## 1 Introduction  
Vietnam is the Southeast Asian country with the highest reliance on coal for electricity generation. Furthermore, with the rapidly increasing demand for electricity in Vietnam, recently the number of coal power plants have been expanded or been constructed. Presently, these coal-fired thermal power plants generate approximately 20% of the total electricity demand in Vietnam. As Vietnamese thermal power plants use domestically mined coal, this increase in coal-fired generation will lead to an increase in coal production within the country by companies such as Vinacomin (Vietnam National Coal and Mineral Industries Group). The stations sampled at for this study, Uong Bi and Pha Lai thermal power stations, procure coal from Vinacomin and have a capacity of approximately 1800 MW of electricity. To satisfy the increasing demands for electric power, the coal power plants are vital, but they also emit a large amount of fly ash and are among the causes of environmental problems, especially, the pollution related to coal fly ash. Currently, coal fly ash is used for various purposes: brick-making, landfilling, construction, soil amendment, and others [1; 2; 3; 4]. However, effective end uses are possible only if a comprehensive characterization of fly ash produced at the plant can be carried out [5; 6]. Also, a comprehensive characterization is expected to give useful information on the combustion processes. Fly ash is a complex heterogeneous form of coal combustion residue. Fly ash is generally regarded as the finest particles that are formed (0.2-90 \(\upmu\)m) because of the transformation of mineral matter (i.e., existing in coal particles) during combustion processes [7; 8]. However, fly ash contains noticeable amounts of large (e.g., 90-300 \(\upmu\)m) char, semi-cooked or cooked carbon matters because of low combustion efficiency in combustors. The main weakness is because of low-quality control in maintaining the particle size of the pulverized coal feed, perhaps more than other factors. Besides, fly ash is characterized by irregular shape and contains vesicular, lacy, solid/hollow Alumino-siliceous spheres as well as an alumino-siliceous matter of complex elements [9]. Based on combustion conditions, the type of coal used, and removal proficiency of devices that control air pollution, the chemical composition of fly ash is variant [10]. As an example, in South African coal fly ash, the combined content of Fe\({}_{2}\)O\({}_{3}\), Al\({}_{2}\)O\({}_{3}\), and SiO\({}_{2}\) in fly ash must be over 70% whereas the CaO content should not exceed 5%. Therefore, the 2-step treated fly ash was added together that contains less than 50% of Fe\({}_{2}\)O\({}_{3}\), Al\({}_{2}\)O\({}_{3}\), and SiO\({}_{2}\)[11]. In this fly ash category, the CaO content changes in the range of 20 to 30%. However, the formulation of fly ash may differ to some extent by whether the same is true of fly ashes with low/high calcium or low/high iron [12]. As a result, they require different utilization schemes. Fly ash's mineralogical composition mainly depends on the geological features that inform coal formation and deposition, as well as the combustion condition [13; 14; 15]. Here, quartz [16; 17; 18; 19], mullite [14; 20; 21; 22], hematite [19; 23; 24; 25], magnetite, and lime are the most common and predominant phases apart from other minor constituents [22; 25; 26; 27; 28]. Also, fly ash's mineralogical composition is determined by the type of coal used. So, anorthite, quartz, hematite, gehlenite, and mullite predominantly constitute the main crystalline phases in lignite fly ash [22; 26; 29; 30; 31]. In fact, quartz and mullite are the primary crystalline phases in low calcium fly ashes while high calcium fly ashes contain quartz, C4 AS, and C3A. The predominant minerals in fly ashes are mullite, quartz, hematite, magnetite, calcite, and dolomite. As an important aspect of fly ash, morphology requires a thorough evaluation.