## 4 Use and Classification of Ash on Pavements  
Bituminous mixtures consist of asphalt binder and mineral aggregates, and their behavior is affected both by the individual properties of these components and by the relationship between them. The binders, which can be asphalt cement or modified asphalt cement (with the addition of modifiers such as polymers and rubber), act as binders for aggregates and as a waterproofing agent, encompassing the mineral particles and forming a cohesive mass. Aggregates, regardless of source, processing method or mineralogy, must be strong (tough and durable) and resist the stresses and abrasion resulting from the application of repeated loads. In this context, the study of the reuse of tailings from the steel industry in the investigation of asphalt mixtures seeks to contribute to reducing the large volumes of this material in landfills. Thus, this solution is an alternative to mitigate environmental liabilities and contribute to the study of an alternative paving material [145; 146]. The demand for road and airport infrastructure materials is enormous, and is often limited to deposits of raw materials such as gravel, soil and sand, among others. Thus, it can be concluded that the supply of so-called alternative materials for the execution of the infrastructure has a great influence on the evaluations of the economic viability of a pavement [147; 148]. Ashes have been used in the industry since the 1930s, when they began to be available in significant quantities. Initially, they were used as raw material incorporated in the manufacture of concrete and pozzolanic Portland cement [149; 150]. However, there is a growing increase in the control criteria by environmental agencies in relation to the exploration of new deposits and the search for raw material, causing the use of waste to increase throughout the world. In general, the two factors being the scarcity of deposits of conventional materials and the availability of large quantities of waste around the world, are the driving forces of the studies that point to the use of these materials [151; 152; 153]. The use of waste in pavement construction is often economically viable, as paving works consume large amounts of material. However, the reuse of materials in this type of work must be done in such a way that the expected performance of the pavement is not compromised, since the residues and by-products differ substantially in their types and properties and, consequently, in their paving applications [154]. The pavement should not be used only as a waste disposal site, only to solve environmental problems. It is necessary to show the advantages of using waste from the point of view of improving the pavement and not only its contribution to nature conservation. Experience and knowledge of the use of waste varies by material. In order to use these materials, engineers, researchers, the generating industry, and environmental and paving agencies must be aware of the properties of the material to be used and the associated limitations in their use and application. The use of waste in paving can be beneficial, both for the generating industry and for the civil construction industry. This use of waste leads to the reduction of costs with the extraction and transport of conventional aggregates, minimizing the environmental impact caused by the construction waste disposal deposits and the preservation of deposits of natural materials. The residues can be reused both in pavements and in the manufacture of pozzolanic Portland cement, in stabilized bases and modified soils for roads, paths and buildings, and also as fillers in bituminous mixtures. Some of the residues already used or studied in pavement layers are mentioned, such as construction, renovation and demolition residues (CRD), steel slag, rubber from waste tires, coal ash from thermoelectric plants, rice husk ash, ornamental rock residues, and petroleum exploration and production residues (drilling gravel, copper slag and oily sludge, among others) [155].