## 4 Use and Classification of Ash on Pavements
Bottom ash (heavy ash or wet ash) originates from the processes of combustion of coal in pulverized form, and from the burning or gasification of coal in a fluidized bed, which generally contain a non-carbon content that is burned from 5 to 10% [173]. They are heavier and coarser-grained, which fall to the bottom of the furnaces and gasifiers. They are extracted by the flow of water, mainly in the large boilers of thermal power plants and steam plants. Slag (or coarse ash / boiler slag) originates in the process of combustion or gasification of coal in fixed and mobile gates [174, 175]. They often occur with coarse-grained particles and synthesized blocks, with a considerable content of unburned carbon (10-20%). They are removed from the bottom of the ovens after cooling with water. It should be noted that Brazil does not have specific legislation for the classification of ashes. Therefore, these are only classified as solid waste, and their use requires the adaptation of the relevant regulations and the approval of environmental agencies. Light or volatile ashes appear in small amounts as hollow spheres. These are called enospheres when they are empty, and plerospheres when they are filled with many small spheres [176, 177]. A large part of these particles is retained by a capture system, such as tissue filters, cyclones and electrostatic precipitators, which are listed in Table 2.  
This type of ash is usually gray, abrasive, mainly alkaline and refractory in nature. These materials are in the category of pozzolanic and/or cementitious materials. The main constituents of the ash are crystals of silica SiO\({}_{2}\), alumina Al\({}_{2}\)O\({}_{3}\), oxide of iron Fe\({}_{2}\)O\({}_{3}\) and lime CaO, in addition to other components in smaller amounts such as MgO, Na\({}_{2}\)O, K\({}_{2}\)O, SO\({}_{3}\), MnO and TiO\({}_{2}\), as well as carbon particles not consumed during combustion. The potential use of fly ash is controlled primarily by its chemical composition, although their small particle size is also an important consideration. Fly ash has cementation characteristics that allow its use for soil stabilization without the use of activators to improve its mechanical properties, such as in soft soils for application on bases and sub-bases. This allows it to contribute to a stable work platform favorable for road construction [178, 179]. Fly ash is capable of developing pozzolanic reactions and has characteristics for soil stabilization, since its composition contains the main elements responsible for pozzolanic activity such as SiO\({}_{2}\), Al\({}_{2}\)O\({}_{3}\), Fe\({}_{2}\)O, CaO, MgO, SO\({}_{3}\), Na\({}_{2}\)O and K\({}_{2}\)O. \begin{table}
\begin{tabular}{c c c} \hline \hline
**CLASS 1** & **CLASS 2** & **CLASS 3** \\ \hline \hline Blast furnace waste & Phosphate residue & Gold scraps \\ \hline Fly ash & Copper residue & Copper scraps \\ \hline Sulfur & Cement manufacturing fines & Lead and zinc waste \\ \hline Refining residue & Quarry waste & Coke mill \\ \hline Boiler and grill ash & Mine scraps & Foundry sand \\ \hline Nicel residue & Slate waste & Refractory and ceramic waste \\ \hline Demolition waste & Feldspar waste & Resin \\ \hline Coal mine rejects & Rubber tires & Lagnina \\ \hline Oil shale waste & Zinc and lead slag & Potash mine waste \\ \hline Taconite waste & Mixed ash & Pyrite ash \\ \hline Clay treatment sand & Incineration waste & Glass waste \\ \hline Slag from the iron mines & & Plastic waste \\ \hline Pyrolysis residue & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Characteristics of Fly Ash [33]. The first recorded study on the use of ash in pavements in Brazil was performed in 1998 [180]. The authors evaluated the behavior of fly ash, produced in a thermoelectric plant in the country's south, as a sand stabilizing agent in order to confirm studies that had been carried out only abroad.