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Geopolymerization kinetics of fly ash based geopolymers using JMAK model
Author’s Accepted Manuscript Geopolymerization Kinetics of Fly Ash Based Geopolymers Using JMAK Model Ahmer Ali Siyal, Khairun Azizi Azizli, Zakaria Man, Lukman Ismail, Muhammad Irfan Khan www.elsevier.com/locate/ceri
PII: DOI: Reference:
S0272-8842(16)31060-4 http://dx.doi.org/10.1016/j.ceramint.2016.07.006 CERI13231
To appear in: Ceramics International Received date: 5 February 2016 Revised date: 1 July 2016 Accepted date: 1 July 2016 Cite this article as: Ahmer Ali Siyal, Khairun Azizi Azizli, Zakaria Man, Lukman Ismail and Muhammad Irfan Khan, Geopolymerization Kinetics of Fly Ash Based Geopolymers Using JMAK Model, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.07.006 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Geopolymerization Kinetics of Fly Ash Based Geopolymers Using JMAK Model Ahmer Ali Siyal1, Khairun Azizi Azizli1, Zakaria Man1, Lukman Ismail2, Muhammad Irfan Khan1 1
Department of Chemical Engineering, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, 32610 Tronoh, Perak, Malaysia 2
Faculty of Agro-Based Industry, Universiti Malaysia Kelantan 17600 Jeli, Kelantan.
[email protected] [email protected] [email protected] [email protected] [email protected]
Abstract Geopolymers are versatile materials possessing excellent mechanical properties and resistance against aggressive environments, these materials present a benefit of improving simultaneously
both the environmental and engineering performance as compared to classical conventional materials. This paper determines the geopolymerization kinetics of fly ash based geopolymers using Johnson-Mehl-Avrami-Kolmogorov (JMAK) model. The experiments were designed using Taguchi method by varying four factors (Si/Al ratio, Na/Al ratio, W/S ratio, and curing temperature). The degree of reaction of fly ash (α) was used as a measure of the changes occurring during geopolymerization reaction. The characterization of the cured geopolymers was also carried out. The values of n were in the range of 0.0931 to 0.2321 while the values of k were in the range of 0.366 to 0.671. According to the JMAK model results, geopolymerization of fly ash based geopolymers is a one dimensional diffusion controlled reaction and its growth follows the mechanism of thickening of large product layers. The mechanism of geopolymerization consists of initial dissolution which is a first order chemical reaction, and further reactions including dissolution, gelation, and polycondensation are the diffusion controlled reactions. The asymmetric stretching band of Si-O-T shifted to 992 cm-1 and increased in intensity indicating the formation of geopolymer. Microstructural analysis showed the heterogeneous nature of geopolymers consisting of geopolymer, unreacted fly ash, and different types of needle like structures while one sample showed plate like morphology consistent with the JMAK model results. The geopolymer was found to be an amorphous material with only few peaks due to unreacted crystalline fly ash.
Keywords: Fly Ash, Geopolymer, Taguchi model, Kinetics, JMAK model.
1. Introduction
Geopolymers are inorganic polymer materials possessing properties superior to conventional materials as well as possessing capability of use in a variety of applications such as adhesives, coatings, refractory applications, heavy metal adsorption and fast setting applications. Along with all these applications geopolymers can also be used for the synthesis of ceramics and hydroceramics. Geopolymers are produced by the geopolymerization of aluminosilicate source materials in the presence of alkali and/or alkali silicate solutions. Geopolymerization is a multistep process involving dissolution, reorganization, and condensation reactions occurring concurrently. The kinetic study is very important for understanding of the reactions taking place and the mechanisms involved during transformation of reactants into products and it enables us to tailor the properties of the final product according to the type of application [1]. Various approaches have been employed to study the geopolymerization kinetics. Two in-situ kinetic studies to determine the growth and kinetics of metakaolin based geopolymers with the changes in the morphology and phases of geopolymers using Environmental Scanning Electron Microscopy (ESEM) [2, 3] and Energy Dispersive X-Ray Diffractrometry (EDXRD) [4] have shown to be effective only during the initial setting period. Use of Attenuated Total Reflectance - Fourier Transform Infrared Spectroscopy (ATR-FTIR) to determine the geopolymer gel formation through the changes in the bonding behavior with time cannot be helpful if excess silica is present in the alkaline solution. The technique used to determine the extent of reaction of metakaolin and slag based geopolymers by the changes in the diffractograms of geopolymers with time using X-Ray Pair Distribution Function Analysis [5] becomes difficult when crystals start to appear. Isothermal Conduction Calorimetry (ICC) [6, 7] used to determine the geopolymerization kinetics of metakaolin based geopolymers by the changes in the heat of
reaction with time cannot be used for fly ash based geopolymers without the availability of thermochemical parameters required for data quantification. Phenomenological kinetics models such as the Exponential and Knudson linear dispersion models [8] and the Modified Jandar model [9] have also been used for the study of geopolymerization. However, these models could not be used to describe the overall geopolymerisation process. Various kinetics models have also been proposed for the mechanism of geopolymerization. Glukhovsky [10] described geopolymer formation as a combination of destruction of raw materials and condensation of the formed products. In the study by Xu et al [11], geopolymerization was shown to consist of dissolution, polymerization, and condensation reactions, whilst
Rees et al [12] described the gelation
mechanism for fly ash based geopolymers consisting of dissolution, polymerization, and nucleation and growth. Despite several studies on the geopolymerization kinetics and mechanisms of geopolymerization of fly ash based geopolymers, a complete understanding of the process of geopolymer phase transformation from paste to solid and the mechanisms involved during the transformation has not been well developed. Johnson-Mehl-Avrami–Kolmogorov (JMAK) model is a phenomenological model and it has been used to describe the kinetics of various processes such as crystallization, polymerization, depositions, hydration kinetics of Portland cement and so on [13-16]. This model can be used for any system which transforms by the process of nucleation and growth [17]. The geopolymerization involves polymerization reaction and it has also been observed that geopolymer grows by the process of nucleation and growth [12, 18], so this model may be helpful for the study of geopolymerization kinetics of fly ash based geopolymers. It is novel to use this model for geopolymerization kinetics as it has not been used yet. JMAK model is an isothermal kinetic model having two parameters; Avrami exponent
(n) and growth rate (k). Avrami exponent (n) describes the mechanism occurring during transformation from paste to a solid and growth rate (k) is related to the nucleation and growth rates and it is temperature dependent. According to JMAK model, the degree of phase conversion (α (t)) is given below, ( )
(1)
where α is the degree of reaction, n is the Avrami exponent, k is the Avrami constant, and t is the time. The values of n are in the range of 1- 4, while fractional values of n also exist due to secondary crystallization and lower n values (<1) indicate linear nuclei growth [19, 20]. This paper determines the geopolymerization kinetics of fly ash based geopolymers using JMAK model. Taguchi design expert was used for designing of experiments. The changes occurring during geopolymerization with time were determined in terms of degree of reaction of fly ash (α) using HCl method. The effects of parameters such as Si/Al ratio, Na/Al ratio, W/S ratio, and temperature on the degree of reaction (α) were also determined. The response index based on signal to noise ratio (S/N) ratio principle [21] was used for analysis of the data. The degree of reaction (α) data was used in JMAK model to determine the kinetics parameters. The bonding, microstructural, and phase analyses of the fully cured samples were determined using Fourier transform infrared spectroscopy (FTIR), Field emission scanning electron microscopy (FESEM), and X-Ray diffraction (XRD) respectively.
2. Experimental Program 2.1 Materials Fly ash (from local thermal power plant) was used for the synthesis of geopolymers. AR
grade NaOH pellets and sodium silicate (SiO2 - 37.79 %, Na2O - 16.36 %) purchased from R & M Chemicals, Malaysia were used for synthesis of geopolymers. AR grade HCl and Na2CO3 purchased from R & M Chemicals, Malaysia were used for determining the degree of reaction of fly ash (α). 2.2 2.2.1
Methods Determination of design of mixtures
Taguchi experimental design method was used for designing of experiments. The application of Taguchi model to geopolymers is limited to only a few studies [22, 23]. Four parameters such as silica to alumina (Si/Al) ratio, sodium to alumina (Na/Al) ratio, water to solid (W/S) ratio, and temperature were selected for this study. The parameters and their levels are shown in Table.1. The ranges and levels of the parameters were selected in a way to represent the best possible compositions based on the presented compositions in the literature [24-26]. We used one of the orthogonal arrays such as L9 (43) developed by Taguchi to represent all the factor or parameter levels. Using Taguchi design expert software a total of 9 trials or samples were obtained. Chemical compositions of the samples are shown in Table.2. *** Insert Table.1 here *** *** Insert Table.2 here *** 2.2.2 Geopolymer Synthesis Alkaline solution was prepared by dissolving sodium hydroxide pellets in distilled water. The solution was gently mixed until the pellets completely dissolved. The solution was left for 2 hours at room temperature to cool to normal temperature. The geopolymer samples were prepared following the design of mixtures suggested by Taguchi method (Table 2). The alkaline solution was poured onto the fly ash and both were mixed for 8 minutes using a hand mixer. The
geopolymer paste was poured into the mold and put in an electric oven at the desired temperature for curing. 2.2.3 Geopolymerization Kinetics Geopolymerization kinetics of fly ash based geopolymers was determined using JMAK model. The degree of reaction (α) was selected as a parameter to detect the changes occurring during geopolymerization. JMAK kinetics parameters such as Avrami exponent (n) and growth rate (k) were determined using the degree of reaction of fly ash (α). Determination of degree of reaction The degree of reaction of fly ash (α) was determined using hydrochloric acid (HCl) method. As geopolymer samples contain some quantity of unreacted sodium hydroxide which can react with HCl so before determining the degree of reaction (α), the geopolymer samples were neutralized to remove the unreacted sodium hydroxide. Phenolphthalein indicator and 0.2 M HCl solution were used for neutralization of samples. Five grams of geopolymer sample was dissolved in distilled water with a few drops of phenolphthalein indicator added to the mixture. Then 0.2 M HCl was added drop by drop until the color of solution disappeared. The neutralized sample was filtered and placed in the oven for 2 hours at 110 ˚C. One gram of the dried and ground neutralized geopolymer sample and 30 ml of 2 M HCl were placed in plastic centrifuge tube and heated to 60 ˚C for 15 minutes in a water bath. Then it was centrifuged for 5 minutes at 7000 r.p.m. The liquid was then removed and hot water was added and centrifuged again. After repeating the washing process with hot water for three times, the tube was then filled with 30 ml of 5 % Na2CO3 solution, stirred and placed in water bath and heated to 80 ˚C for 20 minutes, and then centrifuged for 5 minutes at 7000 r.p.m. The liquid was
decanted and the residue was washed with hot water three times following the same procedure described earlier. The tube with residue was dried at 110 ˚C for 2 hours, and weighed [27]. The residue remaining in the centrifuge tube was the unreacted fly ash. The degree of reaction (α) of the sample was determined in triplets and an average value of three readings was taken as the degree of reaction (α) of a sample. The formula used for determining the degree of reaction is given below, ( )
(
)
(2)
Mc is the initial weight of powder sample, g; Mr is the final weight of the residue, g Determination of curing time The curing time for kinetics study was also determined by selecting three samples (FAGP-1, FAGP-4, and FAGP-7) from Table 2. These samples were selected due to difference of compositions. The degree of reaction (α) of these samples was determined after 1 hour, 2 hours, 3 hours, 4 hours, 5 hours, 24 hours (1 day), 72 hours (3 days), 168 hours (7 days), 504 hours (21 days), and 672 hours (28 days). Fig.1 shows the results of the degree of reaction (α) of three selected samples. It can be observed that the degree of reaction (α) becomes stable after 168 hours (7 days) of reaction and during the period from 168 to 672 hours (7 to 28 days), it follows a straight line path and shows very small changes so it is better to take 168 hours (7 days) of reaction time for geopolymerization kinetics study. The curing time of 168 hours (7 days) was set as the benchmark for this study and further study was carried out for early stage curing period of 7 days. The degree of reaction (α) for all samples was determined after 1 hour, 2 hours, 3 hours, 4 hours, 5 hours, 24 hours, 72 hours, and 168 hours. *** Insert Fig.1 here ***
The effects of parameters such as Si/Al ratio, Na/Al ratio, W/S ratio, and temperature on the degree of reaction (α) were also carried out. The data obtained from the degree of reaction (α) was analyzed using response index principle based on signal to noise (S/N) ratio. The effect of different levels of the synthesis parameters on the degree of reaction (α) was determined by averaging the degree of reaction (α) of the geopolymer samples which contain that level of the parameter to be analyzed. 2.2.3.1 JMAK model kinetics The JMAK model kinetics parameters such as Avrami exponent (n) and growth rate (k) were determined using degree of reaction (α) and JMAK model. The data obtained from the degree of reaction (α) was used in the JMAK model Equation (3) to extract the kinetics parameters such as n and k. (
(
))
(3)
The values of the Avrami exponent (n) and growth rate (k) were obtained from Avrami plots of ln (-ln (1-α)) vs lnt. The slope of the plot is equal to n value and the value of k was determined from the intercept of the plot. 2.2.4 Characterization of Geopolymers Geopolymer samples cured for 7 days were characterized to determine their bonding behavior, microstructure, and the phases present respectively. Fourier Transform Infrared Spectroscopy (FTIR – Perkin Elmer Spectrum One) was used to determine the bonding characteristics in the geopolymer using the KBr pellet technique, 15 mg of geopolymer sample was mixed with 1 g of KBr. Approximately ∼0.25 g of the ground sample was then pressed with a steel die into a pellet for FTIR analysis. Microstructural analysis of the geopolymer sample was done using the Field Emission Scanning Electron Microscope (FESEM Zeiss Supra55 VP,
Germany). Gopolymer sample was mounted on aluminum stubs using conductive glue and then coated with a thin layer of carbon [28] and analyzed using FESEM. A further 0.20 gram of geopolymer sample was ground to a fine powder to less than ~ 10 μm (or 200 mesh) to analyze the phases present in the geopolymer using X-Ray Diffraction (XRD - D8, Advanced Bruker).
3. Results and Discussion 3.1 Effects of parameters on the degree of reaction (α) Fly ash used in this study contains SiO2 - 43.25 %, Al2O3 - 20.58 %, Fe2O3 - 12.41 %, and CaO - 11.11 % in wt. % (XRF results). As the sum of SiO2, Al2O3, and Fe2O3 is greater than 70%, this fly ash is under class F according to ASTM C618. The reactivity of the glassy phase of fly ash depends on synthesis parameters such as Si/Al ratio, Na/Al ratio, W/S ratio, and temperature. The effect of each synthesis parameter on the degree of reaction (α) is described below. Effect of Si/Al ratio Fig.2 shows the effect of Si/Al ratios on the response index (degree of reaction (α)) at different curing times. The response index increases by increasing the Si/Al ratio from 1.8 to 2.2 (level 1 to level 2) while further increasing this ratio to 2.6 (level 3) the response index decreases. The higher response index was obtained at Si/Al ratio of 2.2. The increase of degree of reaction (α) by increasing the Si/Al ratio from 1.8 to 2.2 is due to enhancement in geopolymerization reaction. The geopolymerization reaction at Si/Al ratio of 1.8 is slower and produces lower amount of geopolymer but increasing this ratio to 2.2 provides more silica in the system which enhances geopolymerization and as a result the degree of reaction (α) increases. The soluble silicate enhances polycondensation by changing the nature of
*** Insert Fig.2 here *** silicate species present in alkaline silicate solution. The monomeric chains and cyclic trimmers are the dominant silicate species in the system containing lower soluble silicates, while in the systems with higher soluble silicate concentrations larger rings, complex structures, and polymers are present resulting in the formation of a three dimensional framework which also increases the mechanical properties of geopolymers [29]. At higher Si/Al ratio of 2.6, the degree of reaction decreases due to rapid setting of the paste. At Si/Al ratios of 2.6, the dissolution of fly ash does not complete and solidification of the paste occurs before completion of major portion of dissolution reaction resulting in the lower contents of silica and alumina available for reaction which decreases the geopolymer formation and degree of reaction (α). The degree of reaction (α) at Si/Al ratio of 2.6 is higher than the degree of reaction (α) at Si/Al ratio of 1.8 in the early 4 hours due to higher silica content which enhances initial stage reaction but after 4 hours the reaction becomes slow due to consumption of most of the available silica and alumina particles in the initial stage reaction. The degree of reaction (α) also increases with the increase of curing time due to increase of geopolymer formation. The increase of curing time provides more time to the reaction which produces higher amounts of geopolymer resulting in the increase of degree of reaction (α). Effect of Na/Al ratio Fig.3 shows the effect of Na/Al ratios on the response index (degree of reaction (α)). By increasing the Na/Al ratio from 1 to 1.4 (level 1 to level 3) the response index decreases. The higher response index was obtained at Na/Al ratio of 1.0 (level 1). The Na/Al ratio of 1.0 causes higher dissolution of Si4+ and Al3+ ions from fly ash due to higher content of NaOH and increases the formation of sodium aluminosilicate gel leading to
*** Insert Fig.3 here *** higher geopolymer formation and the increase of degree of reaction (α). NaOH is composed of Na+ cations and OH- anions. During geopolymerization reaction, the Si-O-T bonds in the fly ash are attacked by OH- producing Si-O bonds [30]. After the rupture of first bond, these silanol groups deprotonate and may be charge balanced by an alkali cation. Aluminate dissolution also follows the same mechanism but they are not often deprotonated once released into solution. The optimum Na/Al ratio for geopolymer synthesis is 1, this amount is enough to charge stabilize all the Al3+ available in the tetrahedral coordination in the produced geopolymer [31], and this statement has also been satisfied in this study. Increasing the Na/Al ratio from 1.0 to 1.4 decreases the degree of reaction (α) due to higher pH of activating solution. The higher pH of the solution decreases the dissolution of fly ash resulting in lower geopolymer formation and the degree of reaction (α) decreases. The degree of reaction (α) increases with the increase of curing time due to increase of geopolymer formation with the increase of curing time. Effect of W/S ratio The effect of W/S ratios on the response index (degree of reaction (α)) is shown in Fig.4. The response index increases by increasing the W/S ratio from 0.20 to 0.30 (level 1 to level 3). The higher response index was obtained at W/S ratio of 0.3. Water to solid ratio is very crucial parameter in the synthesis of geopolymers which affects the workability and mechanical properties of geopolymers. Water is involved in the dissolution of aluminosilicate and polycondensation of geopolymers [32]. The difference in the degree of reaction (α) at three levels of W/S ratios is very small but it can be used to explain the effects of different W/S ratios on the degree of reaction (α). By increasing the W/S ratio from 0.20 to 0.30, the degree of reaction (α) increased due to increase in dissolution of fly ash and
*** Insert Fig.4 here *** polycondensation of the geopolymers. The water to solid ratio of 0.30 increases the dissolution of fly ash and results in higher amounts of silica and alumina available for geopolymerization. It also increases the polycondensation of monomers resulting in the increases of geopolymer formation and degree of reaction (α). The degree of reaction (α) also increases with the increase of curing time due to increase of geopolymer formation with the increase of curing time. Effect of Temperature The effect of temperatures on the response index (degree of reaction (α)) is shown in Fig.5. The response index increases by increasing the curing temperature from 40 °C to 60 °C (level 1 to level 2) while further increasing the temperature to 80 °C (level 3) decreases the response index. The higher response index was obtained at a temperature of 60 °C. At the temperature of 40 °C, the geopolymerization reaction is slow resulting in the lower geopolymer formation and degree of reaction (α). Increasing temperature from 40 °C to 60 °C, increases the degree of reaction (α) due to acceleration of geopolymerization reaction. It has been recognized that the total pore volume (V) and surface area (S) increases by increasing the temperature. The increase in the extent of pore volume and surface area depends on the silicate ratio used. The higher surface area will increase the reactivity of fly ash. The higher temperature increases the extent of dissolution of Si and Al from the glassy phase of fly ash, leading to fast nucleation, polymerization, and polycondensation reactions to form a geopolymer material. Smith Songpiriyakij [33] also observed the increase of degree of reaction (α) with the increases of curing temperature (27 °C to 60 °C) using picric acid method on fly ash-biomass based geopolymer. Further increase in temperature from 60 °C to 80 °C decreased the degree of reaction (α) due to evaporation of water. The higher temperature of 80 °C caused faster setting of
*** Insert Fig.5 here *** the paste due to which lower amounts of silica and alumina were leached out from fly ash which decreased the geopolymer formation and degree of reaction (α). The degree of reaction (α) at a temperature of 80 °C is higher than the degree of reaction (α) at 40 °C during the early period due to faster geopolymerization at higher temperature but with the passage of curing time the line due to degree of reaction (α) at 80 °C decreased due to lower amounts of silica and alumina available for further reaction. The degree of reaction (α) also increases with the increase of curing time due to availability of more time for formation of higher quantity of geopolymer material.
3.2
Johnson-Mehl-Avrami-Kolmogorov (JMAK) model kinetic parameters
Johnson-Mehl-Avrami-Kolmogorov (JMAK) model is used to study the geopolymerization kinetics of fly ash based geopolymers in this study. The degree of reaction (α) determined from 1 hour to 7 days was used in the JMAK model (Eq. 3) to determine the kinetic parameters such as Avrami exponent (n) and growth rate (k). According to JMAK model, if a process follows the JMAK model it should yield a straight line. The plots of ln (-ln (1-α) versus lnt are straight lines as shown in Fig.6 (a, b, c) which shows that geopolymerization follows the JMAK model. The slope of the straight line is equal to the value of n while its intercept equals to lnk. The values of n ranges from 0.0931 to 0.2321 while the values of k ranges from 0.366 to 0.671 as shown in Table 3. The value of n is different for each sample due to different compositions of the samples but it always remains below 0.5 which shows that geopolymerization mechanism is same for all geopolymer samples. JMAK model describes the phase transformation processes possessing values of n in the range of 0.5 as the diffusion controlled reactions. The Avrami exponent (n) is
composed of p, s, and q as shown in Equation (4). p is growth dimensionality, s is the type of rate control (s = 1 for phase boundary, s = 2 for diffusion), and q is the type of nucleation (q = 0 for site saturation, q = 1 for continuous nucleation). n = p/s + q
(4)
In this case the values of p, s, and q are 1, 2, and 0. p =1 stands for one dimensional, s = 2 stands for diffusion reaction, and q = 0 stands for site saturation. If the nucleation at the start of the reaction is rapid then quickly stops due to exhaustion of the available nucleation sites or the reduction of driving force due to depletion of the species needed for nucleation, then it can be approximated that all the nuclei were present at the start of the reaction, this condition is the site saturation condition. Thus according to JMAK model, the geopolymerization of fly ash based geopolymers can be described as one dimensional diffusion controlled reaction and it follows the mechanism of thickening of large product layers in the form of plates. The low values of n describe that each individual nuclei does not grow with constant radial growth rate as assumed in the second assumption of JMAK model. The low values of n can be caused by the decrease of radial nuclei growth rate due to hindering of previously formed nuclei and a reduction in molecular mobility [20]. It can also be due to compaction of particles which reduce the mobility of particles and geopolymerization slows. Fast transformation processes from one phase to another develop higher nucleation rates and lower the free energies of nucleation which result in the improper arrangement of molecules. The one dimensional diffusion controlled processes with low values of n (<1) were also observed in other transformations [34-36]. Our results are up to some extent consistent with Chen et al [9] research even though the model and synthesis parameters are different and they only determined the dissolution kinetics.
*** Insert Fig.6 here *** *** Insert Table.3 here *** The higher growth (k) rate was obtained at a temperature of 80 °C. The increase of growth rate with the increase of temperature shows enhancement in the nucleation process. At lower temperatures of 40 °C and 60 °C, the geopolymerization is slow but as temperature is increased to 80 °C, the geopolymerization reaction accelerates which causes faster dissolution of fly ash, polymerization, and condensation reactions resulting in increased production of geopolymers and ultimately the growth rate (k) increases. Increasing temperature also decreases nucleation barrier so nucleation rate increases which results in the production of more nuclei and ultimately geopolymer growth increases. The mechanism of geopolymerization consists of initial dissolution which is a first order chemical reaction and the further reactions including dissolution, gelation, and polycondensation are the diffusion controlled reactions. The initial dissolution is a very fast reaction which was not observed in this study. Chen et al [9] observed the initial dissolution reaction and it was only observed at 22 °C. The increase of temperature accelerates the initial dissolution reaction and the solution quickly saturates with silicon and aluminum species which take part in the geopolymer formation and the initial dissolution cannot be detected. In our study the minimum curing temperature is 40 °C this is the reason due to which we could not observe a separate slope for the initial dissolution of fly ash. After the initial dissolution of fly ash further reactions including dissolution, gelation, and polycondensation are the diffusion controlled reactions. After initial dissolution the viscosity of the paste increases and the reaction takes place by the diffusion of reactants and the geopolymer grows in the form of plate like structures.
3.3 Characterization of Geopolymers Fourier Transform Infrared Spectroscopy (FTIR) Analysis FTIR spectrum provides information regarding different types of chemical bonds in the materials on molecular level. The differences in the FTIR spectra of fly ash and geopolymers show some insights into the reactions occurring during geopolymerization. FTIR spectra of fly ash and geopolymer samples are shown in Fig.7. Fly ash shows a peak at 776 cm-1 due to quartz. The band centered at 1095 cm-1 is due to asymmetric stretching vibrations of Si-O-T bonds (Ttetrahedral Al or Si). Two small bands at 1632 cm-1 and 3445 cm-1 corresponds to bending vibration of H-OH bonds and stretching vibration of –OH bonds. The peak at 472 cm-1 is due to symmetric bending vibrations of Si-O-Si and Al-O-Al in the fly ash. The peaks detected in the fly ash and geopolymer samples are shown in Table 4 and Table 5. Fig.7 shows that the peak due to asymmetric stretching vibration mode of Si-O-T (Ttetrahedral Al or Si, containing multiple overlapping components which sum up to give a broad peak) at 1095 cm-1 increases in intensity, narrows, and moves to lower wavenumbers. This peak shift is due to the formation of geopolymers by geopolymerization of aluminosilicate source material. A shift in the main band shows that some changes are occurring in the angle and length of the Si-O-Si bonds in the gel [37] and it provides the extent of reaction or the formation of geopolymers. The movement of the main band gives clue about the formation of geopolymers and the geopolymerization kinetics also depends on the movement of the main band. Faster the movements of the main band faster the formation of geopolymers and the geopolymerization kinetics. Although this peak shift has also been observed in the formation of zeolites from disordered aluminosilicate precursors but some types of zeolites also form during geopolymerization in addition to geopolymers. Fig. 7 shows that the Si-O-T peak does not move
*** Insert Table.4 here *** *** Insert Table.5 here *** *** Insert Fig.7 here *** again to higher wavenumbers which indicates that the geopolymerization reaction is still continuous and when it will complete, the Si-O-T band will slightly move towards higher wavenumbers. Fig.7 (a) shows the main band at Si/Al ratio of 2.2 is more intense as compared to the band at Si/Al ratio of 2.6 and it is consistent with the degree of reaction (α) results. The temperature has higher effect while Si/Al ratio has lower effect on the movement of the main band. The increase of temperature increases the rate of reaction by accelerating the geopolymerization reaction. Increasing temperature from 40 °C to 80 °C also increased the growth rate (k) due to faster nucleation and growth. With the increase of Si/Al ratio, the shift in the main band from 1095cm-1 to 1032 cm-1 is small as compared to the shift at other parameters which indicates the lower geopolymer formation. The increase in the intensity of band at 3445 cm-1 with the increase of parameters shows the formation of activated products with more surface hydroxyl groups hydrogen bonded to adsorbed water. The disappearance of 776 cm-1 band which is due to the vibrations of AlO4, shows the reorganization of geopolymeric gel. Microstructural Analysis Field Emission Scanning Electron Microscope (FESEM) images provide information regarding the morphology as well as the surface texture of individual particles. The microstructures of 7 days cured geopolymer samples are shown in Fig.8. Geopolymer, unreacted fly ash, and pores can be seen in Fig. 8. FAGP-5 shows that the whole surface of fly ash is covered with the geopolymer material which is due to higher amount of geopolymer. FAGP-5
produces higher amount of geopolymer due to higher degree of reaction (α), (68%). The presence of unreacted fly ash particles indicates incomplete geopolymerization. The unreacted fly ash is the refractory part of fly ash which remains unreacted during geopolymerization due to its crystalline nature. It is composed of quartz, mullite, and ferrite. The unreacted fly ash particles are strongly bonded with the geopolymer matrix as shown in Fig.8. No any particular method is available for determining the quantity of unreacted fly ash in the geopolymer using FESEM. Pores are also visible in some of the geopolymer samples (Fig.8 (FAGP-1, FAGP-2, and FAGP-7)) which were produced during polycondensation and curing process. The curing at higher temperature causes fast setting and arrangement of the paste in a heterogeneous fashion due to which pores form in the geopolymer material. The presence of pores decreases the strength of geopolymer. Different types of needle like crystalline structures can also be seen in Fig.8 (FAGP-6 and FAGP-8) which may be some types of zeolite materials. The crystalline materials in FAGP-6 and FAGP-8 are produced due to higher curing temperature and higher silica content. It can be observed that with the increase of Si/Al ratio from 1.8 (FAGP-1, FAGP2, and FAGP-3) to 2.2 (FAGP-4, FAGP-5, and FAGP-6) and then to 2.6 (FAGP-7, FAGP-8, and FAGP-9), the geopolymer microstructure becomes more and more compact. This behavior is due to increase of silica content which compacts the paste and decreases the degree of reaction (α) as shown in Fig.2. A homogeneous and compact geopolymer microstructure can be seen in Fig.8 (FAGP-4). This micrograph shows that the hardened geopolymer is made up of plates. This result is consistent with the JMAK model results which state that the geopolymer growth follows the mechanism of thickening of large product layers. The plate type morphology is not observed in
*** Insert Fig.8 here *** other samples may be due to very small size of the plates that FESEM cannot detect. These micrographs are consistent with other micrographs reported in literature [38, 39]. All geopolymer samples show different types of microstructure which indicates the heterogeneous nature of produced geopolymers and it is consistent with past researchers result [40]. Phase Analysis Phase analysis of geopolymer samples was carried out to determine the phases present in the geopolymer after 7 days of curing. Phase analysis results of fly ash and geopolymers are shown in Fig.9 (a and b). The final geopolymer gel structure depends on the composition of starting materials, dissolution behavior of raw materials, and reaction conditions. The broad halo hump from 20° 2 θ to 35° 2 θ observed in fly ash is the characteristic of amorphous nature of fly ash. Few broad peaks observed at 20° 2 θ, 27° 2 θ, and 33° 2 θ, 42° 2 θ, and 37° 2 θ in the spectrum of fly ash are due to crystalline phases such as quartz, mullite, and ferrite present in fly ash as shown in Fig.9(a). The XRD spectra of geopolymers shows a broad diffuse halo between 25° 2θ to 40° 2θ rather than sharp diffraction peaks as shown in Fig.9(b), which is the characteristic of amorphous aluminosilicate geopolymers. The shift of broad halo from 20° 2θ - 35° 2θ in the fly ash to 25° 2θ - 40° 2θ in the geopolymer is due to the formation of aluminosilicate geopolymers. The XRay spectra of geopolymers shows few peaks at 14° 2θ, 25° 2θ, and 27° 2θ, 33° 2θ and 43° 2θ, and 38° 2θ which are due to quartz, mullite, and ferrite already present in the fly ash in crystalline form. Phase analysis shows that higher portion of geopolymers is in amorphous form which is consistent with the previous research results [31]. It is very difficult to see the exact proportions
*** Insert Fig.9 here *** of the amorphous phases in case of material containing multiple X-ray amorphous phases; this is the acknowledged shortcoming of this characterization technique. The smaller peaks due to quartz, mullite, and ferrite are not visible in some of the geopolymer samples. This may be due to slight dissolution of crystalline compounds present in fly ash during geopolymerization to form X-ray amorphous geopolymer phases which will later transform to zeolite type crystalline compounds [41]. By increasing the temperature from 40 °C (FAGP-1, FAGP-5, and FAGP-9) to 80 °C (FAGP-3, FAGP-4, and FAGP-8), few new peaks appear in the XRD spectra. These peaks show the formation of crystalline material in geopolymer. The higher temperature enhances the condensation of oligomers to form crystalline material. Higher temperature enhances geopolymerization by increasing the number of Al and Si dissolved from the fly ash which will produce more oligomers to participate in geopolymerization. The higher temperature also increased the degree of reaction (α) and growth rate (k). During microstructural analysis, the plate like morphology was detected in some samples but phase analysis does not show any peaks due to crystalline material may be due to smaller size of the produced crystals and plates which XRD cannot detect or due to interference of the crystalline material with the fly ash impurities [9]. Phase analysis results does not show much difference between the XRD spectra of geopolymer samples synthesized using different compositions. Phase analysis results shows that geopolymerization reaction does not change the amorphous nature of fly ash.
4. Conclusions The JMAK model is suitable for determining the geopolymerization kinetics of fly ash based
geopolymers. The Si/Al ratio of 2.2, Na/Al ratio of 1.0, W/S ratio of 0.30, and temperature of 60 °C produced higher degree of reaction (α). According to JMAK model, the geopolymerization of fly ash based geopolymers is one dimensional diffusion controlled reaction and the growth follows the mechanism of thickening of large product layers. The mechanism of geopolymerization consists of initial dissolution which is a first order chemical reaction and the further reactions including dissolution, gelation, and polycondensation are diffusion controlled reactions. The shifting of the main Si-O-T band to lower wavenumbers indicated the formation of geopolymers. The effect of temperature was higher while the Si/Al ratio was lower on the shifting of the main band. The microstructures of geopolymers showed plate like morphology in some samples as described by JMAK model. The phase analysis showed only few peaks due to crystalline material already present in the fly ash in crystalline form while the higher portion of geopolymer was in amorphous form.
Acknowledgements Authors are very thankful to Universiti Teknologi PETRONAS, Malaysia for financial support and research facilities and the Ministry of Higher Education (MOHE), Malaysia for financial support under Fundamental Research Grant Scheme (FRGS) 015-3AB-I67.
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Figure 1: Degree of reaction of fly ash (28 days)
Figure 2: Effects of Si/Al ratios on the degree of reaction (α)
Figure 3: Effects of Na/Al ratios on the degree of reaction (α)
Figure 4: Effects of W/S ratios on the degree of reaction (α)
Figure 5: Effects of temperatures on the degree of reaction (α)
Figure 6: Avrami plots of geopolymer samples at (a) 40 °C, (b) 60 °C, (c) 80 °C
Figure 7: FTIR spectra of geopolymer samples after 7 days of curing at different (a) Si/Al, (b) Na/Al, (c) W/S ratios, and (d) temperature.
Figure 8: FESEM images of geopolymer samples
Figure 9: XRD spectra of, (a) fly ash, and (b) geopolymer samples
Table.1: Factors and levels
Level 1
Level 2
Level 3
Na/Al ratio
1
1.2
1.4
Si/Al ratio
1.8
2.2
2.6
W/S ratio
0.20
0.25
0.30
Curing Temperature
40 °C
60 °C
80 °C
Factors
Table.2. Suggested experimental series by Taguchi method Trail Number
Sample Name
Si/Al
Na/Al
W/S
Temperature ° C
T1
FAGP-1
1.8
1
0.20
40
T2
FAGP-2
1.8
1.2
0.25
60
T3
FAGP-3
1.8
1.4
0.30
80
T4
FAGP-4
2.2
1
0.25
80
T5
FAGP-5
2.2
1.2
0.30
40
T6
FAGP-6
2.2
1.4
0.20
60
T7
FAGP-7
2.6
1
0.30
60
T8
FAGP-8
2.6
1.2
0.20
80
T9
FAGP-9
2.6
1.4
0.25
40
Table 3: The values of JMAK model parameters n and k
Sample Name
n
k
(Avrami exponent)
(Avrami growth rate)
FAGP-1
0.1855
0.366
FAGP-2
0.0931
0.502
FAGP-3
0.1009
0.483
FAGP-4
0.1092
0.671
FAGP-5
0.1371
0.385
FAGP-6
0.2321
0.373
FAGP-7
0.1623
0.467
FAGP-8
0.1632
0.482
FAGP-9
0.1481
0.382
Table 4 Bands in fly ash Band
cm-1
Attribution
1
776
Quartz band
2
683
Quartz band
3
1095
Asymmetric stretching band of Si-O-T
4
1636
Bending vibration of H-OH bonds
5
3445
Stretching vibration of –OH bonds
Table 5: Bands in the alkali activated fly ash Band
cm-1
Attribution
1
462
Asymmetric stretching vibrations of SiO-Si and Al-O-Al
2
683
Quartz band
3
992
Asymmetric stretching band of Si-O-T
4
1636
Bending vibration of H-OH bonds
5
3409
Stretching vibration of –OH bonds
PII: DOI: Reference:
S0272-8842(16)31060-4 http://dx.doi.org/10.1016/j.ceramint.2016.07.006 CERI13231
To appear in: Ceramics International Received date: 5 February 2016 Revised date: 1 July 2016 Accepted date: 1 July 2016 Cite this article as: Ahmer Ali Siyal, Khairun Azizi Azizli, Zakaria Man, Lukman Ismail and Muhammad Irfan Khan, Geopolymerization Kinetics of Fly Ash Based Geopolymers Using JMAK Model, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.07.006 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Geopolymerization Kinetics of Fly Ash Based Geopolymers Using JMAK Model Ahmer Ali Siyal1, Khairun Azizi Azizli1, Zakaria Man1, Lukman Ismail2, Muhammad Irfan Khan1 1
Department of Chemical Engineering, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, 32610 Tronoh, Perak, Malaysia 2
Faculty of Agro-Based Industry, Universiti Malaysia Kelantan 17600 Jeli, Kelantan.
[email protected] [email protected] [email protected] [email protected] [email protected]
Abstract Geopolymers are versatile materials possessing excellent mechanical properties and resistance against aggressive environments, these materials present a benefit of improving simultaneously
both the environmental and engineering performance as compared to classical conventional materials. This paper determines the geopolymerization kinetics of fly ash based geopolymers using Johnson-Mehl-Avrami-Kolmogorov (JMAK) model. The experiments were designed using Taguchi method by varying four factors (Si/Al ratio, Na/Al ratio, W/S ratio, and curing temperature). The degree of reaction of fly ash (α) was used as a measure of the changes occurring during geopolymerization reaction. The characterization of the cured geopolymers was also carried out. The values of n were in the range of 0.0931 to 0.2321 while the values of k were in the range of 0.366 to 0.671. According to the JMAK model results, geopolymerization of fly ash based geopolymers is a one dimensional diffusion controlled reaction and its growth follows the mechanism of thickening of large product layers. The mechanism of geopolymerization consists of initial dissolution which is a first order chemical reaction, and further reactions including dissolution, gelation, and polycondensation are the diffusion controlled reactions. The asymmetric stretching band of Si-O-T shifted to 992 cm-1 and increased in intensity indicating the formation of geopolymer. Microstructural analysis showed the heterogeneous nature of geopolymers consisting of geopolymer, unreacted fly ash, and different types of needle like structures while one sample showed plate like morphology consistent with the JMAK model results. The geopolymer was found to be an amorphous material with only few peaks due to unreacted crystalline fly ash.
Keywords: Fly Ash, Geopolymer, Taguchi model, Kinetics, JMAK model.
1. Introduction
Geopolymers are inorganic polymer materials possessing properties superior to conventional materials as well as possessing capability of use in a variety of applications such as adhesives, coatings, refractory applications, heavy metal adsorption and fast setting applications. Along with all these applications geopolymers can also be used for the synthesis of ceramics and hydroceramics. Geopolymers are produced by the geopolymerization of aluminosilicate source materials in the presence of alkali and/or alkali silicate solutions. Geopolymerization is a multistep process involving dissolution, reorganization, and condensation reactions occurring concurrently. The kinetic study is very important for understanding of the reactions taking place and the mechanisms involved during transformation of reactants into products and it enables us to tailor the properties of the final product according to the type of application [1]. Various approaches have been employed to study the geopolymerization kinetics. Two in-situ kinetic studies to determine the growth and kinetics of metakaolin based geopolymers with the changes in the morphology and phases of geopolymers using Environmental Scanning Electron Microscopy (ESEM) [2, 3] and Energy Dispersive X-Ray Diffractrometry (EDXRD) [4] have shown to be effective only during the initial setting period. Use of Attenuated Total Reflectance - Fourier Transform Infrared Spectroscopy (ATR-FTIR) to determine the geopolymer gel formation through the changes in the bonding behavior with time cannot be helpful if excess silica is present in the alkaline solution. The technique used to determine the extent of reaction of metakaolin and slag based geopolymers by the changes in the diffractograms of geopolymers with time using X-Ray Pair Distribution Function Analysis [5] becomes difficult when crystals start to appear. Isothermal Conduction Calorimetry (ICC) [6, 7] used to determine the geopolymerization kinetics of metakaolin based geopolymers by the changes in the heat of
reaction with time cannot be used for fly ash based geopolymers without the availability of thermochemical parameters required for data quantification. Phenomenological kinetics models such as the Exponential and Knudson linear dispersion models [8] and the Modified Jandar model [9] have also been used for the study of geopolymerization. However, these models could not be used to describe the overall geopolymerisation process. Various kinetics models have also been proposed for the mechanism of geopolymerization. Glukhovsky [10] described geopolymer formation as a combination of destruction of raw materials and condensation of the formed products. In the study by Xu et al [11], geopolymerization was shown to consist of dissolution, polymerization, and condensation reactions, whilst
Rees et al [12] described the gelation
mechanism for fly ash based geopolymers consisting of dissolution, polymerization, and nucleation and growth. Despite several studies on the geopolymerization kinetics and mechanisms of geopolymerization of fly ash based geopolymers, a complete understanding of the process of geopolymer phase transformation from paste to solid and the mechanisms involved during the transformation has not been well developed. Johnson-Mehl-Avrami–Kolmogorov (JMAK) model is a phenomenological model and it has been used to describe the kinetics of various processes such as crystallization, polymerization, depositions, hydration kinetics of Portland cement and so on [13-16]. This model can be used for any system which transforms by the process of nucleation and growth [17]. The geopolymerization involves polymerization reaction and it has also been observed that geopolymer grows by the process of nucleation and growth [12, 18], so this model may be helpful for the study of geopolymerization kinetics of fly ash based geopolymers. It is novel to use this model for geopolymerization kinetics as it has not been used yet. JMAK model is an isothermal kinetic model having two parameters; Avrami exponent
(n) and growth rate (k). Avrami exponent (n) describes the mechanism occurring during transformation from paste to a solid and growth rate (k) is related to the nucleation and growth rates and it is temperature dependent. According to JMAK model, the degree of phase conversion (α (t)) is given below, ( )
(1)
where α is the degree of reaction, n is the Avrami exponent, k is the Avrami constant, and t is the time. The values of n are in the range of 1- 4, while fractional values of n also exist due to secondary crystallization and lower n values (<1) indicate linear nuclei growth [19, 20]. This paper determines the geopolymerization kinetics of fly ash based geopolymers using JMAK model. Taguchi design expert was used for designing of experiments. The changes occurring during geopolymerization with time were determined in terms of degree of reaction of fly ash (α) using HCl method. The effects of parameters such as Si/Al ratio, Na/Al ratio, W/S ratio, and temperature on the degree of reaction (α) were also determined. The response index based on signal to noise ratio (S/N) ratio principle [21] was used for analysis of the data. The degree of reaction (α) data was used in JMAK model to determine the kinetics parameters. The bonding, microstructural, and phase analyses of the fully cured samples were determined using Fourier transform infrared spectroscopy (FTIR), Field emission scanning electron microscopy (FESEM), and X-Ray diffraction (XRD) respectively.
2. Experimental Program 2.1 Materials Fly ash (from local thermal power plant) was used for the synthesis of geopolymers. AR
grade NaOH pellets and sodium silicate (SiO2 - 37.79 %, Na2O - 16.36 %) purchased from R & M Chemicals, Malaysia were used for synthesis of geopolymers. AR grade HCl and Na2CO3 purchased from R & M Chemicals, Malaysia were used for determining the degree of reaction of fly ash (α). 2.2 2.2.1
Methods Determination of design of mixtures
Taguchi experimental design method was used for designing of experiments. The application of Taguchi model to geopolymers is limited to only a few studies [22, 23]. Four parameters such as silica to alumina (Si/Al) ratio, sodium to alumina (Na/Al) ratio, water to solid (W/S) ratio, and temperature were selected for this study. The parameters and their levels are shown in Table.1. The ranges and levels of the parameters were selected in a way to represent the best possible compositions based on the presented compositions in the literature [24-26]. We used one of the orthogonal arrays such as L9 (43) developed by Taguchi to represent all the factor or parameter levels. Using Taguchi design expert software a total of 9 trials or samples were obtained. Chemical compositions of the samples are shown in Table.2. *** Insert Table.1 here *** *** Insert Table.2 here *** 2.2.2 Geopolymer Synthesis Alkaline solution was prepared by dissolving sodium hydroxide pellets in distilled water. The solution was gently mixed until the pellets completely dissolved. The solution was left for 2 hours at room temperature to cool to normal temperature. The geopolymer samples were prepared following the design of mixtures suggested by Taguchi method (Table 2). The alkaline solution was poured onto the fly ash and both were mixed for 8 minutes using a hand mixer. The
geopolymer paste was poured into the mold and put in an electric oven at the desired temperature for curing. 2.2.3 Geopolymerization Kinetics Geopolymerization kinetics of fly ash based geopolymers was determined using JMAK model. The degree of reaction (α) was selected as a parameter to detect the changes occurring during geopolymerization. JMAK kinetics parameters such as Avrami exponent (n) and growth rate (k) were determined using the degree of reaction of fly ash (α). Determination of degree of reaction The degree of reaction of fly ash (α) was determined using hydrochloric acid (HCl) method. As geopolymer samples contain some quantity of unreacted sodium hydroxide which can react with HCl so before determining the degree of reaction (α), the geopolymer samples were neutralized to remove the unreacted sodium hydroxide. Phenolphthalein indicator and 0.2 M HCl solution were used for neutralization of samples. Five grams of geopolymer sample was dissolved in distilled water with a few drops of phenolphthalein indicator added to the mixture. Then 0.2 M HCl was added drop by drop until the color of solution disappeared. The neutralized sample was filtered and placed in the oven for 2 hours at 110 ˚C. One gram of the dried and ground neutralized geopolymer sample and 30 ml of 2 M HCl were placed in plastic centrifuge tube and heated to 60 ˚C for 15 minutes in a water bath. Then it was centrifuged for 5 minutes at 7000 r.p.m. The liquid was then removed and hot water was added and centrifuged again. After repeating the washing process with hot water for three times, the tube was then filled with 30 ml of 5 % Na2CO3 solution, stirred and placed in water bath and heated to 80 ˚C for 20 minutes, and then centrifuged for 5 minutes at 7000 r.p.m. The liquid was
decanted and the residue was washed with hot water three times following the same procedure described earlier. The tube with residue was dried at 110 ˚C for 2 hours, and weighed [27]. The residue remaining in the centrifuge tube was the unreacted fly ash. The degree of reaction (α) of the sample was determined in triplets and an average value of three readings was taken as the degree of reaction (α) of a sample. The formula used for determining the degree of reaction is given below, ( )
(
)
(2)
Mc is the initial weight of powder sample, g; Mr is the final weight of the residue, g Determination of curing time The curing time for kinetics study was also determined by selecting three samples (FAGP-1, FAGP-4, and FAGP-7) from Table 2. These samples were selected due to difference of compositions. The degree of reaction (α) of these samples was determined after 1 hour, 2 hours, 3 hours, 4 hours, 5 hours, 24 hours (1 day), 72 hours (3 days), 168 hours (7 days), 504 hours (21 days), and 672 hours (28 days). Fig.1 shows the results of the degree of reaction (α) of three selected samples. It can be observed that the degree of reaction (α) becomes stable after 168 hours (7 days) of reaction and during the period from 168 to 672 hours (7 to 28 days), it follows a straight line path and shows very small changes so it is better to take 168 hours (7 days) of reaction time for geopolymerization kinetics study. The curing time of 168 hours (7 days) was set as the benchmark for this study and further study was carried out for early stage curing period of 7 days. The degree of reaction (α) for all samples was determined after 1 hour, 2 hours, 3 hours, 4 hours, 5 hours, 24 hours, 72 hours, and 168 hours. *** Insert Fig.1 here ***
The effects of parameters such as Si/Al ratio, Na/Al ratio, W/S ratio, and temperature on the degree of reaction (α) were also carried out. The data obtained from the degree of reaction (α) was analyzed using response index principle based on signal to noise (S/N) ratio. The effect of different levels of the synthesis parameters on the degree of reaction (α) was determined by averaging the degree of reaction (α) of the geopolymer samples which contain that level of the parameter to be analyzed. 2.2.3.1 JMAK model kinetics The JMAK model kinetics parameters such as Avrami exponent (n) and growth rate (k) were determined using degree of reaction (α) and JMAK model. The data obtained from the degree of reaction (α) was used in the JMAK model Equation (3) to extract the kinetics parameters such as n and k. (
(
))
(3)
The values of the Avrami exponent (n) and growth rate (k) were obtained from Avrami plots of ln (-ln (1-α)) vs lnt. The slope of the plot is equal to n value and the value of k was determined from the intercept of the plot. 2.2.4 Characterization of Geopolymers Geopolymer samples cured for 7 days were characterized to determine their bonding behavior, microstructure, and the phases present respectively. Fourier Transform Infrared Spectroscopy (FTIR – Perkin Elmer Spectrum One) was used to determine the bonding characteristics in the geopolymer using the KBr pellet technique, 15 mg of geopolymer sample was mixed with 1 g of KBr. Approximately ∼0.25 g of the ground sample was then pressed with a steel die into a pellet for FTIR analysis. Microstructural analysis of the geopolymer sample was done using the Field Emission Scanning Electron Microscope (FESEM Zeiss Supra55 VP,
Germany). Gopolymer sample was mounted on aluminum stubs using conductive glue and then coated with a thin layer of carbon [28] and analyzed using FESEM. A further 0.20 gram of geopolymer sample was ground to a fine powder to less than ~ 10 μm (or 200 mesh) to analyze the phases present in the geopolymer using X-Ray Diffraction (XRD - D8, Advanced Bruker).
3. Results and Discussion 3.1 Effects of parameters on the degree of reaction (α) Fly ash used in this study contains SiO2 - 43.25 %, Al2O3 - 20.58 %, Fe2O3 - 12.41 %, and CaO - 11.11 % in wt. % (XRF results). As the sum of SiO2, Al2O3, and Fe2O3 is greater than 70%, this fly ash is under class F according to ASTM C618. The reactivity of the glassy phase of fly ash depends on synthesis parameters such as Si/Al ratio, Na/Al ratio, W/S ratio, and temperature. The effect of each synthesis parameter on the degree of reaction (α) is described below. Effect of Si/Al ratio Fig.2 shows the effect of Si/Al ratios on the response index (degree of reaction (α)) at different curing times. The response index increases by increasing the Si/Al ratio from 1.8 to 2.2 (level 1 to level 2) while further increasing this ratio to 2.6 (level 3) the response index decreases. The higher response index was obtained at Si/Al ratio of 2.2. The increase of degree of reaction (α) by increasing the Si/Al ratio from 1.8 to 2.2 is due to enhancement in geopolymerization reaction. The geopolymerization reaction at Si/Al ratio of 1.8 is slower and produces lower amount of geopolymer but increasing this ratio to 2.2 provides more silica in the system which enhances geopolymerization and as a result the degree of reaction (α) increases. The soluble silicate enhances polycondensation by changing the nature of
*** Insert Fig.2 here *** silicate species present in alkaline silicate solution. The monomeric chains and cyclic trimmers are the dominant silicate species in the system containing lower soluble silicates, while in the systems with higher soluble silicate concentrations larger rings, complex structures, and polymers are present resulting in the formation of a three dimensional framework which also increases the mechanical properties of geopolymers [29]. At higher Si/Al ratio of 2.6, the degree of reaction decreases due to rapid setting of the paste. At Si/Al ratios of 2.6, the dissolution of fly ash does not complete and solidification of the paste occurs before completion of major portion of dissolution reaction resulting in the lower contents of silica and alumina available for reaction which decreases the geopolymer formation and degree of reaction (α). The degree of reaction (α) at Si/Al ratio of 2.6 is higher than the degree of reaction (α) at Si/Al ratio of 1.8 in the early 4 hours due to higher silica content which enhances initial stage reaction but after 4 hours the reaction becomes slow due to consumption of most of the available silica and alumina particles in the initial stage reaction. The degree of reaction (α) also increases with the increase of curing time due to increase of geopolymer formation. The increase of curing time provides more time to the reaction which produces higher amounts of geopolymer resulting in the increase of degree of reaction (α). Effect of Na/Al ratio Fig.3 shows the effect of Na/Al ratios on the response index (degree of reaction (α)). By increasing the Na/Al ratio from 1 to 1.4 (level 1 to level 3) the response index decreases. The higher response index was obtained at Na/Al ratio of 1.0 (level 1). The Na/Al ratio of 1.0 causes higher dissolution of Si4+ and Al3+ ions from fly ash due to higher content of NaOH and increases the formation of sodium aluminosilicate gel leading to
*** Insert Fig.3 here *** higher geopolymer formation and the increase of degree of reaction (α). NaOH is composed of Na+ cations and OH- anions. During geopolymerization reaction, the Si-O-T bonds in the fly ash are attacked by OH- producing Si-O bonds [30]. After the rupture of first bond, these silanol groups deprotonate and may be charge balanced by an alkali cation. Aluminate dissolution also follows the same mechanism but they are not often deprotonated once released into solution. The optimum Na/Al ratio for geopolymer synthesis is 1, this amount is enough to charge stabilize all the Al3+ available in the tetrahedral coordination in the produced geopolymer [31], and this statement has also been satisfied in this study. Increasing the Na/Al ratio from 1.0 to 1.4 decreases the degree of reaction (α) due to higher pH of activating solution. The higher pH of the solution decreases the dissolution of fly ash resulting in lower geopolymer formation and the degree of reaction (α) decreases. The degree of reaction (α) increases with the increase of curing time due to increase of geopolymer formation with the increase of curing time. Effect of W/S ratio The effect of W/S ratios on the response index (degree of reaction (α)) is shown in Fig.4. The response index increases by increasing the W/S ratio from 0.20 to 0.30 (level 1 to level 3). The higher response index was obtained at W/S ratio of 0.3. Water to solid ratio is very crucial parameter in the synthesis of geopolymers which affects the workability and mechanical properties of geopolymers. Water is involved in the dissolution of aluminosilicate and polycondensation of geopolymers [32]. The difference in the degree of reaction (α) at three levels of W/S ratios is very small but it can be used to explain the effects of different W/S ratios on the degree of reaction (α). By increasing the W/S ratio from 0.20 to 0.30, the degree of reaction (α) increased due to increase in dissolution of fly ash and
*** Insert Fig.4 here *** polycondensation of the geopolymers. The water to solid ratio of 0.30 increases the dissolution of fly ash and results in higher amounts of silica and alumina available for geopolymerization. It also increases the polycondensation of monomers resulting in the increases of geopolymer formation and degree of reaction (α). The degree of reaction (α) also increases with the increase of curing time due to increase of geopolymer formation with the increase of curing time. Effect of Temperature The effect of temperatures on the response index (degree of reaction (α)) is shown in Fig.5. The response index increases by increasing the curing temperature from 40 °C to 60 °C (level 1 to level 2) while further increasing the temperature to 80 °C (level 3) decreases the response index. The higher response index was obtained at a temperature of 60 °C. At the temperature of 40 °C, the geopolymerization reaction is slow resulting in the lower geopolymer formation and degree of reaction (α). Increasing temperature from 40 °C to 60 °C, increases the degree of reaction (α) due to acceleration of geopolymerization reaction. It has been recognized that the total pore volume (V) and surface area (S) increases by increasing the temperature. The increase in the extent of pore volume and surface area depends on the silicate ratio used. The higher surface area will increase the reactivity of fly ash. The higher temperature increases the extent of dissolution of Si and Al from the glassy phase of fly ash, leading to fast nucleation, polymerization, and polycondensation reactions to form a geopolymer material. Smith Songpiriyakij [33] also observed the increase of degree of reaction (α) with the increases of curing temperature (27 °C to 60 °C) using picric acid method on fly ash-biomass based geopolymer. Further increase in temperature from 60 °C to 80 °C decreased the degree of reaction (α) due to evaporation of water. The higher temperature of 80 °C caused faster setting of
*** Insert Fig.5 here *** the paste due to which lower amounts of silica and alumina were leached out from fly ash which decreased the geopolymer formation and degree of reaction (α). The degree of reaction (α) at a temperature of 80 °C is higher than the degree of reaction (α) at 40 °C during the early period due to faster geopolymerization at higher temperature but with the passage of curing time the line due to degree of reaction (α) at 80 °C decreased due to lower amounts of silica and alumina available for further reaction. The degree of reaction (α) also increases with the increase of curing time due to availability of more time for formation of higher quantity of geopolymer material.
3.2
Johnson-Mehl-Avrami-Kolmogorov (JMAK) model kinetic parameters
Johnson-Mehl-Avrami-Kolmogorov (JMAK) model is used to study the geopolymerization kinetics of fly ash based geopolymers in this study. The degree of reaction (α) determined from 1 hour to 7 days was used in the JMAK model (Eq. 3) to determine the kinetic parameters such as Avrami exponent (n) and growth rate (k). According to JMAK model, if a process follows the JMAK model it should yield a straight line. The plots of ln (-ln (1-α) versus lnt are straight lines as shown in Fig.6 (a, b, c) which shows that geopolymerization follows the JMAK model. The slope of the straight line is equal to the value of n while its intercept equals to lnk. The values of n ranges from 0.0931 to 0.2321 while the values of k ranges from 0.366 to 0.671 as shown in Table 3. The value of n is different for each sample due to different compositions of the samples but it always remains below 0.5 which shows that geopolymerization mechanism is same for all geopolymer samples. JMAK model describes the phase transformation processes possessing values of n in the range of 0.5 as the diffusion controlled reactions. The Avrami exponent (n) is
composed of p, s, and q as shown in Equation (4). p is growth dimensionality, s is the type of rate control (s = 1 for phase boundary, s = 2 for diffusion), and q is the type of nucleation (q = 0 for site saturation, q = 1 for continuous nucleation). n = p/s + q
(4)
In this case the values of p, s, and q are 1, 2, and 0. p =1 stands for one dimensional, s = 2 stands for diffusion reaction, and q = 0 stands for site saturation. If the nucleation at the start of the reaction is rapid then quickly stops due to exhaustion of the available nucleation sites or the reduction of driving force due to depletion of the species needed for nucleation, then it can be approximated that all the nuclei were present at the start of the reaction, this condition is the site saturation condition. Thus according to JMAK model, the geopolymerization of fly ash based geopolymers can be described as one dimensional diffusion controlled reaction and it follows the mechanism of thickening of large product layers in the form of plates. The low values of n describe that each individual nuclei does not grow with constant radial growth rate as assumed in the second assumption of JMAK model. The low values of n can be caused by the decrease of radial nuclei growth rate due to hindering of previously formed nuclei and a reduction in molecular mobility [20]. It can also be due to compaction of particles which reduce the mobility of particles and geopolymerization slows. Fast transformation processes from one phase to another develop higher nucleation rates and lower the free energies of nucleation which result in the improper arrangement of molecules. The one dimensional diffusion controlled processes with low values of n (<1) were also observed in other transformations [34-36]. Our results are up to some extent consistent with Chen et al [9] research even though the model and synthesis parameters are different and they only determined the dissolution kinetics.
*** Insert Fig.6 here *** *** Insert Table.3 here *** The higher growth (k) rate was obtained at a temperature of 80 °C. The increase of growth rate with the increase of temperature shows enhancement in the nucleation process. At lower temperatures of 40 °C and 60 °C, the geopolymerization is slow but as temperature is increased to 80 °C, the geopolymerization reaction accelerates which causes faster dissolution of fly ash, polymerization, and condensation reactions resulting in increased production of geopolymers and ultimately the growth rate (k) increases. Increasing temperature also decreases nucleation barrier so nucleation rate increases which results in the production of more nuclei and ultimately geopolymer growth increases. The mechanism of geopolymerization consists of initial dissolution which is a first order chemical reaction and the further reactions including dissolution, gelation, and polycondensation are the diffusion controlled reactions. The initial dissolution is a very fast reaction which was not observed in this study. Chen et al [9] observed the initial dissolution reaction and it was only observed at 22 °C. The increase of temperature accelerates the initial dissolution reaction and the solution quickly saturates with silicon and aluminum species which take part in the geopolymer formation and the initial dissolution cannot be detected. In our study the minimum curing temperature is 40 °C this is the reason due to which we could not observe a separate slope for the initial dissolution of fly ash. After the initial dissolution of fly ash further reactions including dissolution, gelation, and polycondensation are the diffusion controlled reactions. After initial dissolution the viscosity of the paste increases and the reaction takes place by the diffusion of reactants and the geopolymer grows in the form of plate like structures.
3.3 Characterization of Geopolymers Fourier Transform Infrared Spectroscopy (FTIR) Analysis FTIR spectrum provides information regarding different types of chemical bonds in the materials on molecular level. The differences in the FTIR spectra of fly ash and geopolymers show some insights into the reactions occurring during geopolymerization. FTIR spectra of fly ash and geopolymer samples are shown in Fig.7. Fly ash shows a peak at 776 cm-1 due to quartz. The band centered at 1095 cm-1 is due to asymmetric stretching vibrations of Si-O-T bonds (Ttetrahedral Al or Si). Two small bands at 1632 cm-1 and 3445 cm-1 corresponds to bending vibration of H-OH bonds and stretching vibration of –OH bonds. The peak at 472 cm-1 is due to symmetric bending vibrations of Si-O-Si and Al-O-Al in the fly ash. The peaks detected in the fly ash and geopolymer samples are shown in Table 4 and Table 5. Fig.7 shows that the peak due to asymmetric stretching vibration mode of Si-O-T (Ttetrahedral Al or Si, containing multiple overlapping components which sum up to give a broad peak) at 1095 cm-1 increases in intensity, narrows, and moves to lower wavenumbers. This peak shift is due to the formation of geopolymers by geopolymerization of aluminosilicate source material. A shift in the main band shows that some changes are occurring in the angle and length of the Si-O-Si bonds in the gel [37] and it provides the extent of reaction or the formation of geopolymers. The movement of the main band gives clue about the formation of geopolymers and the geopolymerization kinetics also depends on the movement of the main band. Faster the movements of the main band faster the formation of geopolymers and the geopolymerization kinetics. Although this peak shift has also been observed in the formation of zeolites from disordered aluminosilicate precursors but some types of zeolites also form during geopolymerization in addition to geopolymers. Fig. 7 shows that the Si-O-T peak does not move
*** Insert Table.4 here *** *** Insert Table.5 here *** *** Insert Fig.7 here *** again to higher wavenumbers which indicates that the geopolymerization reaction is still continuous and when it will complete, the Si-O-T band will slightly move towards higher wavenumbers. Fig.7 (a) shows the main band at Si/Al ratio of 2.2 is more intense as compared to the band at Si/Al ratio of 2.6 and it is consistent with the degree of reaction (α) results. The temperature has higher effect while Si/Al ratio has lower effect on the movement of the main band. The increase of temperature increases the rate of reaction by accelerating the geopolymerization reaction. Increasing temperature from 40 °C to 80 °C also increased the growth rate (k) due to faster nucleation and growth. With the increase of Si/Al ratio, the shift in the main band from 1095cm-1 to 1032 cm-1 is small as compared to the shift at other parameters which indicates the lower geopolymer formation. The increase in the intensity of band at 3445 cm-1 with the increase of parameters shows the formation of activated products with more surface hydroxyl groups hydrogen bonded to adsorbed water. The disappearance of 776 cm-1 band which is due to the vibrations of AlO4, shows the reorganization of geopolymeric gel. Microstructural Analysis Field Emission Scanning Electron Microscope (FESEM) images provide information regarding the morphology as well as the surface texture of individual particles. The microstructures of 7 days cured geopolymer samples are shown in Fig.8. Geopolymer, unreacted fly ash, and pores can be seen in Fig. 8. FAGP-5 shows that the whole surface of fly ash is covered with the geopolymer material which is due to higher amount of geopolymer. FAGP-5
produces higher amount of geopolymer due to higher degree of reaction (α), (68%). The presence of unreacted fly ash particles indicates incomplete geopolymerization. The unreacted fly ash is the refractory part of fly ash which remains unreacted during geopolymerization due to its crystalline nature. It is composed of quartz, mullite, and ferrite. The unreacted fly ash particles are strongly bonded with the geopolymer matrix as shown in Fig.8. No any particular method is available for determining the quantity of unreacted fly ash in the geopolymer using FESEM. Pores are also visible in some of the geopolymer samples (Fig.8 (FAGP-1, FAGP-2, and FAGP-7)) which were produced during polycondensation and curing process. The curing at higher temperature causes fast setting and arrangement of the paste in a heterogeneous fashion due to which pores form in the geopolymer material. The presence of pores decreases the strength of geopolymer. Different types of needle like crystalline structures can also be seen in Fig.8 (FAGP-6 and FAGP-8) which may be some types of zeolite materials. The crystalline materials in FAGP-6 and FAGP-8 are produced due to higher curing temperature and higher silica content. It can be observed that with the increase of Si/Al ratio from 1.8 (FAGP-1, FAGP2, and FAGP-3) to 2.2 (FAGP-4, FAGP-5, and FAGP-6) and then to 2.6 (FAGP-7, FAGP-8, and FAGP-9), the geopolymer microstructure becomes more and more compact. This behavior is due to increase of silica content which compacts the paste and decreases the degree of reaction (α) as shown in Fig.2. A homogeneous and compact geopolymer microstructure can be seen in Fig.8 (FAGP-4). This micrograph shows that the hardened geopolymer is made up of plates. This result is consistent with the JMAK model results which state that the geopolymer growth follows the mechanism of thickening of large product layers. The plate type morphology is not observed in
*** Insert Fig.8 here *** other samples may be due to very small size of the plates that FESEM cannot detect. These micrographs are consistent with other micrographs reported in literature [38, 39]. All geopolymer samples show different types of microstructure which indicates the heterogeneous nature of produced geopolymers and it is consistent with past researchers result [40]. Phase Analysis Phase analysis of geopolymer samples was carried out to determine the phases present in the geopolymer after 7 days of curing. Phase analysis results of fly ash and geopolymers are shown in Fig.9 (a and b). The final geopolymer gel structure depends on the composition of starting materials, dissolution behavior of raw materials, and reaction conditions. The broad halo hump from 20° 2 θ to 35° 2 θ observed in fly ash is the characteristic of amorphous nature of fly ash. Few broad peaks observed at 20° 2 θ, 27° 2 θ, and 33° 2 θ, 42° 2 θ, and 37° 2 θ in the spectrum of fly ash are due to crystalline phases such as quartz, mullite, and ferrite present in fly ash as shown in Fig.9(a). The XRD spectra of geopolymers shows a broad diffuse halo between 25° 2θ to 40° 2θ rather than sharp diffraction peaks as shown in Fig.9(b), which is the characteristic of amorphous aluminosilicate geopolymers. The shift of broad halo from 20° 2θ - 35° 2θ in the fly ash to 25° 2θ - 40° 2θ in the geopolymer is due to the formation of aluminosilicate geopolymers. The XRay spectra of geopolymers shows few peaks at 14° 2θ, 25° 2θ, and 27° 2θ, 33° 2θ and 43° 2θ, and 38° 2θ which are due to quartz, mullite, and ferrite already present in the fly ash in crystalline form. Phase analysis shows that higher portion of geopolymers is in amorphous form which is consistent with the previous research results [31]. It is very difficult to see the exact proportions
*** Insert Fig.9 here *** of the amorphous phases in case of material containing multiple X-ray amorphous phases; this is the acknowledged shortcoming of this characterization technique. The smaller peaks due to quartz, mullite, and ferrite are not visible in some of the geopolymer samples. This may be due to slight dissolution of crystalline compounds present in fly ash during geopolymerization to form X-ray amorphous geopolymer phases which will later transform to zeolite type crystalline compounds [41]. By increasing the temperature from 40 °C (FAGP-1, FAGP-5, and FAGP-9) to 80 °C (FAGP-3, FAGP-4, and FAGP-8), few new peaks appear in the XRD spectra. These peaks show the formation of crystalline material in geopolymer. The higher temperature enhances the condensation of oligomers to form crystalline material. Higher temperature enhances geopolymerization by increasing the number of Al and Si dissolved from the fly ash which will produce more oligomers to participate in geopolymerization. The higher temperature also increased the degree of reaction (α) and growth rate (k). During microstructural analysis, the plate like morphology was detected in some samples but phase analysis does not show any peaks due to crystalline material may be due to smaller size of the produced crystals and plates which XRD cannot detect or due to interference of the crystalline material with the fly ash impurities [9]. Phase analysis results does not show much difference between the XRD spectra of geopolymer samples synthesized using different compositions. Phase analysis results shows that geopolymerization reaction does not change the amorphous nature of fly ash.
4. Conclusions The JMAK model is suitable for determining the geopolymerization kinetics of fly ash based
geopolymers. The Si/Al ratio of 2.2, Na/Al ratio of 1.0, W/S ratio of 0.30, and temperature of 60 °C produced higher degree of reaction (α). According to JMAK model, the geopolymerization of fly ash based geopolymers is one dimensional diffusion controlled reaction and the growth follows the mechanism of thickening of large product layers. The mechanism of geopolymerization consists of initial dissolution which is a first order chemical reaction and the further reactions including dissolution, gelation, and polycondensation are diffusion controlled reactions. The shifting of the main Si-O-T band to lower wavenumbers indicated the formation of geopolymers. The effect of temperature was higher while the Si/Al ratio was lower on the shifting of the main band. The microstructures of geopolymers showed plate like morphology in some samples as described by JMAK model. The phase analysis showed only few peaks due to crystalline material already present in the fly ash in crystalline form while the higher portion of geopolymer was in amorphous form.
Acknowledgements Authors are very thankful to Universiti Teknologi PETRONAS, Malaysia for financial support and research facilities and the Ministry of Higher Education (MOHE), Malaysia for financial support under Fundamental Research Grant Scheme (FRGS) 015-3AB-I67.
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Figure 1: Degree of reaction of fly ash (28 days)
Figure 2: Effects of Si/Al ratios on the degree of reaction (α)
Figure 3: Effects of Na/Al ratios on the degree of reaction (α)
Figure 4: Effects of W/S ratios on the degree of reaction (α)
Figure 5: Effects of temperatures on the degree of reaction (α)
Figure 6: Avrami plots of geopolymer samples at (a) 40 °C, (b) 60 °C, (c) 80 °C
Figure 7: FTIR spectra of geopolymer samples after 7 days of curing at different (a) Si/Al, (b) Na/Al, (c) W/S ratios, and (d) temperature.
Figure 8: FESEM images of geopolymer samples
Figure 9: XRD spectra of, (a) fly ash, and (b) geopolymer samples
Table.1: Factors and levels
Level 1
Level 2
Level 3
Na/Al ratio
1
1.2
1.4
Si/Al ratio
1.8
2.2
2.6
W/S ratio
0.20
0.25
0.30
Curing Temperature
40 °C
60 °C
80 °C
Factors
Table.2. Suggested experimental series by Taguchi method Trail Number
Sample Name
Si/Al
Na/Al
W/S
Temperature ° C
T1
FAGP-1
1.8
1
0.20
40
T2
FAGP-2
1.8
1.2
0.25
60
T3
FAGP-3
1.8
1.4
0.30
80
T4
FAGP-4
2.2
1
0.25
80
T5
FAGP-5
2.2
1.2
0.30
40
T6
FAGP-6
2.2
1.4
0.20
60
T7
FAGP-7
2.6
1
0.30
60
T8
FAGP-8
2.6
1.2
0.20
80
T9
FAGP-9
2.6
1.4
0.25
40
Table 3: The values of JMAK model parameters n and k
Sample Name
n
k
(Avrami exponent)
(Avrami growth rate)
FAGP-1
0.1855
0.366
FAGP-2
0.0931
0.502
FAGP-3
0.1009
0.483
FAGP-4
0.1092
0.671
FAGP-5
0.1371
0.385
FAGP-6
0.2321
0.373
FAGP-7
0.1623
0.467
FAGP-8
0.1632
0.482
FAGP-9
0.1481
0.382
Table 4 Bands in fly ash Band
cm-1
Attribution
1
776
Quartz band
2
683
Quartz band
3
1095
Asymmetric stretching band of Si-O-T
4
1636
Bending vibration of H-OH bonds
5
3445
Stretching vibration of –OH bonds
Table 5: Bands in the alkali activated fly ash Band
cm-1
Attribution
1
462
Asymmetric stretching vibrations of SiO-Si and Al-O-Al
2
683
Quartz band
3
992
Asymmetric stretching band of Si-O-T
4
1636
Bending vibration of H-OH bonds
5
3409
Stretching vibration of –OH bonds