## 2 Literature Review
The bottom ash referred to as the agglomerated ash is made up of coarser particles that are too heavy to be carried along with the hot fuse gases, the bottom ash particle size ranges from approximately 2 \(\upmu\)m to 20 mm with an angular shape and a porous morphology [33; 34]. The bottom ash is mostly used in the construction industry, underground mining, or as landfills in open mines and quarries [35]. The coal ash physical and chemical properties are dependent on the reactions occurring during high-temperature coal combustion in the boiler furnace and subsequent cooling of the flue gases, and the type of coal [35]. The fly ash is made up of fine, smooth, powdery particles that are predominantly spherical in shape, amorphous in nature, and either solid or hollow in form. Generally, coal ash specific gravity varies largely at an approximate range between 1.6 to 3.1 but lies mostly around 2.0 [36]. Coal ash-specific gravity variation results from several factors, including particle shape, graduation, and chemical composition [36]. Coal fly ash particles can be classified as silty sand to sandy silt containing some clay-sized fractions based on their grain size distribution [37]. Coal fly ash comprises a high specific surface area with a low bulk density [38]. Coal fly ash contains a minimum percentage of thin-walled hollow particles known as cenospheres, these are typically fined-grained silt-sized particles with a diameter range between 1 \(\upmu\)m to 100 \(\upmu\)m, and a median particle diameter of 20 \(\upmu\)m to 25 \(\upmu\)m [39]. Coal fly ash is usually dark grey or tan. The quantity of unburnt carbon and iron within the fly ash affects its colour, the colour might vary from either orange to deep red, brown, or white to yellow [40]. Coal ash chemical composition is primarily determined based on the coal chemistry during the combustion process. Ash is mainly derived from coal inorganic materials such as clays, feldspars, quartz, and metal oxides. Major coal ash elemental makeup is similar to a wider range of the composition of rocks in the earth's crust. Typical coal fly ash major mineral components comprise more than 90% of oxides of silicon, aluminium, iron, and calcium [41; 42] as seen from the result obtain in this study. Elements such as magnesium, potassium, sodium, titanium, and sulphur are the minor constituents, making up approximately 8% of the fly ash mineral components. The fly ash is also made up of surface mineral constituents that comprise less than 1% of its total composition; this includes elements such as arsenic, cadmium, lead, mercury, and selenium [41; 42; 43]. Coal fly ash is recognised as the most complex material in terms of its characteristics containing 316 individual minerals amongst 188 mineral groups [44]. All fly ash is made up of substantial amounts of both amorphous and crystalline silicon dioxide (SiO\({}_{2}\)), aluminium oxide (Al\({}_{2}\)O\({}_{3}\)), and calcium oxide (CaO) the main mineral compounds in coal-bearing rock strata [32]. Fly ashes are classified as acidic depending on the pH value and calcium/sulphur ratio, with a pH of 1.2 up to 7 it is acidic, a pH of 8 up to 9 is mildly alkaline ash and it is more strongly alkaline ash with a pH 11 up to 13 [45]. Depending on the boiler combustion conditions, the fly ash might also contain some amount of unburnt carbon. Loss-on-ignition (LOI) is the laboratory test used to determine the quantity of unburnt carbon in fly ash. LOI values range from less than 1% to more than 20% [42; 43]. The fly ash's fundamental chemical properties and reactivity are influenced by its relative Ca, Fe, and S contents. Ashes from Subbituminous and lignite coals relatively contain a greater concentration of Ca with an exceeding 15% (as CaO), this ash forms an alkaline solution when it contacts water (i.e., pH 11-12), whereas bituminous coal ash relatively contains a lesser Ca concentration leading to slightly acidic and alkaline solutions when contacts water (i.e., pH 5-10) [44; 45].