## 4 Use and Classification of Ash on Pavements
In Brazil, the generation of ash derived from coal, without counting other sources, is quite high. It is estimated at four million tons per year, and the trend is for this value to increase due to the development of the industrial sector. The production of residues from the burning of fossil coal is an inevitable consequence of the use of this non-renewable natural resource as fuel. Of the total amount of coal burned in Brazil, only 30% of the ash is sold, justifying the importance of developing research, products and techniques that seek to take advantage of this waste [156; 157]. It is known that the ash has a high contaminating potential, related to its hydraulic drag and with two determining factors, being the alkaline pH present in its composition and the solubilization of its elements when it comes into contact with the drag water [158; 159; 160]. In large areas of settling basins, elements including heavy metals are leached into the environment. This can contaminate the soil and subsoil, reach the water table and adversely affect animals and plants. Considering the cumulative effects of the heavy metals, these effects can be serious [161; 162]. However, when evaluating the contaminating potential of the ash, it should be considered that not all ash has the same composition. Its physical, chemical and mineralogical characteristics depend on a variety of factors, including the composition of the source material of the matter, combustion conditions, type and efficiency of the emission control system and disposal methods used. Contrary to investigations that only consider the contaminating potential of the ashes, it is stated that the ashes can be considered an important mineral resource, due to their physicochemical and mineralogical properties (Table 1). Rarely found in other materials, they are characterized by a high reaction capacity with binders of the lime. Furthermore, its potentially dangerous or toxic elements are immobilized when it is used in a stabilized form with lime [163]. In addition, other factors must be considered. For example, the large amounts of material available ready for use after its formation, requiring no processing processes, except for occasional drying in the presence of excess moisture. The main effort to reduce the environmental impacts resulting from the disposal of ash in the environment has been aimed at evaluating the potential of ash for use in different industrial processes [164]. \begin{table}
\begin{tabular}{c c c c} \hline
**WASTE** & **CHARACTERISTICS** & **USE** & **ADVANTAGES** \\ \hline \multirow{4}{*}{**Background ash**} & By-product of coal & & \\  & combustion: particles with a & & Cement & Energy saving \\  & size of 0.09 mm to 20 mm; & light aggregate & Increased production capacity for \\  & angular shape; very porous & Sub-base & relatively lower capital expenditure \\ \hline \multirow{4}{*}{**Fly ash**} & By-product of coal & & & Energy saving \\  & combustion, smaller particles & & & Increased production capacity for \\  & carried by flue gases into & & & relatively lower capital expenditure \\  & chimneys. & asphalt filler & & Fine texture \\  & & Brick & & Low specific mass \\  & & & Brick & Ease of combination with free lime \\ \hline \end{tabular}
\end{table} 
Table 1: Overall assessment of waste and by-products [165; 166]. Ash is a by-product that is classified, according to the standard, as industrial waste. That is, it originates in the activities of the different branches of industry, such as metallurgical, chemical, petrochemical, paper and food. However, according to the standard, for its classification the leaching and solubilization tests must first be carried out. These tests will indicate if the waste presents any type of danger, flammability, toxicity, reactivity or pathogenicity, all reactions that can be harmful to the environment. Solid waste is classified as Class I Waste, which represents hazardous waste, Class II Waste, which represents non-inert waste, and Class III waste, which is inert waste [15].