**1. Introduction**

Road construction is an essential activity for the economic development of a nation and the enhancement of social welfare. Moreover, road transport accounts in different countries for a high percentage of total goods transport, being essential for short and medium distance communication. Therefore, the construction of higher quality and with greater safety roads for vehicles is an unquestionable fact [1,2]. However, this type of infrastructure affects the environment throughout its life cycle assessment [3].

The environmental impact produced by the construction of roads begins with their laying out, altering the landscape. Subsequently, for construction a series of materials are required in significant quantities which are mainly extracted from nearby quarries. In turn, during the manufacture of bituminous mixtures creates CO2 emissions and fossil fuels are consumed. Transport equipment, extension of the bituminous mixture and compaction also represent an important source of greenhouse

gas emissions. Once the infrastructure has been executed, the conservation and maintenance work [4], as well as the continuous flow of vehicles, implies a significant effect on the environment during their working life. At the end of their working life [5], the aged materials are removed and dumped in landfills in most cases, without taking advantage of the usefulness they still offer [6].

According to the scheme detailed above, corresponding to the so-called Linear Economy, significant greenhouse gas emissions are produced throughout the life cycle assessment of the road. Consequently, and in line with the new Circular Economy [7], these emissions must be reduced with different methods [8]. Among these different forms of reducing environmental impact is the use of industrial by-products as raw materials [9]. In this way, the extraction of natural materials is reduced, with the consequent decrease in gas emissions, and the deposition of industrial waste in landfills is avoided [10]. Furthermore, the use of the techniques of manufacturing bituminous mixtures more sustainable with the environment and with a much more optimized processes it also offers a significant reduction in environmental impact. In turn, the development of sustainable materials with industrial by-products, with a longer working life and with a higher quality, also creates the reduction of greenhouse gas emissions [11,12]. Finally, the use of aged materials for the manufacture of new materials avoids the dumping of waste in landfills and reduces the extraction of new raw materials. In this manner, the environmental impact is significantly reduced and the flow of materials is closed [13].

In line with the comments above, various investigations have been carried out in which waste has been incorporated for the manufacture of bituminous mixtures and as a substitute for traditional aggregates. Among these wastes are recycled concrete waste [14], copper slag [15], ceramic and brick dust [16], polymer waste [17], recycled glass [18], recovered asphalt pavement [19] and crumb tire rubber [20], among others.

The use of waste is therefore a good option within the Circular Economy that tries to obtain final products of similar quality. However, in this research, Stone Mastic Asphalt (SMA) type bituminous mixtures are developed with electric arc furnace slags and ladle furnace slags in order to improve the properties of the final mixture with respect to those made with virgin materials [21]. This is made possible by optimizing the strength characteristics of the electric arc furnace slag and the cementitious qualities of the ladle furnace slag. Furthermore, the use of industrial by-products derived from the steel of the siderurgical industry allows it to be considered a sustainable material.

Stone Mastic Asphalt (SMA) bituminous mixtures have a discontinuous grading. This discontinuous grading gives them greater resistance to plastic strains, a better surface texture, greater friction of the tire with the pavement [22], greater permeability to evacuate rainwater, and even greater absorption of noise caused by the contact of the tire with the pavement. At the same time, the incorporation of a higher percentage of bitumen compared to other types of discontinuous grading bituminous mixtures, gives it greater resistance to repetitive traction loads and consequently a longer working life [23,24]. This higher percentage of bitumen is achieved by the addition of fibers. These fibers absorb the excess bitumen and prevent it from bleeding out during the working life of the pavement. Therefore, Stone Mastic Asphalt has a high quality and the resistance suitable for use on roads with important traffic during their working lives.

However, the discontinuous grading of the detailed mixture, as well as the required quality, make the use of high-strength aggregates necessary. Aggregates of higher quality and mechanical resistance mainly correspond to siliceous rocks that are difficult to extract and process, producing important CO2 gas emissions in their extraction and continuous wear of the equipment during processing [25]. Therefore, the use of high resistance electric arc furnace slags, with excellent shape and reduced price [26], means an important reduction of the environmental impact [27]. In addition, the coating of the electric arc furnace slag with bitumen, reduces in most cases, the possible leaching of contaminating elements that it may contain.

The electric arc furnace slag has been used in road infrastructures as an aggregate in concrete pavements [28,29], demonstrating good mechanical behavior of the resulting material. They have also been used as substitutes for natural aggregate in different percentages in hot mix asphalts, showing excellent results in terms of workability, rigidity and fatigue resistance [30,31]. At the same time, warm mix asphalt has been developed with electric arc furnace slag [32,33], reflecting the improvement in the mechanical properties of the bituminous mixes manufactured [34]. Stone mastic asphalt mixtures have even been made with partial replacement of the aggregate with electric arc furnace slag [35], demonstrating that bituminous mixtures with slag were more resistant to cracking at low temperatures than those that incorporated natural aggregate.

In turn, siliceous aggregates have less adhesion with the bitumen than calcareous aggregates, mainly due to their chemical composition and compatibility between materials. Therefore, to execute a correct mastic that coats the aggregates, that supports the traction loads during the working life and avoids the bleeding of bitumen, calcareous filler or cement is usually used. Cement is one of the materials which provides greater resistance to mixing; however, its manufacturing is a process with a significant environmental impact, as it is a high source of greenhouse gas emissions. To solve this fact, in this research, ladle furnace slag was used as a filler. Ladle furnace slags have been studied in different investigations as additives to cement [36–38] or even for soil stabilization [39], showing very interesting cementitious properties [40,41]. Nevertheless, very few investigations have been carried out in which ladle furnace slag is used as a filler in bituminous mixtures [39] and even fewer in mixtures of such high quality as Stone Mastic Asphalt.

On the other hand, for bituminous mixture containing a higher percentage of bitumen to have adequate resistance to repeated traction loads and that no bitumen bleeding occurs, cellulose fibers must be incorporated. These cellulose fibers, introduced in a low percentage into the bituminous mix, are capable of retaining the bitumen in the mix and forming a quality mastic in conjunction with the bitumen and filler. Specially treated commercial fibers are usually used for this purpose; however, in this research and with the aim of making a sustainable mix, cellulose fibers that have been discarded by the papermaking industry were incorporated. These cellulose fibers discarded by the papermaking industry have no current use, so in most cases they are deposited in landfills.

In conclusion, this research develops a quality hot mix asphalt, Stone Mastic Asphalt type, for roads with important traffic with electric arc furnace slag as a coarse and fine aggregate, with ladle furnace slag as a filler and with discarded cellulose fibers from the papermaking industry as an additive. For this purpose, the waste was initially characterized and its properties compared with conventional materials. Subsequently, different families of samples were conformed by increasing percentages of bitumen and the physical properties and Marshall Stability of the mixtures obtained were evaluated. Finally, an optimal material combination was obtained for the asphalt mixtures developed, and the advantages of using the waste over virgin materials were compared.

The tests carried out, as well as their quality limits, will be governed by Spanish regulations, which in turn coincide with European regulations. This Spanish regulation corresponds to the Circular Order OC 3/2019 [42] and was selected because of the profusion of these techniques that have reached the Spanish territory, there existing an infinity of success cases. However, the comparison of the results obtained in bituminous mixtures with waste and bituminous mixtures with traditional materials, objectively reflects the quality of the incorporation of the by-products, and the results can easily be extrapolated to other international regulations.

The results showed that the incorporation of electric arc furnace slag, ladle furnace slag and cellulose fibers created an SMA mix with a higher percentage of bitumen and better mechanical performance, compared to the use of traditional aggregates and fillers.
