash geopolymer [29,30]. The band near 1657 cm1 can be ascribed to the water molecules occluded inside the aluminosilicate structure [30,31]. The small band at around 2360 cm1 is due to the infrared band
Fig. 6. Variation of compressive strength of size fractioned fly ash based geopolymer cured at ambient temperature.
Fig. 8. FTIR spectrum showing structural changes in geopolymer derived from size fractioned fly ash by elevated temperature curing.
position of HCO ions [27,28]. The absorption bands at 3 1403 cm1 corresponds to stretching vibrations of C@O, confirming the presence of carbonate groups i.e. Na2CO3 [32,33]. This band was more intense in FA1 and FA2 which indicates the more carbonation due to unreacted alkali. 3.4.2. Microstructural studies Fig. 9 shows the XRD patterns of geopolymers samples subjected to elevated temperature curing. For all the studied samples, peaks belonging to the following two categories were found, (a) remnants of original fly ash, and (b) formed during geopolymerisation reaction. In the unreacted fly ash, the order of crystallinity was FA2 > FA1 > FA3 > FA4 (Fig. 3) however this order has changed after geopolymerisation. FA4 which has shown minimum peak intensity has shown some very prominent peaks of sodalite (PDF 37-476) and thermonatrite (PDF 08-0448) after geopolymerisation. Similar type of peaks were reported by earlier researchers while studying the geopolymerisation of low calcium ferronickel slag with kaolinite [27,28]. They have also discussed the reaction path for formation of sodalite (Na4Al3(SiO4)3Cl) and thermonatrite (Na2CO3H2O) in geopolymer system. In contrast FA3 has shown low peak intensity of quartz and mullite but prominent hump after geopolymerisation. Peaks of sodalite and thermonatrite were found, but not as prominent as in FA4. A large area of polished section has been scanned and the morphological features have been obtained using both scattered and backscattered electron mode. Fig. 10(a–d) shows the representative microstructure of geopolymer samples cured at 60 °C. Energy
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Fig. 9. X-ray diffractogram of geopolymers showing change in amorphosity and crystallinity.
dispersive X-ray microanalysis has been used to get the elemental composition of selected features. In general, the homogeneity of microstructure was better in FA3 and FA4 with pore volume and uniform distribution of gel phase. EDS analysis revealed that the composition of these gels corresponds to NASH gel with varying Si/Al ratio. In general, the following major morphological features were observed in the polished sections, dense gel phase (Fig. 10d), scattered gel phase (Fig. 10b) and partly reacted cenosphere (Fig. 10a and c). In addition, angular quartz particles, fibrous
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gel structure and carbon rich phase were also observed. To understand the chemistry of reaction phases, EDS analysis has been carried out and the results of both microstructure and EDS are summarized in Table 3. A wide variation in particle size and chemical composition has been found in different fraction of fly ash. These variations are associated with the principle of size separation in ESP where separation efficiency depends on particle size, conductivity and gas viscosity. The migration velocity is proportional to particle size and inversely proportional to gas viscosity, thus carries the fly ash of different size to different distance [23]. In air classification system size separation depends on air flow rate and density. By comparing the results of FA4 with our previous reported work on air classified fly ash of similar D50, it was found that the size fractioned fly ash from ESP shows a wider particle size distribution pattern [16]. As the chemistry of fly ash varies with particle size, this might have also resulted into different chemistry than air classified fly ash. FA1 and FA2 which were coarser have shown similar SiO2/Al2O3 ratio and glass content. Similarly FA3 and FA4 which were finer have shown closer values of SiO2/Al2O3 ratio and glass content. The geopolymerisation reaction (Figs. 4 and 5) has shown linear relationship with glass content. As the glass content is also related with size fractions, it can also be correlated with particle size of fly ash. Interestingly the FA3 sample has shown the three distinct sub peaks showing presence of three reaction mechanism. These sub-peaks were less evident in FA4 and almost negligible in FA1 and FA2. However the peak widening increased in coarser fractions and peak position shifted from right to left which indicates that the reaction was less intense and started finishing early due to the availability of less reactive glass content. The compressive strength development at both ambient and elevated temperature was found dependent on three parameters (a) SiO2/Al2O3 ratio, size fraction and glass content. The final strength was comparable in both ambient and elevated temperature cured samples. The same position of FTIR spectrum in all the samples indicated similar structural changes, but the variation in intensity was due to different degree of changes. The intense peak of carbonate bond near 1403 cm1 in FA1 and FA2 was due to the carbonation of free alkali
Fig. 10. Scanning electron micrographs of (a) FA1, (b) FA2, (c) FA3 and (d) FA4 fly ash derived geopolymer.
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Table 3 Summary of microstructural features obtained using SEM-EDS. Sample
Morphological features
EDS summary
FA1
Porous structure with lot of unreacted quartz, iron rich phase, calcium rich phase, carbon rich phase, scattered gel
FA2
Porous structure with lot of unreacted quartz, carbon rich phase, scattered gel Compact microstructure with dense gel phase and low porosity, partially reacted cenosphere and unreacted quartz particles Dense gel phase uniformly distributed, partially reacted cenosphere and unreacted quartz particles
Gel composition varies from Si/Al = 1.5–1.8 and Na/Si = 1.4–3.0 Iron rich phase is probably hematite and Ca rich phase is having Ca/Si = 0.6–0.7, Ca/ Al = 1.3–1.6 Gel composition varies from Si/Al = 1.3–1.7 and Na/Si = 0.4–1.5, carbon rich phase is having Ca/Si = 0.16–0.18, Ca/Al = 0.4 – 0.6 and Ca/Na is 0.9–1.0 Gel composition varies from Si/Al = 1.5–1.8 and Na/Si = 1.4–3.0
FA3 FA4
which formed the thermonatrite, a member of the ‘‘soda minerals’’ group which is formed by atmospheric carbonation of the excess sodium hydroxide as per the given reaction [27]:
2NaOH þ CO2 ¼ Na2 CO3 þ H2 O
ð1Þ
This observation is also in the agreement with XRD results which has shown the presence of thermonatrite phase. It is very interesting to note the change in trend of XRD pattern after geopolymerisation (Fig. 9). FA4, which was originally characterized by low intensity peaks of quartz, mullite, hematite and hump corresponding to amorphous and low crystalline phases, has shown high intensity peaks of sodalite and thermonatrite, after geopolymerisation. FA3, which was originally characterized by high intensity peaks, has shown significant reduction in peak intensity and more amorphization, which is typical of geopolymerisation. It can be stated that zeolite formation was initiated in FA4 sample but FA3 has shown better geopolymerisation. Similarly FA2 has shown higher intensity peaks than FA1. SEM-EDS (Table 3) analysis indicated that the gel characteristic and composition was different in finer size fractions than the coarser size fractions. Also coarser size fractions have shown more unreacted particles like aluminosilicate cenospheres, quartz and hematite. Due to the lesser reactivity of coarser size fractions, some alkali remained free after geopolymerisation which in contact with atmospheric carbon converted into thermonatrite. By correlating the compressive strength results with characterization results, it is coming out clearly that strength development is not the function of a single parameter but depends on multiple factors such as ratio of silica to alumina, particle fineness, glass content, types and quantity of reaction products formed and homogeneity of microstructure. 4. Conclusions Based on the above results, the following conclusions may be drawn: (1) Different size fractions of fly ash collected from different hoppers have significant variation in chemistry, mineralogy, particle size distribution and glass content. (2) SiO2/Al2O3 ratio has shown the increasing trend with increasing fineness, whereas Fe2O3, CaO and loss on ignition decreased. (3) The heat flow curve at 60 °C which indicates the geopolymerisation reaction has shown linear correlation with the glass content of fly ash. (4) The finer fractions FA3 and FA4 have shown more intense reaction and formation of more reaction product and homogeneous microstructure. (5) The early strength development in elevated temperature cured sample was more evident but the final strength development in both ambient and elevated temperature cured samples was almost the same.
Gel composition varies from Si/Al = 1.3–1.5 and Na/Si = 1.1–1.6
(6) The strength development was influenced by combined actions of SiO2/Al2O3 ratio, particle size and glass content which determined the course of geopolymerisation reaction and consequently microstructure development.
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[26] J.W. Phair, J.S.J. Van-Deventer, J.D. Smith, Mechanism of polysialation in the incorporation of zirconia into fly ash-based geopolymers, Ind. Eng. Chem. Res. 39 (2000) 2925–2934. [27] K. Komnitsas, D. Zaharaki, V. Perdikatsis, Geopolymerisation of low calcium ferronickel slags, J. Mater. Sci. 42 (9) (2007) 3073–3082. [28] D. Zaharaki, K. Komnitsas, V. Perdikatsis, Use of analytical techniques for identification of inorganic polymer gel composition, J. Mater. Sci. 45 (10) (2010) 2715–2724. [29] J.A. Armstrong, S.E. Dann, Investigation of zeolite scales formed in the Bayer process, Microporous Mesoporous Mater. 41 (2000) 89–97. [30] A. Gok, M. Omastova, J. Proke, Synthesis and characterization of red mud/ polyaniline composites: electrical properties and thermal stability, Eur. Polym. J. 43 (2007) 2471–2480. [31] C.F. Linares, S. Sanchez, C. Urbina-de-Navarro, K. Rodrıguez, M.R. Goldwasser, Study of cancrinite-type zeolites as possible antiacid agents, Microporous Mesoporous Mater. 77 (2005) 215–221. [32] C.K. Huang, P.F. Kerr, Infrared study of the carbonate minerals, Am. Mineral. 45 (1960) 311–324. [33] C.K. Yip, G.C. Lukey, J.L. Provis, J.S.J. van Deventer, Carbonate mineral addition to metakaolin-based geopolymers, Cem. Concr. Compos. 30 (2008) 979–985.
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Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt
Original Research Paper
Geopolymerisation behaviour of size fractioned fly ash Sanjay Kumar a,⇑, Ferenc Kristály b, Gabor Mucsi b a b
CSIR-National Metallurgical Laboratory, Jamshedpur 831007, India University of Miskolc, Miskolc 3515, Hungary
a r t i c l e
i n f o
Article history: Received 1 April 2014 Received in revised form 22 July 2014 Accepted 31 July 2014 Available online xxxx Keywords: Fly ash Size fractioned Geopolymerisation Microstructural characterization Strength development
a b s t r a c t Fly ash is a fine powder residue resulted from combustion of pulverized coal in thermal power plants. Different size fractions of fly ash have different properties. Four size fractions (with characteristic particle diameter D50 of 40.37, 23.64, 10.33 and 2.98 lm respectively) collected from different fields of an electrostatic precipitator and representing the entire particle spectrum of fly ash has been selected for the study. These fractions have been characterized for their granulometry, chemistry, glass content and mineralogical phases. Geopolymerisation of size fractioned fly ash has been carried out at ambient (27 °C) and elevated (60 °C) temperature using isothermal conduction calorimetry (ICC) and the microstructure has been studied using X-ray diffractometry (XRD), scanning electron microscopy with X-ray microanalysis probe (SEM-EDS) and Fourier transform infrared spectroscopy (FTIR). Calorimetric studies showed that the heat flow curve during geopolymerisation has linear correlation with the glass content of fly ash. The compressive strength development at both ambient and elevated temperature was due to the combined effect of SiO2/Al2O3 ratio, particle size and glass content. SEM-EDS studies have shown more reaction product in finer fractions and unreacted particles in coarser fractions. Formation of more thermonatrite phase was due to poor reactivity of coarse size fraction resulting into free alkali which in presence of atmospheric carbon formed Na2CO3H2O. Ó 2014 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
1. Introduction Fly ash is a by-product of thermal power plants resulted from the combustion of pulverized coal. It is a granular material with particle size typically varying from submicron size to 250 lm [1]. Interestingly the different size fractions of fly ash vary considerably in terms of chemistry, mineralogy and reactivity [2]. Due to its pozzolanic property, small particle size, flow characteristic and combination of crystalline and amorphous phase, fly ash is preferred material for many applications such as production of cement, concrete and brick [3]. Due to its unique combination of properties, uses of fly ash not merely as filler material, but as reactive component are continuously being explored. One such potential application where fly ash actively participates in reaction is its use for geopolymer synthesis. Geopolymers are new class of inorganic polymer materials synthesized by reaction between alumino-silicate and alkali
⇑ Corresponding author at: Recycling & Waste Utilization, CSIR-National Metallurgical Laboratory, Jamshedpur 831007, India. Tel.: +91 657 2345049, mobile: +91 9939326346. E-mail address: [email protected] (S. Kumar).
compounds at ambient or near ambient temperature [4]. Due to easy process of synthesis, low environmental impact and versatility in properties, they are fast emerging materials of choice for a range of building materials, fire resistant ceramics, composites, matrix for immobilization of toxic wastes, and many others. Traditionally metakaolin has been used as raw material for geopolymer synthesis [5,6]. Recently, there has been a change in trend where emphasis on raw material has been shifted from pure and naturally occurring materials to waste and by-products, more specifically fly ash. The reason for increased attention on fly ash is associated with easy availability, technical superiority, environmental and economic benefits [7–10]. The mechanism of geopolymerisation of fly ash is considered similar to metakaolin based geopolymer, but the degree of reaction varies due to the reactive and non-reactive fractions of fly ash [11]. The property of resulting geopolymer is influenced by the chemistry and reactivity of fly ash. The effect on role of chemistry on the properties of fly ash based geopolymer is well documented [12–15]. The reactivity of the fly ash in geopolymer system is mainly governed by two factors, (a) presence of reactive amorphous phase, and (b) fine particle size. Geopolymerisation of different size fractions of fly ash and its influence on the properties has been studied by many researchers [16–19]. However, in the majority of the studies, the different sizes have
http://dx.doi.org/10.1016/j.apt.2014.09.001 0921-8831/Ó 2014 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
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been obtained either by air classification or by milling of fly ash [16–19]. In the present work, the different size fractions of fly ash have been collected from the different hoppers of an electrostatic precipitator (ESP). The rationale for collecting from hoppers is based on ESP structure and existing practice of fly ash collection. Typically ESP has many hoppers in the direction of gas flow for the collection of fly ash. The fly ash collected in different hoppers has different properties. For example, the particle size decreases and specific surface area increases as the collection hopper gets distance from the boiler [20–22]. In practice, the fly ash collected from different hoppers is blended together before storage or disposal. Thus the distinct advantage of the properties of different size fractions is lost. Again for getting those properties, either fly ash is classified or milled. Also the chemistry and particle size distribution of fly ash collected in hoppers of ESP is not similar to those of air classifiers as they operate on different principles. In ESP, the separation efficiency depends on particle size, conductivity and gas viscosity, whereas in air classifiers, it depends on air flow rate and density [23]. Thus the resulting fly ash is different in chemistry and reactivity, even if they are close in particle size distribution. We have carried out studies on suitability of air classified and mechanically activated fly ash for synthesis of geopolymers [16]. The objective of the present work was to develop geopolymers from size classified fly ash directly collected from hoppers of ESP. The focus of the study is to elucidate the effect of different size fractions on geopolymerisation reaction using isothermal conduction calorimeter. The microstructure obtained after reactions were studied using XRD, FTIR and SEM-EDS. Attempt has been made to correlate the size, reaction, structure and properties.
2. Experimental The different size fractions of fly ash were collected from different fields of ESP at Tata Power, Jamshedpur, India. Out of samples of various size fractions, four representative samples were selected with characteristic particle diameter D50 of 40.37, 23.64, 10.33 and 2.98 lm and labeled as FA1, FA2, FA3 and FA4 respectively. The chemical analysis of the fly ash was carried out using combination of X-ray fluorescence, atomic absorption spectrophotometer and classical analysis. The glass content of the fly ash was determined by counting the grains in a polarizing microscope. The particle size analysis of fly ash was carried out using a laser particle size analyzer (MASTERSIZER S, Malvern, U.K.). The rate of heat evolution during the reaction (dq/dt) was measured using an eight channel isothermal conduction calorimeter (TAM AIR, Thermometric AB, Jarafalla, Sweden). The process involved the preparation of the alkaline activator solution at least 24 h before use, mixing of an alkaline activator with the powder sample, and loading of the mix in the calorimeter. Analytical grade sodium hydroxide in flake form (98% purity) was used to prepare the alkaline activator solution. An alkaline activator of 6 M concentration was prepared in distilled water. The choice of 6 M concentration was based on our previous works [7,9] where it was found that higher concentration often leads to free alkalies which are more susceptible to carbonation. 7 g Solid sample and 3.5 ml of activator solution were used throughout the study. The samples were mixed and then loaded into the calorimeter. Calorimetric studies were carried out at 60 °C. The results obtained were presented in offset mode. Fourier Transform Infrared Spectroscopy (FT-IR-410 JASCO, U.S.A) was used for structural characterization of geopolymers. The samples were prepared by mixing the powder with KBr. XRD patterns were recorded on a SIEMENS X-ray diffractometer (Model D500), using Co Ka radiation with a Fe-filter. The scanning speed of 1 deg/min was used and the samples were scanned from
10° to 60° 2h angle. Morphological characterization of the fractured samples was done by a scanning electron microscope (JEOL SEM 840A) fitted with a Kevex Energy Dispersive Spectrometer (EDS) for X-ray micro-analysis after carbon coating on the fractured surface. The X-ray micro-analysis of areas of interest was determined from the average of minimum six analyses and used to calculate elemental ratios. For all the physical testing, liquid (alkali solution) to solid ratio was kept at 0.35. The samples were prepared at 27 ± 2 °C and under relative humidity of 65%. For compressive strength 70 70 70 mm cubic samples were prepared by vibro-casting of geopolymer paste. Compressive strength was tested on an Automatic Compression Testing Machine (AIMIL COMPTEST 2000, India) at age of 3, 7, 14 and 28 days after casting the samples at 27 °C followed by curing at 60 °C for 24 h. Averages of six samples were tested for each result. 3. Results and discussion 3.1. Characterization of size fractioned fly ash Fig. 1 shows the schematic diagram of a typical electrostatic precipitator. The coarser fraction of fly ash is collected close to entry side whereas the finer fraction is collected close to exit side of ESP. The fly ash used in the present study was collected from different fields of the hoppers. Table 1 shows chemical analysis of size fractioned fly ash. The glass content of fly ash has also been included in the table. It is interesting to note that SiO2 content decreased with the fineness whereas the Al2O3 content remained more or less constant. Fe2O3 concentration was also found to be higher towards coarser size fractions. The increase in LOI in coarser particles was mainly due to the presence of free unburned carbon. The composition of glass fraction was mainly silicate with little amount of alumino-silicates. No clear trend of glass content with size fraction has been observed. However, the finer fractions have shown higher and coarser fractions have shown lower glass content. Fig. 2 shows the particle size distribution of FA1, FA2, FA3 and FA4 samples. The characteristic particle diameters D10, D50 and D90 have been tabulated in Table 2. A wide variation in particle size was observed in the fly ash. In FA4, 90% particles were <20 lm size whereas in the FA2, the coarser size just exceeded 100 lm. The mineralogical composition of fly ash is given in Fig. 3. FA4 sample, which has the finest particle side distribution, has shown the low intensity of quartz peak. The featureless hump between 15° and 40° 2h was more prominent in FA3 and FA4, which was due to the presence of amorphous and poorly crystalline phase. In all the size fractions, similar crystalline phases but with different peak intensity were detected. The most common phases were quartz (PDF 46-1045), hematite (PDF 88-2359) and Mullite (PDF 851460). 3.2. Geopolymerisation reaction To study the effect of size fractions on geopolymerisation reaction, isothermal conduction calorimeter was used under the following two conditions: (a) at ambient temperature (27 °C), and
Fig. 1. Schematic diagram of a typical electrostatic precipitator.
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S. Kumar et al. / Advanced Powder Technology xxx (2014) xxx–xxx
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Table 1 Chemical analysis of size fractionated fly ash. Radicals
SiO2 Al2O3 Fe2O3 TiO2 CaO MgO Na2O K2O P MnO S LOI SiO2/Al2O3 ratio Glass content
Fly ash samples (wt%) FA4
FA3
FA2
FA1
59.42 28.6 2.83 1.62 0.59 0.04 0.36 1.06 0.21 0.03 0.02 1.17 2.07 60
58.36 28.23 3.18 1.64 0.76 0.42 0.36 1.09 0.25 0.03 0.03 1.30 2.06 62
51.42 28.52 4.99 1.72 0.95 0.70 0.05 1.45 0.44 0.03 0.19 1.75 1.80 41
50.98 28.06 4.92 1.75 0.99 0.74 0.06 1.45 0.52 0.04 0.17 2.37 1.81 42.5
Fig. 3. X-ray diffractogram of size fractioned fly ash showing crystalline and amorphous phases.
Fig. 2. Particle size distribution of FA1, FA2, FA3 and FA4 samples.
Table 2 Characteristic particle diameter of fly ash samples. Characteristic diameter
D10 D50 D90
Fly ash samples (lm) FA4
FA3
FA2
FA1
0.34 2.98 19.86
0.41 10.33 69.76
8.89 40.37 103.21
1.74 23.64 91.49
(b) elevated temperature (60 °C). The peak intensity at ambient temperature was almost negligible so only the result of 60 °C was included in the study (Fig. 4). The first peak (which appears more like a straight line) in the beginning corresponds to wetting and partial dissolution of glassy content followed by small induction period as a consequence of low reactivity [24]. Although the peak intensity varied in all the samples, the peak behaviour was the same in FA1 and FA2 samples, and FA3 and FA4 samples. In FA3 and FA4, the second exothermic peak after induction period has shown the presence of three sub peaks (P1, P2 and P3). These sub peaks possibly correspond to different reactions such as dissolution–precipitation, formation of NASH (N = Na2O, A = Al2O3, S = SiO2, H = H2O) gel and polycondensation. It is interesting to note that FA4, which has the finest particle size, have shown less intense peak than FA3, which has the more glass content. But the peak P3 appeared early in FA4 sample. In FA1 and FA2, only one peak was observed with a small shoulder at the position of P1. To understand the degree of reaction in fly ash of different size fraction, the cumulative area under peak was measured by
Fig. 4. Isothermal conduction calorimetric curve of size fractioned fly ash geopolymerised at 60 °C.
Fig. 5. Cumulative heat flow curve derived from calorimetric data obtained in Fig. 4 showing rate of geopolymerisation.
integrating the calorimetric curve and results are compiled in Fig. 5. The order of fly ash fraction reactivity was:
FA3 > FA4 > FA1 > FA2
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3.3. Physical properties of geopolymer The variation in compressive strength with curing time was measured for the following two sets of samples: (a) sample cured at ambient temperature for different periods of time, (b) sample cured at 60 °C for 24 h followed by curing at ambient temperature for different periods of time. Fig. 6 shows the variation of compressive strength of the samples cured at ambient temperature. The strength development trend in FA3 and FA4 was different from FA1 and FA2. The strength development accelerated significantly after 7 days in FA3 and FA4 samples and reached to maximum value to 44–45 MPa after 28 days. However, the strength development was not so significant in FA1 and FA2 and obtained the maximum strength of 18 and 20 MPa respectively. The trend of strength development was different in the samples cured at 60 °C followed by ambient temperature curing (Fig. 7). All the samples have achieved 70% of ultimate strength on the 1st day itself. FA3 and FA4 have shown significant strength from beginning. By comparing Figs. 6 and 7, it can be inferred that the early strength development was faster in the samples cured at elevated temperature but the final strength was similar in both ambient and elevated temperature cured samples.
Fig. 7. Variation of compressive strength of size fractioned fly ash based geopolymer cured at elevated temperature.
3.4. Characterization of geopolymer samples 3.4.1. FTIR studies Fig. 8 shows the FTIR spectra of geopolymer matrixes derived from different size fractions of fly ash after elevated temperature curing. The bands at 450 cm1 and 555 cm1 is related to AlAO/ SiAO in plane and bending modes and 1004 cm1 with asymmetric AlAO/SiAO stretching [25,26]. The change in intensity of spectra with size fraction is associated with the extent of structural reorganization or aluminium incorporation while lowering the energy of the band and can be assigned to either AlAO or SiAO bonds [3,27,28]. The maximum peak intensity at 1004 cm1 corresponds to FA4 which is finest fly ash. The broad band at 3458 cm1 and a weak peak at 1657 cm1 is due to the stretching vibrations of OAH bonds and HAOAH bending vibrations of interlayer adsorbed H2O molecule, respectively. The change in intensity of these peaks can be attributed to the presence of structural water in fly ash geopolymer [29,30]. The band near 1657 cm1 can be ascribed to the water molecules occluded inside the aluminosilicate structure [30,31]. The small band at around 2360 cm1 is due to the infrared band
Fig. 6. Variation of compressive strength of size fractioned fly ash based geopolymer cured at ambient temperature.
Fig. 8. FTIR spectrum showing structural changes in geopolymer derived from size fractioned fly ash by elevated temperature curing.
position of HCO ions [27,28]. The absorption bands at 3 1403 cm1 corresponds to stretching vibrations of C@O, confirming the presence of carbonate groups i.e. Na2CO3 [32,33]. This band was more intense in FA1 and FA2 which indicates the more carbonation due to unreacted alkali. 3.4.2. Microstructural studies Fig. 9 shows the XRD patterns of geopolymers samples subjected to elevated temperature curing. For all the studied samples, peaks belonging to the following two categories were found, (a) remnants of original fly ash, and (b) formed during geopolymerisation reaction. In the unreacted fly ash, the order of crystallinity was FA2 > FA1 > FA3 > FA4 (Fig. 3) however this order has changed after geopolymerisation. FA4 which has shown minimum peak intensity has shown some very prominent peaks of sodalite (PDF 37-476) and thermonatrite (PDF 08-0448) after geopolymerisation. Similar type of peaks were reported by earlier researchers while studying the geopolymerisation of low calcium ferronickel slag with kaolinite [27,28]. They have also discussed the reaction path for formation of sodalite (Na4Al3(SiO4)3Cl) and thermonatrite (Na2CO3H2O) in geopolymer system. In contrast FA3 has shown low peak intensity of quartz and mullite but prominent hump after geopolymerisation. Peaks of sodalite and thermonatrite were found, but not as prominent as in FA4. A large area of polished section has been scanned and the morphological features have been obtained using both scattered and backscattered electron mode. Fig. 10(a–d) shows the representative microstructure of geopolymer samples cured at 60 °C. Energy
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Fig. 9. X-ray diffractogram of geopolymers showing change in amorphosity and crystallinity.
dispersive X-ray microanalysis has been used to get the elemental composition of selected features. In general, the homogeneity of microstructure was better in FA3 and FA4 with pore volume and uniform distribution of gel phase. EDS analysis revealed that the composition of these gels corresponds to NASH gel with varying Si/Al ratio. In general, the following major morphological features were observed in the polished sections, dense gel phase (Fig. 10d), scattered gel phase (Fig. 10b) and partly reacted cenosphere (Fig. 10a and c). In addition, angular quartz particles, fibrous
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gel structure and carbon rich phase were also observed. To understand the chemistry of reaction phases, EDS analysis has been carried out and the results of both microstructure and EDS are summarized in Table 3. A wide variation in particle size and chemical composition has been found in different fraction of fly ash. These variations are associated with the principle of size separation in ESP where separation efficiency depends on particle size, conductivity and gas viscosity. The migration velocity is proportional to particle size and inversely proportional to gas viscosity, thus carries the fly ash of different size to different distance [23]. In air classification system size separation depends on air flow rate and density. By comparing the results of FA4 with our previous reported work on air classified fly ash of similar D50, it was found that the size fractioned fly ash from ESP shows a wider particle size distribution pattern [16]. As the chemistry of fly ash varies with particle size, this might have also resulted into different chemistry than air classified fly ash. FA1 and FA2 which were coarser have shown similar SiO2/Al2O3 ratio and glass content. Similarly FA3 and FA4 which were finer have shown closer values of SiO2/Al2O3 ratio and glass content. The geopolymerisation reaction (Figs. 4 and 5) has shown linear relationship with glass content. As the glass content is also related with size fractions, it can also be correlated with particle size of fly ash. Interestingly the FA3 sample has shown the three distinct sub peaks showing presence of three reaction mechanism. These sub-peaks were less evident in FA4 and almost negligible in FA1 and FA2. However the peak widening increased in coarser fractions and peak position shifted from right to left which indicates that the reaction was less intense and started finishing early due to the availability of less reactive glass content. The compressive strength development at both ambient and elevated temperature was found dependent on three parameters (a) SiO2/Al2O3 ratio, size fraction and glass content. The final strength was comparable in both ambient and elevated temperature cured samples. The same position of FTIR spectrum in all the samples indicated similar structural changes, but the variation in intensity was due to different degree of changes. The intense peak of carbonate bond near 1403 cm1 in FA1 and FA2 was due to the carbonation of free alkali
Fig. 10. Scanning electron micrographs of (a) FA1, (b) FA2, (c) FA3 and (d) FA4 fly ash derived geopolymer.
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Table 3 Summary of microstructural features obtained using SEM-EDS. Sample
Morphological features
EDS summary
FA1
Porous structure with lot of unreacted quartz, iron rich phase, calcium rich phase, carbon rich phase, scattered gel
FA2
Porous structure with lot of unreacted quartz, carbon rich phase, scattered gel Compact microstructure with dense gel phase and low porosity, partially reacted cenosphere and unreacted quartz particles Dense gel phase uniformly distributed, partially reacted cenosphere and unreacted quartz particles
Gel composition varies from Si/Al = 1.5–1.8 and Na/Si = 1.4–3.0 Iron rich phase is probably hematite and Ca rich phase is having Ca/Si = 0.6–0.7, Ca/ Al = 1.3–1.6 Gel composition varies from Si/Al = 1.3–1.7 and Na/Si = 0.4–1.5, carbon rich phase is having Ca/Si = 0.16–0.18, Ca/Al = 0.4 – 0.6 and Ca/Na is 0.9–1.0 Gel composition varies from Si/Al = 1.5–1.8 and Na/Si = 1.4–3.0
FA3 FA4
which formed the thermonatrite, a member of the ‘‘soda minerals’’ group which is formed by atmospheric carbonation of the excess sodium hydroxide as per the given reaction [27]:
2NaOH þ CO2 ¼ Na2 CO3 þ H2 O
ð1Þ
This observation is also in the agreement with XRD results which has shown the presence of thermonatrite phase. It is very interesting to note the change in trend of XRD pattern after geopolymerisation (Fig. 9). FA4, which was originally characterized by low intensity peaks of quartz, mullite, hematite and hump corresponding to amorphous and low crystalline phases, has shown high intensity peaks of sodalite and thermonatrite, after geopolymerisation. FA3, which was originally characterized by high intensity peaks, has shown significant reduction in peak intensity and more amorphization, which is typical of geopolymerisation. It can be stated that zeolite formation was initiated in FA4 sample but FA3 has shown better geopolymerisation. Similarly FA2 has shown higher intensity peaks than FA1. SEM-EDS (Table 3) analysis indicated that the gel characteristic and composition was different in finer size fractions than the coarser size fractions. Also coarser size fractions have shown more unreacted particles like aluminosilicate cenospheres, quartz and hematite. Due to the lesser reactivity of coarser size fractions, some alkali remained free after geopolymerisation which in contact with atmospheric carbon converted into thermonatrite. By correlating the compressive strength results with characterization results, it is coming out clearly that strength development is not the function of a single parameter but depends on multiple factors such as ratio of silica to alumina, particle fineness, glass content, types and quantity of reaction products formed and homogeneity of microstructure. 4. Conclusions Based on the above results, the following conclusions may be drawn: (1) Different size fractions of fly ash collected from different hoppers have significant variation in chemistry, mineralogy, particle size distribution and glass content. (2) SiO2/Al2O3 ratio has shown the increasing trend with increasing fineness, whereas Fe2O3, CaO and loss on ignition decreased. (3) The heat flow curve at 60 °C which indicates the geopolymerisation reaction has shown linear correlation with the glass content of fly ash. (4) The finer fractions FA3 and FA4 have shown more intense reaction and formation of more reaction product and homogeneous microstructure. (5) The early strength development in elevated temperature cured sample was more evident but the final strength development in both ambient and elevated temperature cured samples was almost the same.
Gel composition varies from Si/Al = 1.3–1.5 and Na/Si = 1.1–1.6
(6) The strength development was influenced by combined actions of SiO2/Al2O3 ratio, particle size and glass content which determined the course of geopolymerisation reaction and consequently microstructure development.
Acknowledgements The research work was carried out in the framework of a TÉT Indo-Hungarian collaboration project GEOPOL10. The fly ash used in the study was supplied by Tata Power Co. Ltd, Jamshedpur, India and is greatly acknowledged. The first author also acknowledges Dr. S. Srikanth, Director, CSIR-National Metallurgical Laboratory, Jamshedpur, India for his permission to publish this work. References [1] H.L. Seung, J.K. Hong, S. Etsuo, D. Masaki, Effect of particle size distribution of fly ash–cement system on the fluidity of cement pastes, Cem. Concr. Res. 33 (2003) 763–768. [2] M. Tiwari, S.K. Sahu, R.C. Bhangare, P.Y. Ajmal, G.G. Pandit, Elemental characterization of coal, fly ash, and bottom ash using energy dispersive Xray fluorescence technique applied radiation and isotopes, in press,
Please cite this article in press as: S. Kumar et al., Geopolymerisation behaviour of size fractioned fly ash, Advanced Powder Technology (2014), http:// dx.doi.org/10.1016/j.apt.2014.09.001
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Please cite this article in press as: S. Kumar et al., Geopolymerisation behaviour of size fractioned fly ash, Advanced Powder Technology (2014), http:// dx.doi.org/10.1016/j.apt.2014.09.001