Characteristics of Fly Ash as a Constituent Material for Geopolymer

Fly ash waste from the Asam-Asam Power Plant in South Kalimantan continues to increase. Various attempts have been made to utilize fly ash waste, among others. as a basic material for making geopolymers. Geopolymer is environmentally friendly because it replaces all cement with high Silica (Si) and Alumina (Al) wastes such as fly ash. This study aims to characterize fly ash as a raw material of geopolymer. Fly ash was filtered using 200 mesh and then heated in an oven at 105°C for 24 hours. The prepared fly ash was then tested for physical properties and compound analysis using XRF, identification of chemical bonds and functional groups using FTIR, morphology, and elements using SEM EDX. Initial and final setting time was investigated using the Vicat apparatus to determine e whether cement is undergoing proper hydration. The results indicated that the fly ash was classified to class F. That was based on the amount of SiO2 + Al2O3 + Fe2O3 of 85.91%, and the CaO content was less than 10%. Moreover, fly ash’s initial and final setting time was 1072.5 and 3180 minutes, respectively. Fly ash in this research can be used as a raw material for geopolymers.


Introduction
Coal is commonly utilized by industry and Power Plants as boiler fuel to generate steam for use as a heating medium or in power plants.About 5% of solid waste is produced from coal combustion through fly ash and bottom ash.Fly ash is a solid waste consisting of silicon dioxide (SiO2), aluminum oxide (Al2O3) and iron oxide (Fe2O3).The factors that influence the mineral content of coal fly ash are: (a) The chemical composition of coal, (b) Coal burning process, (c) Additives used include oil additives for flame stabilization and additives for corrosion control.Most of the ash produced during the coal combustion process is in the form of fly ash as much as 55% -85% and the rest is in the form of bottom ash [1].
According to ASTM C618 [2], fly ash is classified as follows: class F and class C. When anthracite or bituminous coal is burned, Class F fly ash is produced, which possesses pozzolanic properties and less than 10% lime concentration (CaO).In the Asam-asam Power Plant in Tanah Laut Regency, one of the products made using fly ash classified as class F may be discovered.Haryanti [1] tested fly ash from Asam-asam Power Plant as a material for producing lightweight bricks and discovered that it included silica (74.2% SiO2), alumina (5.7% Al2O3), and Fe2O3 at a level of about 14.4%.Alkali metals (2.4% CaO and 2.03% MgO) support Aluminosilicate materials' bond formation.
Various studies on using coal fly ash waste are being carried out to increase its economic value and reduce its negative impact on the environment.Efforts are being made to maximize the economic worth of fly ash waste while reducing its negative environmental impact.The utilization of fly ash material as a geopolymer-forming material is based on the properties of this material which are similar to the properties of cement and have a positive impact from an environmental perspective [3].Geopolymer has advantages such as cost-effectiveness due to the abundant availability of materials, acid resistance, good heat resistance, simple synthesis.It does not produce CO2 emissions as in the manufacture of Portland cement.Besides that, it is also energy efficient and environmentally friendly because geopolymerization only requires heating at relatively low temperatures, and the energy required is only approximately 3/5 compared to making Portland cement.Hence, geopolymer emerges as a potentially profitable material [4].
Geopolymers from alumino-silicates (SiO2 and Al2O3) are made from natural materials such as kaolin, fly ash waste, and other silica and alumina materials [5].Geopolymers have the advantage of having excellent mechanical strength and durability.Samadhi & Pratama [6] made a geopolymer based on kaolin and fly ash to obtain a compressive strength value of 25.1 MPa.The geopolymer gluing process is also inseparable from the role of the activator.Geopolymer activation using an activator solution with a binding reaction that occurs is a polymerization reaction [5].Geopolymer with an 8 M NaOH activator has the greatest compressive strength compared to concentrations of 6 M and 10 M [7].The characterization of fly ash in the production of geopolymers is an intriguing issue for further investigation.

Fly Ash Preparation
The fly ash waste used in this research is from the Asam-asam PLTU Tanah Laut Regency.It was dried in the sun and then heated in an oven at 105 ± 5 o C for 24 hours, after which it was filtered through a 200-mesh sieve.The prepared fly ash was then tested for physical properties and compound analysis using XRF, identification of chemical bonds and functional groups using FTIR, morphology, and elements using SEM EDX.

Fly Ash Characteristic
Fly ash that has passed a 200-mesh sieve is characterized by its physical properties and oxide compounds using XRF and aimed at determining SiO2, Al2O3, and CaO, which will then be categorized into a specific class based on ASTM C18.In addition, the characterization of fly ash with FTIR to obtain data in the form of functional groups and bond structures of fly ash.The results from XRF and FTIR are intended to see the presence of silica and alumina as the essential ingredients in the manufacture of geopolymers.The characterization was done with SEM to observe the morphology and EDX to determine the % mass of elements contained in fly ash.

Fly ash characteristics
Fly ash used as the primary material for production of geopolymers was measured for its physical properties, oxide compounds, functional groups, and morphology, with the characteristics shown in Table 1.The moisture content of fly ash is essential for producing of geopolymers.The moisture content of the fly ash measured at 2.7% indicates that fly ash is dry enough to be used as a raw material for geopolymers because a water content that is too high can affect the quality of the final product [8].According to ACI Committee 26, the suitable moisture content in fly ash is a maximum of 4%, fulfilled in this study.The fly ash-moisture content test results in this study were less than the research [9] using PLTU Asam-asam fly ash with a moisture content result of 4%.
The fly ash density value is shown in Table 1 at 1.33 g/cm 3 .According to Haryadi [10], the density of fly ash ranges between 1.3 g/cm 3 and 4.8 g/cm 3 , where the density can vary depending on the chemical elements and porosity that occur in it.This work is consistent with the findings of with the research by Bhatt et al. [11], which obtained a fly ash density between 1.01 to 1.78 g/cm 3 .The porosity of fly ash is quite large, namely 44.06%.According to Hariska et al. [12], the large porosity of fly ash results in high porosity in concrete.

Oxidizing compounds and particle of fly ash
The X-Ray Fluorescence Spectrometer (XRF) and Electron Dispersive X-ray spectroscopy (EDX) methods show quantitative results of the fly ash components.The XRF method shows the percentage of oxide compounds from the main constituent components in Table 2.Then, the EDX method shows the results in percentage by mass of the constituent elements shown in Table 3.The outcomes of the XRF test showed that fly ash contained 31.90%SiO 2 and only 6.30% Al 2 O 3 .Iron oxide (Fe2O3) is relatively high.It can be formed in fly ash from burning coal containing iron.The content of Fe2O3 in fly ash can vary depending on the type of coal used, combustion conditions, and the process of controlling exhaust emissions at power plants [4].The high percentage of Fe was also confirmed by the results of the XRF study by Ghofur et al. [13] on PLTU Asam-Asam fly ash to obtain Fe of 58.84%.According to Zailani et al. [14], the crystallization of iron oxide (Fe2O3) contained in fly ash through a geopolymerization technique will be able to convert fly ash waste into a concrete material.
In addition, there is also a CaO content of 9.33% which influence the qualities of the resulting geopolymer.During the activation process with an alkaline solution, calcium ions (Ca 2+ ) in fly ash can react with silica and aluminum oxide to form calcium silicate and calcium aluminum silicate salts to strengthen the geopolymer structure.However, if the CaO content in the fly ash is too high, it can decrease the stability of the alkaline activator solution used.It is caused by the formation of hydrated calcium oxide compounds (Ca(OH)2), which can neutralize alkaline solutions and inhibit the geopolymer activation reaction [8].
Based on the data from the XRF test results shown in Table 2, the fly ash used belongs to class F because the amount of SiO2 + Al2O3 + Fe2O3 is more than 70% namely 85.91% and the CaO content <10% based on ASTM C18.Compared to research on the chemical content of fly ash in PLTU Asamasam, there are several differences in the quantity of composition, which in this case is also influenced by the origin of the coal that may differ each year, such as SiO2 (74.20%) and Al2O3 (5.70%) [1]; SiO2 (40.92%) and Al2O3 (10.08%) [15]; SiO2 (39.85%) and Al2O3 (4.49%) [16,17]; and SiO2 (48.86%) and Al 2 O 3 (11.29%)[18].
The results of the XRF test were also supported by the analysis results from EDX, showing that Fe, Si, Al, and Ca were the main elements contained in this fly ash sample.Oxygen (O) is the most significant mass percentage (38.65%)because the combustion results will always be oxides containing oxygen.As may be seen, the dominating elements in this fly ash sample are Fe, Si, Al, and Ca with 25.8 respectively; 18.83; 7.26; and 6.14 percent by mass.They were then followed by Mg (1.98%), Ti (0.52%), Mn (0.50%), and K (0.47%).The elements can also be seen in the graph of the EDX contained in Figure 1.

Chemical bonds and functional groups of fly ash
FTIR carried out identification of functional groups in fly ash in the range of wavenumbers 4000 cm -1 -500 cm -1 .Figure 2 shows the FTIR spectra of fly ash.The chemical bonds of fly ash are shown in table 4.There are absorption bands at 1161 cm -1 , 1084 cm -1 , 1024 cm -1 , 792 cm -1 , 774 cm -1 , 721 cm -1 , 688 cm -1 , 618 cm -1 , which is the IR absorption area of fly ash.The absorption band at 1161 cm -1 shows Si-O-Al (symmetric stretching vibration) in the presence of aluminum in an octahedral configuration [19]; 1084 cm -1 shows Si-O-C stretching and intense absorption at 1024 cm -1 is associated with Si-O-Si asymmetric stretching; both absorption bands indicate the presence of silica with Si-O groups [20]; the absorption band of 800 cm -1 shows a C-O stretching vibration related to the lime content in the ash [21]; the absorption band at 774 cm -1 [22], 688 cm -1 , 618 cm -1 is related to the stretching of Fe-O, indicating the presence of iron as a phase of magnetite, maghemite, and hematite [23].
The characterization of chemical bonds and functional groups of PLTU Asam-asam fly ash conducted by Ghofur et al. [13] only detected the presence of Si and Al did not detect the presence of other elements such as Ca and Fe, where Fe was the most dominant element in the study.This study also explained that FTIR was used to determine active clusters in Asam-asam Power Plant coal fly ash.Table 4 in this study supports bonds and elements in the formed geopolymer.Based on the findings of the FTIR spectrum obtained, it can be concluded that fly ash is a material containing SiO2, Al, Fe, and Ca that can be used for making geopolymers.

Morphology and particle of fly ash
The surface morphology of fly ash with SEM results is shown in Figure 3.The shape of the fly ash is relatively the same as that used as a geopolymer base material.Analysis of the identification of the surface morphology of fly ash was observed at two variations of magnification values, namely 1000 and 2000 times, with the results of Scanning Electron Microscopy (SEM) showing that the structure's shape is spherical with a regular shape also the more the image is enlarged, the more there is agglomeration at several points.This was also found by Hanum et al. [24].By obtaining the surface morphology of the fly ash, the particle diameter size can also be known, as shown in Figure 4.The comparison of the number of particles is used in terms of frequency.The diameter of the fly ash was taken from the SEM results with an average particle size of 18.32μm.This particle analysis confirms the results of the fly ash preparation, that is, a minimum of passing a 200-mesh sieve with a size of 75μm.The small size of fly ash particles was also stated by ACI Committee 226, which explained that fly ash has quite refined grains, which are a maximum of around 45μm [12].This is clarified by the low fly ash fineness of 16.4% (maximum 34% based on ASTM C618).The fly ash particle size in this study is of small value compared to other research [25] which obtained a size of around 20-50μm.The fly ash particle size affects the ability of the material to fill the space in the geopolymer mixture.Finer fly ash particles tend to have better filling abilities than coarser particles [19].For the application of fly ash as raw material on geopolymer.The particle size of fly ash affects the compressive strength of geopolymer paste [26].Nurwidayati et al. [27] stated that low fineness showed that fly ash had a higher specific surface area and greater reactivity, which increased the compressive strength of the geopolymer paste.

Initial and final setting time of geopolymer
The initial setting time of fly ash-based geopolymer paste can be defined as the condition when the paste starts hardening or the time required for the geopolymer paste to change its properties from a liquid state to a solid state, and final setting time can be interpreted when geopolymer paste has hardened sufficiently or Vicat needle penetration is not visually apparent.The setting time of the paste can be tabulated in Table 5.Table 5 shows the comparison setting time value of various fly ash for geopolymer.Composition of those geopolymers paste for molarity of NaOH of 8 M, the ratio of Na2SiO3/NaOH is 2,5, and the fly ash to alkaline solution ratio is 65 to 35.According to the initial and setting time value, it can be concluded that fly ash in this research can be applied to raw geopolymer material.

Conclusion
The XRF results show fly ash has a SiO2 content of 31.90% and Al2O3 of 6.30%.In addition, there is also a CaO content of 9.33 which can affect the properties of the resulting geopolymer.The fly ash used belongs to class F because the amount of (SiO2 + Al2O3 + Fe2O3) is more than 70%, which is 85.91%, and the CaO content is <10% based on ASTM C618.Fly ash is a material that contains SiO2, Al, Fe, and Ca.Moreover, the initial and final setting time of fly ash was 1072.5 and 3180 minutes It is a substance that can be used to produce geopolymers.

Figure 2 .
Figure 2. FTIR Spectra of Fly ash

Figure 4 .
Figure 4. Particle Size Distribution of fly ash.

Table 1 .
Fly ash physical properties.

Table 2 .
Percentage of fly ash oxide compounds.

Table 4 .
Chemical bonds and functional groups fly ash.

Table 5 .
The initial and setting time of various fly ash for geopolymer.