## 1 Introduction
Four coal power plants are operating in Peninsula Malaysia, namely, Kapar in Selangor, Manjung in Perak, Tanjung Bin in Johor, and Jimah in Negeri Sembilan [14]. These plants generate about 40% of Malaysia's electricity from coal. Due to the high demand for coal, a great amount of industrial waste has been generated. Bottom ash accounts for 100 million metric tonnes (Mt) of the millions of tonnes of coal ash waste that is produced yearly, with the remainder being fly ash [15]. According to the World of Coal Ash (WOCA), coal-fired power stations produce 780 million metric tonnes of CBA, with Asian countries contributing 66% of this amount, followed by Europe and the United States [16]. As the largest producer, China alone produced 395 Mt of coal ash in one year. The United States follows with 118 Mt, India with 105 Mt, Europe with 52.6 Mt, and Africa with 31.1 Mt [17]. Unlike fly ash, which has been widely used as a raw material replacement in the concrete and bricks industry, CBA is still treated as useless solid waste material and is disposed of on open land [18]. At present, bottom ash is dumped at landfills [19; 20]. Disposing of CBA in landfills and the open environment may result in the depletion of valuable land and groundwater resources and air contamination [21]. Coal ash waste typically contains trace quantities of heavy metals and metalloids, including lead (Pb), arsenic (As), zinc (Zn), copper (Cu), manganese (Mn), cadmium (Cd), nickel (Ni), chromium (Cr), manganese (Mn), and selenium (Se) [22; 23; 24]. The concentration of heavy metals in bottom ash is higher than that in fly ash. Under acidic conditions, these elements can be extracted and then pollute the nearby soils, surface water, and groundwater sources [25; 26]. Furthermore, in the worst-case scenario, heavy metal contamination can enter the food chain, resulting in a genotoxic effect [27]. With the increment in CBA waste production, the scarcity of land, the costs of operating existing landfills, and the hazardous impact of CBA on humans and the environment, a dire need exists to recycle and utilize CBA [20; 28; 29]. With the increasing recognition of sustainable pavement, the pavement sector has started incorporating a larger variety of industrial waste and recycled materials into pavement to preserve natural resources [30]. On the other hand, the enormous use of natural aggregates, especially in pavement construction, has led researchers to find other alternative materials to reduce the depletion of natural aggregates. Natural aggregate, a non-renewable source, is the most significant material used in construction, especially pavement construction [31; 32]. Base and sub-base courses all use natural aggregates, whereas bituminous pavement uses 95% natural aggregates and concrete pavement uses 87%. The aggregate needed for one km of a bituminous mixture's surface course can surpass 15,000 tonnes [33]. The demand for natural resources has increased due to the increased consumption of raw materials [34]. One possible approach to this issue is to utilise waste materials. Through the replacement of natural aggregates with waste aggregates in asphalt pavement construction, the construction cost of pavement can be reduced, as well as the necessity of extracting aggregates from natural resources. In past and current research, researchers have studied the use of waste materials as aggregate in asphalt mixtures [35]. Some of the materials used were mining waste, recycled concrete, steel slag, palm oil shell, palm oil clinker, recycled asphalt, and glass [34; 35; 36; 37; 38; 39; 40; 41; 42]. Previous research has revealed that bottom ash exhibits desirable engineering properties that make it suitable for use in construction materials, including as a substitute for aggregates, as a replacement for cement, as a bitumen additive, and as a filler in asphalt pavement [43; 44; 45].