## 1 Introduction  
CBA is a coal-combustion byproduct that is created in electricity generation. The specific properties of CBA are determined by elements such as the origin and type of coal that it results from. Coal is classified into four distinct categories: anthracite, bituminous, sub-bituminous, and lignite [1]. This categorisation is based on the coal's carbon composition, heat energy production, and moisture content, as well as the presence of other chemical components in the coal [2]. With its high carbon concentration, anthracite is often the least suitable for energy production among all coal types. The chemical composition of coal is influenced by its geological formation. Different forms of coal exhibit varied levels of silica oxide (SiO\({}_{2}\)), alumina oxide (Al\({}_{2}\)O\({}_{3}\)), and ferric oxide (Fe\({}_{2}\)O\({}_{3}\)), which in turn affect their chemical properties [2,3]. Coal is a precious resource, and the main use of coal is its consumption for electricity generation [4]. To produce electricity in coal power plants, a huge amount of coal is combusted within designated boilers, resulting in the formation of different types of ashes, including fly ash and CBA. Coal-based electricity generation contributes to air pollution through releasing gas emissions and producing ash as a byproduct of the combustion process [5]. CBA refers to coarser particles removed from the furnace bottom, whereas fly ash (FA) is finer ash that is produced and collected at various stages of the burning process [6]. Coal ash waste consists of fly ash and bottom ash, which show different variations in their physical and chemical properties. Fly ash accounts for around 70-80% of the total ash produced by the burning of coal, while bottom ash makes up about 10-20% [7]. There are two types of bottom ash that are obtained based on the boiler that is applied: wet and dry. Typically, the bottom ash consists of particles that fall within the size range of sand and gravel. After grinding and milling, one study found that the particle size distribution of bottom ash changed from a maximum of 9.5 mm to as fine as 300 \(\mu\)m [8]. Bottom ash is sharp-edged and irregular in shape, has a porous texture, and has angular particles. CBA has a particle distribution ranging from the size of fine sand to that of gravel particles. Particles of coal bottom ash that are in the form of fine particles will constitute 50-90% of the overall bottom ash. The specific gravity of bottom ash varies between 1.39 and 2.41. Because of its porous nature, CBA has a high water absorption value ranging from 11.61 to 32.23%. The AASHTO classification system categorises CBA as A-1-a, while the USCS classification system classifies it as well-graded sand [9]. The chemical composition of CBA depends upon the particular type of coal that is employed. The primary mineral elements present in CBA are silicon dioxide (SiO\({}_{2}\)), aluminium oxide (Al\({}_{2}\)O\({}_{3}\)), and iron oxide (Fe\({}_{2}\)O\({}_{3}\)) [10]. SiO\({}_{2}\) has the highest percentage with 45.3-68.9%, followed by Al\({}_{2}\)O\({}_{3}\) (15.89-29.24%) and Fe\({}_{2}\)O\({}_{3}\) (3.57-19.84%). The total amount of these three chemical oxides was higher than 70% in the CBA used in this study, which means that this ash belongs to ASTM Type F ash [11]. Coal-fired power generation is currently more economically appealing than other options, such as oil and natural gas, particularly in countries with abundant coal reserves like India, the United States of America (US), and China [12]. For example, coal consumption in China was half of the world's total coal consumption in 2012, followed by the US, India, Japan, the Russian Federation, South Africa, South Korea, Germany, Poland, and Indonesia, with coal consumptions of 11.7%, 8.0%, 3.3%, 2.5%, 2.4%, 2.2%, 2.1%, and 1.4%, respectively [7]. In Malaysia, electricity generation comprises 33% coal and 9% hydro. The percentage of electricity that is generated by coal was expected to increase due to the increase in the gas price [13].