## 4 Use and Classification of Ash on Pavements
This classification serves to define the procedures necessary for the treatment to be given in the disposal of solid waste resulting from industrial processes. Based on these tests, and comparing the leached and solubilized concentrations with the standards established as the norm, a solid residue is classified as inert or not inert. When the concentration of a chemical element that is considered dangerous exceeds the established limit, the waste is classified as dangerous, i.e., Class I. Characteristics such as corrosivity, flammability and violent reaction with water also classify the waste as dangerous. It can be concluded that, although there are many beneficial applications for ash, they can only be used if they meet the regulatory requirements established in the legislation on solid waste [15]. Since ash is industrial waste, it is classified according to the Brazilian standards that govern the classification of solid waste in terms of how dangerous it is. In the case of coal ash, the ash formation process occurs by direct combustion of coal. Coal is a solid raw material, consisting of two closely mixed fractions: an organic (volatile material plus fixed carbon), and a mineral (clay, quartz, pyrites, carbonates, etc.). The action of heat causes the organic fraction to generate volatiles and coke, while the mineral turns into ash with modified mineralogy. There is loss of water from the clays, the decomposition of carbonates and the oxidation of sulfides. Broadly speaking, the ash is considered to consist of non-combustible carbon components and unburned particles due to the incomplete combustion of pulverized coal [167; 168; 169]. The combustion characteristic of pulverized coal occurs at high temperatures, between 1200 \({}^{\circ}\)C and 1600 \({}^{\circ}\)C, in an oxidizing gaseous environment. The residence time of the particles in the oxidizing flame is an average of two seconds, a sufficient condition for the total or partial fusion of mineral matter. The different temperature zones within the boilers cause the pulverized coal particles to present, after combustion, different characteristics. This results in ash fractions with different physical, chemical, mineralogical and microstructural characteristics, which allows the ashes to be classified into two different types, being light or flying (dry) and heavy (wet). All of the physical and chemical properties of coal ash are influenced by several factors such as coal composition, degree of processing and grinding of coal, type, design and operation of the boiler, ash extraction and handling system, and transformations that occur according to the combustion temperature [170]. Due to the factors presented, the ash will present variations in its composition and physicochemical properties, not only from plant to plant, but from boiler to boiler in the same plant and even in the same boiler at different times. Ash is generated from various organic and inorganic components of the coal feed. Due to the scale of a variety of components, they constitute one of the most complex anthropogenic materials that can be characterized. Approximately 316 individual minerals and 188 mineral groups have been identified in different ash analyzed as documented in Table 2. The ashes are classified according to the gasification or coal burning process in three different types: light ash (or fly ash), bottom ash (heavy ash or wet ash), and finally slag or coarse ash. Light ash (or fly ash) is composed of extremely fine light particles (100% less than 0.15 mm), and which are entrained by the combustion gases of furnaces or gases generated in industrial gasifiers [171; 172]. A large part of these particles is retained by a capture system such as fabric filters, cyclones, electrostatic precipitators, etc. The large units that produce this type of ash are thermoelectric plants and steam plants.