**2. Materials and Methods**

This section describes the materials used for the development of the research, as well as the scientific methodology followed to reach the final conclusions. The final objective is the study of the benefits of incorporating the waste mentioned for the manufacture of SMA-type bituminous mixtures.

## *2.1. Materials*

The materials used in this project are mainly waste and commercial materials. These materials are detailed in Section 2.1, defining their origin, production process and particular characteristics, making possible the reproduction of the present tests.

It should be noted that the industrial waste from this research (electric arc furnace slag (EAFS), ladle furnace slag (LFS) and cellulose fibers) was supplied by the producing company in an unaltered form. The process that has been carried out on these wastes is detailed in the following sections.

In turn, it should also be mentioned that the tests carried out have been executed for different production batches of the waste. In this way it has been confirmed that the physical and chemical properties of the waste are maintained over time. This fact is essential, since if the characteristics of the waste were to be modified to a large extent it would make their use in the construction of road infrastructure unfeasible, since large quantities of materials are consumed and could lead to changes in the final characteristics of the bituminous mixtures. It can therefore be stated that the waste studied maintains its physical and chemical properties over time, unlike other wastes such as sewage sludge, cutting sludge, etc.

Finally, it should be mentioned that the ladle furnace slag, electric arc furnace slag, cellulose fibers, as well as the hornfels aggregates and calcareous filler, were dried at a temperature of 105 ± 2 ◦C for 24 h in order to eliminate the humidity in them. The elimination of the humidity from the materials is intended to avoid introducing more variables into the methodology and to provide objective results. In the subsequent manufacturing process in industry, this humidity of the materials should simply be taken into account in order to take the appropriate corrections, if it was necessary.

#### 2.1.1. Electric Arc Furnace Slag (EAFS)

The electric arc furnace slag used comes from the siderurgical industry located in the region of Andalucía, Spain. These slags have a continuous grading with different particle sizes up to a maximum of 22 mm. The existence of particles smaller than 0.063 mm is negligible, and there are mainly coarse and fine aggregates, in smaller quantities. An irregular shape of the particles can be observed by the processes of their formation.

It may be pointed out that electric arc furnace slag is formed in the metallurgical industry in the first stage called melting and in the electric arc furnace. These furnaces are fed with soft iron or steel scrap. In this melting stage, a series of phases are carried out such as oxidation, to remove manganese and silicon impurities, dephosphorization and the formation of foaming slag. All the impurities are accumulated in this foaming slag. The slag is extracted, forming the electric arc furnace slag after cooling and watering with water.

The production company then crushes the material and performs an economical particle size classification for filler of embankments. These slags are used in the present investigation.

In turn, the mission of the electric arc furnace slag is to replace the traditionally used coarse and fine siliceous aggregate. Therefore, it provides the necessary mineral skeleton of the bituminous mix, and it must be sufficiently resistant to support the repeated compressive loads of the traffic, as well as the roughness to provide good friction between the tire and the pavement. The slag from the electric arc furnace was washed and sieved by different sieves, obtaining the grading fractions necessary for the conformation of the grading curve.

#### 2.1.2. Ladle Furnace Slag (LFS)

The ladle furnace slag comes, like the electric arc furnace slag, from the area of Andalucía, Spain. These slags have a very fine particle size derived directly from their formation process.

Ladle furnace slag is produced in the refining stage, after the melting stage in which the electric arc furnace slag is produced. The refining stage includes a series of phases such as deoxidation, allowing the removal of metal oxides from the furnace, desulphurization and decarburization of the

steel. For this phase to take place, the liquid from the electric arc furnace is transferred to the ladle furnace, being covered with slag, and continuously stirred by blowing inert gas, usually argon. Finally, this slag of much smaller particle size is removed and deposited in the vicinity for cooling.

The ladle furnace slag was taken directly from the producing industry as an undisturbed sample and will serve as a filler for the bituminous mixtures conformed. These ladle furnace slags provide the desired cementitious characteristics, which have been confirmed by various authors. For this purpose, they were sieved after drying at 105 ± 2 ◦C for 24 h by the 0.063 mm sieve.

#### 2.1.3. Cellulose Fiber from the Papermaking Industry

Cellulose fibers are currently an unused waste produced from the cardboard manufacturing industry. These fibers are formed in the process of producing packaging paper from recycled paper. The recycled paper is grinded with water to put the fibers in suspension, and then submitted to a physical separation with different sieves. Finally, a cyclonic separation is carried out. The waste from this cyclonic separation is transferred to a press to remove some of the water contained in the waste. This waste, after being pressed, is the one used in this research and is called cellulose fiber discarded by the paper industry.

Detailed cellulose fibers are the additive that was incorporated into the bituminous mix for the retention of a higher percentage of bitumen in the mix. These fibers have been taken from the production industry and have undergone a process of adaptation for use in bituminous mixtures. This process consists of a washing with a 30% sodium hydroxide solution. This pre-treatment is carried out with a double objective; on the one hand, the organic reactions that could be produced are paralyzed; on the other hand, any natural waxes that could be adhered to the fibers and that would prevent the correct adhesion with the bitumen of the bituminous mix are removed. Once this pre-treatment has been carried out, they are ground to achieve the smallest possible fiber size, making it possible to homogenize them during the mixing process in the bituminous mixture.

#### 2.1.4. Bitumen

The bitumen used is a 50/70 bitumen as defined by European regulations, both numbers being the penetration rate at which it oscillates. This hard penetration bitumen is usually used in the Spanish regions due to the existing hot climates. It is a commercial bitumen without additives. Its technical data can be seen in Table 1.


**Table 1.** Technical specifications of the bitumen used.

#### 2.1.5. Hornfels Aggregate

Hornfels aggregate is a commonly used aggregate on important traffic roads mainly due to its excellent characteristics. This aggregate comes from the area of Andalucía, Spain, just like the other materials.

Hornfels rocks are a type of contact metamorphic rocks, very hard and with great resistance to the cycles of freezing and thawing. It contains a high proportion of quartz, graphite, biotite, iron oxide or feldspars, so it can be considered a quality siliceous rock.

The extraction of this material in quarries, being a hard rock, consumes a great quantity of explosives, since its resistance to fragmentation is high. In addition, its siliceous composition means that treatment and processing equipment often wear out, compared to limestone stone.

It is therefore an aggregate of excellent quality, in which significant greenhouse gas emissions are emitted during its extraction and with which electric arc furnace slag is to be compared. Therefore, its function within the bituminous mixtures created is that of a coarse and fine aggregate. To this end, as with the slag, the aggregate is received from the quarry and washed, to be subsequently sieved by different sieves that can form the selected grading curve.

#### 2.1.6. Calcareous Filler

The problems derived from the lack of adhesion between the siliceous aggregates and the bitumen make the use of filler of limestone type common. Calcareous aggregates have much lower resistance than siliceous ones, as well as a lower resistance to the abrasion caused by the tire. Therefore, its use in important traffic roads is not usual or recommended.

However, its use as an inert filler makes possible the formation of a mastic of acceptable quality that coats the siliceous aggregates and forms a structure capable of withstanding the loads of traffic. Therefore, the function of the calcareous filler in the present investigation is the comparison with the properties of the bituminous mixtures conformed with it, with those of the mixtures conformed with ladle furnace slag.

The calcareous filler supplied by the producing company had a very fine particle size and did not need to be sieved, unlike the ladle furnace slag. This filler was dried at 105 ± 2 ◦C for 24 h to avoid the existence of water during the conformation of the bituminous mixtures.

#### *2.2. Methodology*

The methodology followed in the present investigation is composed of a series of logically ordered tests to obtain objective results on the quality of the execution of Stone Mastic Asphalt type bituminous mixtures with electric arc furnace slag, ladle furnace slag and cellulose fibers from the papermaking industry. To this end, bituminous mixtures manufactured were compared with bituminous mixtures conformed with commercial materials.

Based on this, the wastes were analyzed to determine their physical properties and chemical composition. In this way, the suitability of the materials for forming SMA mixtures for roads with high vehicle traffic was evaluated.

Subsequently, mixtures were conformed with traditional aggregates and with waste, as well as increasing percentages of bitumen. The groups of samples conformed were analyzed to obtain the physical and resistant properties, through the Marshall test.

Finally, and after evaluating the properties of the different mixtures, the optimum combination of materials was obtained for each family of samples studying the advantages of using electric arc furnace slags, ladle furnace slags and cellulose fibers.

The following sub-sections describe each of the research phases in detail.
