## 1 Introduction
However, if the temperature is too high, the healing level decreases, probably due to the expansion of the asphalt mixture, which could cause structural defects in the pavement [17; 18; 19; 20]. In the field, due to the continuous traffic flow, usually, the rest periods are not long enough to allow the self-healing to occur, and the pavement temperature rarely reaches the temperature needed for flowing. For this reason, in the last years, researchers have studied several technologies to promote the self-healing process. An example is the capsule healing, in which capsules containing rejuvenor oil are mixed with the asphalt materials [21; 22]. When crack damage appears next to the capsules, they open and release the oil. The bitumen will be rejuvenated and the life of the asphalt mixture extended [23]. Another type of technology consists of heating the pavement, through induction or microwave heating, in order to reduce the viscosity of the bitumen and heal the cracks. In the case of induction heating, electrical currents are induced by adding conductive particles in the composition of the mixture, and the heat is generated by the Joule effect [24; 25; 26]. Several additives and respective dosages were studied in order to maximize the conductivity of the mixture, such as steel wool, steel fibers, graphite, carbon black and carbon fibers [24; 27; 28; 29; 30]. Another heating technique is microwave heating, that was found to be more effective than induction heating to heal cracks in asphalt roads [19]. Microwaves are electromagnetic waves with frequencies ranging from 300 MHz to 300 GHz, and wavelengths from 1 m to 1 mm. In industrial applications, the frequencies 915 MHz and 2.45 GHz are the most commonly used, but for special applications, the frequency 5.8 GHz is increasingly used [31]. When microwave radiation is applied, the polar molecules of the asphalt mixture attempt to line up (polarization) with the alternating electromagnetic field. The inability of this polarization to follow the extremely rapid reversals of the electromagnetic field generates random motion and inter-molecular friction that produces heat [32; 33]. Microwave heating basically depends on the strength and the frequency of electromagnetic field, the dielectric properties of the matter, which represents the efficiency of material in absorbing microwave energy, the conductivity losses and some thermal properties [34]. Several authors studied the dielectric properties of asphalt mixture for different applications, such as deicing [35], density measurement [36], recycling [33], and maintenance purposes [34; 37]. Since the microwave susceptibility of conventional asphalt mixture is low, some authors have added microwave absorbing additives to the mixture for healing purposes [38], such as steel wool fibers [1; 19; 39; 40], steel slag [2; 37; 41], steel shaving [42], ferrite [17; 18] and carbon nanotubes [43]. [PERSON] et al. proposed a classification of different aggregates frequently used for pavements construction in terms of microwave heating efficiency [44]. Steel slag is a byproduct of the steel production process and is widely used in road pavements, due to his excellent mechanical properties, in terms of roughness, shape, angularity, hardness, polishing and wear resistance [45]. From the environmental point of view, the use of steel slag allows to reduce the amount of material to dispose of, and therefore, the incorporation in asphalt mixtures permits to convert a waste material into a resource. Additionally, the inclusion of slag does not imply any additional cost because the cost of the slag is similar to the prices of natural aggregates [38]. However, only a few studies focused on its microwave absorbing properties and its use for healing purposes in asphalt mixtures. [PERSON] et al. proposed a method to increase the content of ferric oxide of steel slag particles, improving the microwave heating efficiency of asphalt mixture [37]. [PERSON] et al.