MINERALOGICAL, MICROSTRUCTURAL AND COMPRESSIVE STRENGTH CHARACTERIZATION OF FLY ASH AS MATERIALS IN GEOPOLYMER CEMENT

: This study was designed to examine the mineral, microstructural, and mechanical strength properties of fly ash and its feasibility as a raw material for geopolymer cement. The study used an experimental method by examining the characteristics of fly ash by X-ray Fluorescence Spectrometer (XRF), Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), hydrometer method, Scanning electron microscopy (SEM), and compressive strength testing. For creating the geopolymer cement paste, a concentration of NaOH 10M was used, with a ratio of water/solid = 0.4 and a ratio of Na 2 SiO 3 /NaOH = 1 using curring at room temperature. The results showed the geopolymer pastes have a compressive strength of 18.1 MPa and 21.5 MPa after 7 days and 28 days. The XRD results showed a decrease in the peak of 2θ at 26.54° because the amorphous part had transformed into a C-S-H solution in geopolymer cement. This finding was supported by the FTIR spectra results showing Si-O-Si bending vibration and the functional group of AlO 2 . It showed that Nagan Raya fly ash-based geopolymer is a potential construction material.


Introduction
The use of portland cement (PC) as the critical binder material in concrete creates many environmental concerns because of its energy consumption and carbon dioxide (CO 2 ) emissions. Depending on the power station, the output of one ton of cement is expected to release between 0.7 and 1.0 tons of CO 2 (Davidovits, 1991) (Peng et al., 2013. Therefore it is important to look for other materials that are more environmentally friendly, and that can replace cement. The application of sustainable cementitious materials replacing the Portland cement in the building industry will considerably contribute to the relief of the environmental problems related to CO 2 emission and global warming. Geopolymer concrete (GPC) is a sustainable cement-less concrete prepared using industrial by-product materials, such as slag, fly ash, and other waste. Previous studies showed that the GPC has better mechanical properties than OPC concrete, the durability performance is a crucial concern for its application in infrastructure.
In the geopolymeric reaction, silica and alumina in the fly ash react with alkaline solutions. The process begins by dissolving fly ash particles using the high-alkaline solution. Then, the polymerization process occurs and eventually precipitates from the solution, forming the product phase. At the same time, precipitation occurs and eventually creates a three-dimensional solid matrix. The main outcome of this geopolymeric reaction is a sodium-aluminosilicate gel (Kovalchuk et al., 2008), (Ryu et al., 2013).
Fly ash has been widely applied in composite cement (Agrawal et al., 2019) (Lahoti et al., 2018. Most previous studies used fly ash as a precursor in geopolymer cement. Fly ash that will be utilized as the precursor in geopolymer must be characterized completely to determine the fly ash's ability in producing geopolymerization. This study investigated the possibility of using the fly ash from the Power Plant in Nagan Raya, Aceh Province, Indonesia. This article reports the characterization results of fly ash, the compressive strength of the geopolymeric material produced, and geopolymerization. Low calcium fly ash is commonly used in geopolymer because it has a better setting time, even though high-calcium fly ash is abundant and harmful to the environment. High-calcium fly ash is rarely used due to its poor characteristics to be applied as a base material for geopolymer concrete (Diaz et al., 2010;Guo et al., 2010;Rattanasak et al., 2011;Temuujin et al., 2014;Tho-in et al., 2012). The high calcium content in fly ash makes the geopolymer concrete harden too fast; thus, it is challenging to implement it as a large-scale construction material. Fly ash from the Nagan Raya Power Plan is high calcium fly ash. To utilize local resources as a source of construction material, fly ash from the Nagan Raya power plant must be examined for its feasibility on geopolymer cement. This paper focuses on studying fly ash properties as a precursor in the cement composite binder for geopolymers. This study aims to examine the raw material characteristics of fly ash as the material for geopolymer cement. Besides, this research also assessed the geopolymerization produced by geopolymer cement. The characterization assessment of the waste materials, such as fly ash, informs the feasibility of the material as a construction material. It can also provide an opportunity to mitigate the disposal and its associated environmental impacts.

Material and Methods Materials
This study used high calcium fly ash from the Nagan Raya power plant in Aceh Province, Indonesia (NFA). The NFA had specific gravity and average particle sizes of 2.457 and 30 µm, respectively. Fly ash was prepared by first passing it through 200 mesh. It was physically activated at 105 °C for one hour to evaporate the water and impurities in its pores. Table 1 displays the chemical compositions of the NFA. The liquid portion in the mixture consisted of 10 M sodium hydroxide (NaOH) (99% purity-Merck) and sodium silicate (Na 2 SiO 3 ), with 12.81% Na 2 O, 24.19% SiO 2 , and 63% H 2 O. The Epoxy resin used was made in America, which was purchased from the Indonesia Chemical Reagent Company. All chemicals were purchased from Indonesia Chemical Reagent Company and were used without further purification.

Mix Proportion
The process of mixing geopolymer paste was conducted by preparing the materials shown in Table 1. NaOH solution was made of 10 Molar NaOH mixed with Na 2 SiO 3 and then left for 24 hours so that the solution was completely mixed into an alkaline solution. Figure 1 showed the specimen preparation process. In the first step, fly ash was mixed with an alkaline solution and stirred for 20 minutes. The epoxy resin was mixed after the geopolymerization process was complete. Next was molding and curring, in which the paste was put into a 5x5x5 cm cube mold and treated at room temperature. After three days, the paste was removed from the mold.

Testing Procedures Hydrometer Method
The hydrometer method part ASTM D7928 was used to determine the particle size distribution used. It was performed by calculating the size of soil particles from the speed they settle in a liquid suspension. The results showed that the particle size distribution of soil is finer than the 75µm sieve.

Scanning Electron Microscopy (SEM)
A scanning electron microscope (SEM) microscopy study was carried out to classify the microstructural characteristics of fly ash and geopolymerization. The microscopy analysis was done using Field Electron and Ion Company, Hillsboro, and Oregon. The specimen with double stick carbon tape was mounted on a brass stub sample holder. Using a blazer sputtering coater, the specimen was dried using infrared light for five minutes and then coated between 20-25A thick. The micrographs were registered at 15 kV and 5000x magnification.

Fourier Transform Infrared (FTIR) Spectroscopy
After 28 days, geopolymer cement paste samples were cut into thin slices, then grounded into fine particles, and filtered using a 75-μm sieve to obtain the required powder samples for FTIR tests. Fourier transform infrared (FT-IR) absorption spectra were recorded in the range of 4,000−450 cm −1 . Spectra were recorded at a spectral resolution of 4 cm −1 , a scan speed of 0.2 cm/s, and they were analyzed with Spectrum software.

X-Ray Diffraction (XRD)
Quantitative XRD analysis was conducted consistently using the Reference Intensity Method (RIM) described by Chung (Chung, 1974). The XRD scans were performed at 10 to 50° 2 with a 0.5 s/step scan speed. The phases of the NFA and geopolymer paste at 28 days were determined by XRD analysis.
The crystallinity index (Crl) used is decided based on the reflected depth data following the means of Segal (1959).

Compressive Strength
The compressive strength test was performed on the 50 mm 3 paste specimens based on ASTM C109 using a Technotest concrete testing machine. The reported seven days and 28 days compressive strength values are the average of three samples for each mix design.

Results and Discussion Characterization of Fly Ash Chemical Characterization
To determine the quality of fly ash used in this study, the NFA was compared to several fly ash from other countries. The NFA was compared to fly ash from Sarawak (SFA) and fly ash from Glastone, Australis (GFA), to assess its mineralogical.
The chemical compositions of fly ash types were examined using X-ray Fluorescence Spectrometer (XRF), and the results are shown in Table 2. According to ASTM C618, fly ash classification is divided into two classes: class F and class C; the main differences between these classes are the amount of calcium, silica, aluminum, and iron oxide in them. Class F fly ash consists of <10% CaO. Table 2 shows that the largest composition of NFA is CaO, Fe 2 O 3 , and SiO 2 . Both SFA and GFA are classified as class F fly ash, while and NFA is class C.
Based on the visual observation, NFA is gray, SFA is darker in the shade (i.e., gray color), and GFA is brownish, as shown in Figure 2.  Figure 2 showed that fly ash with a lighter color is produced from anthracite or bituminous coal and has a finer particle size. This color indicates better fly ash quality (Chindaprasirt et al., 2004), (Wattimena et al., 2017). The basicity index (Kb) and hydration modulus (HM) of NFA, SFA, and GFA are evaluated using the following equations.
……………………… (3) ……………………… (4) The basicity index of NFA, SFA dan GFA, is 1.06, 0.08, and 0.07, respectively. The Kb of SFA and GFA is ≤1, indicating pozzolanic properties, quick lime, hydrated lime, or cement should be added to obtain cementitious properties. Meanwhile, the basicity index of NFA indicates pozzolanic and self-cementing c b a properties (no need to add lime). It contrasts with the self-cementing properties of SFA and GFA, which are very poor due to the lack of CaO content (Leong et al., 2016), (Wilińska & Pacewska, 2018). The hydration modulus of NFA, SFA, and GFA is 2, 0.51, 0.62, respectively. This shows that the hydraulicity of the NFA is high, and it will harden faster, while it will take longer for the SFA and GFA to set.

Mineralogy
The mineralogy of NFA was studied based on the XRD test results, and the peaks represent the presence of the crystalline phases. The XRD result of NFA is similar to the corresponding XRD results of SFA and GFA, as shown in Figure  3. The dominant mineral observed in NFA, SFA, and GFA is quartz. NFA's highest peak was observed at approximately 26.65°at 2θ, suggesting that a greater amount of amorphous material was present. The XRD pattern of NFA shows the wider band from 25° to 30°. GFA band was ranged from 20° to 40°.  Figure 3 showed that the results indicate that the amorphous nature of GFA is more prominent than NFA and SFA. The amorphous property of fly ash is due to the high temperature of coal combustion, which induces the material phase from crystalline to amorphous (Itskos et al., 2010) (Fuller et al., 2018). It is possible to dissolve more amorphous silica and alumina from fly ash and contribute to geopolymerization, thus creating a stronger geopolymer structure.
Eqs (1) and Table 3 show that NFA fly ash contains 64.91% crystalline, and the remaining is amorphous.  The percentage of the amorphous part strongly correlates with the compressive strength. The more amorphous part in fly ash, the more SiO 2 is dissolved with alkaline solutions, and it increases the C-S-H gel needed in the geopolymer process (

Particle Size Distribution
The particle size distribution in NFA is presented in Figure 4. While, Table 4 shows the results of the mean diameter and particle size distribution at 10% (d10), 50% (d50) and 90% (d90).  Figure 4 showed that based on the particle size analysis, it could be seen that the NFA fly ash particle size is mostly 0.074-0.002 mm (93.7%). Compared to the original Portland cement with 70% passing the 200 mesh, the NFA particle size is smoother, with 93.7% passing the 200 mesh. Smaller particle sizes will influence the higher hydration rate. Another advantage of smaller size is that the smoother cement, the less chance of bleeding, and the better workability. However, fine fly ash also has a drawback: the finer the fly ash, the greater the possibility of shrinkage and cracking. Hence, it is necessary to note in using NFA for binder materials in geopolymers. The Coefficient of Curvature (Cc) and Coefficients of Uniformity (Cu) were also used to compare the variety of particle size gradation of fly ash, as given in Eqs. 5 and 6.
Based on Table 5, the analysis revealed that Cu and Cc for NFA were 1.43 and 0.80, respectively. The NFA's Cu value is lower than those of SFA (9.4) and GFA (7.3), showing the NFA particle size is uniform and in a small range. A higher Cu value reflects a more non-uniform particle size distribution and broader ranges of particle sizes (Leong et al., 2016). In this case, the higher number of fine particles in the NFA increases the filler effect, although the structure formed tends not to be more compact and rigid.
Cc value shows the curvature degree of the plot for the particle size distribution. The Cc value of the NFA is 0.8 or close to 1, while it is between 1-3 for the SFA and GFA. These values indicate that the NFA, SFA, and GFA have a well-graded particle size distribution. Figure 5 showed an SEM image of NFA, SFA, and GFA. NFA fly ash particles, as seen in the SEM micrographs, are very small. Figures 5a and b showed the distribution of NFA fly ash looks uniform. The distinct particle size distribution between large and small particles is evident on the NFA. Figure 5c-f showed the particles' sizes of Cut Rahmawati, Sri Aprilia, Taufiq Saidi, & Teuku Budi Aulia: Mineralogical, Microstructural and Compressive Strength Characterization of Fly Ash as Materials In Geopolymer Cement SFA and GFA are distributed equally. From a workability point of view, the shape of NFA fly ash particles that are less spherical will increase the internal friction.

Scanning Electron Microscopy
The irregular particles will increase absorption ability and fluid demand (Lynn et al., 2016). Therefore, a higher amount of aqueous solution is necessary to obtain better workability of NFA. This is different from the SFA and GFA; both have a smooth spherical particle shape; the ball bearing and the lubricant effects are enhanced, improving the mixture's ability to work and flow.

Compressive Strength and Microstructure of Geopolymer Cement Compressive Strength
The compressive strength of the Geopolymer paste was tested after seven days and 28 days; the results are presented in Figure 6. The compressive strength shows an increase after 28 days (21.5 MPa). This compressive strength is slightly below the compressive strength reported by (Phoo-ngernkham et al., 2014;Saidi & Hasan, 2020). (e) GFA-distribution from fine to coarser portions is in decreasing order (magnification to 20µm). Source: (Leong et al., 2016) This study (Phoo-ngernkham, Chindaprasirt, Sata, Hanjitsuwan, & Hatanaka, 2014) geopolymer suspensions to create new chains consisting of [AlO4] 5− , [SiO4] 4− tetrahedral, and polyamines (Du et al., 2016). Figure 7 showed the reaction of the epoxy resin with the geopolymer that occurred by OH-binding. This mixture showed an excellent dispersion to the micro-level with the increasing initial compressive strength. Figure 7. The schematic interactions between geopolymer and the epoxy resin (Ferone et al., 2013). Figure 8 shows the XRD patterns of the NFA and the geopolymer cement containing 20 wt% of epoxy resin. A decrease in peak 2θ at 26.54° indicates that the amorphous part of the fly ash has transformed into a C-S-H solution on the geopolymer cement. The XRD graph shows a similar diffraction pattern between the NFA and geopolymer cement; only the peak at an angle of 2θ seems to decrease.

X-Ray Diffraction Characterization
The amorphous part of fly ash is only 33.23%, but the results show that the amorphous part could be transformed into a C-S-H solution on the geopolymer cement. An increase in compressive strength also supports this.   Figure 9 displays the FTIR spectra of the geopolymer-based epoxy and fly ash as raw material. The intensity of the peak at 521 cm -1 was associated with Si-O-Si bending vibration (Aouan et al., 2021), (Sarkar & Dana, 2021). The small band of fly ash at around 671 cm -1 represents the functional group of AlO 2 , which is its chemical composition. The band of geopolymer pastes at about 795 cm -1 is characterized as the crystalline phase of quartz (Ranjbar et al., 2014), (Arioz et al., 2020). However, it only shows a small intensity at the fly ash band because the amorphous part of fly ash has been transformed into the crystalline phase. The wide vibration bands around 1145 cm -1 exhibited in spectra were attributed to T-O-Si (T: Si or Al) (Komnitsas et al., 2007) (Da Luz et al., 2019.

FTIR Analysis
Consequently, other small bands at about 1,467 cm -1 that emerge in the geopolymer specimens are assigned to carbonate asymmetric stretching. The presence of sodium carbonate due to the atmospheric carbonation of alkaline O-C-O-O is indicated by this band (Lee & Deventer, 2002), (Erfanimanesh & Sharbatdar, 2020). The wide vibration bands around 1,703 and 3,629 cm -1 exhibited in spectra were attributed to O-H stretching and bending, respectively (Gharzouni et al., 2016) (Gouny et al., 2012, (Troëdec et al., 2009). It is predicted that epoxy contributed to these bonds as interactions between geopolymer and the epoxy resin. Another band was around 1,145 cm -1 , corresponding to the Si-O-Al and Si-O-Si vibration bands of the geopolymer. This band is a major fingerprint for the geopolymer matrix and defines the extent of polysialylation.  Figure 9 shows that several changes are observed when the values are compared to the same characteristic bands of NFA. The change of vibrational and deformation bands of Si-O-Si to high and low values is noticeable. At the vibration bands of 950-1,200 cm -1 attributed to T-O-Si (T: Si or Al) (Peyne et al., 2017), (Jose et al., 2020), it also seems to change. Besides, the NFA used as raw material for Si-O-Al shows a low intensity. Still, after the geopolymerization process, a C-S-H gel is formed, increasing the intensity of vibration bands. This result is consistent with the XRD analysis of geopolymer cement that shows the depolymerization. It indicates a decrease in peak 2θ at 26.54° and NFA transformed into a C-S-H solution.

Conclusion
This experimental study proves that geopolymers can be formed from high calcium fly ash from Nagan Raya. The NFA mineral indicates pozzolanic and self-cementing properties. However, NFA has some disadvantages. The hydration modulus of NFA is 2, so it will harden faster, and microstructural formed tends not to be more compact and rigid. Also, the percentage of the amorphous part of fly ash is 35.09%, affecting C-S-H gel formation; therefore, another additional material, epoxy resin, should be added to the hydroxyl bond. This study shows that epoxy can correct the weakness of NFA by contributing hydroxyl bonds to geopolymers. This aligns with XRD analysis results, indicating that NFA has transformed into a C-S-H solution on the geopolymer cement. These XRD results are also supported by FTIR analysis results showing a hydroxyl O-H bond at vibration bands around 1,703 and 3,629 cm -1 , which is estimated from the epoxy resin. The compressive strength shows an increase after 28 days (21.5 MPa), so that it is concluded that NFA can be used as a raw material in geopolymers.