1. Introduction
Asphalt materials are regarded as crucial components of contemporary pavement structures. Their exemplary service performances, encompassing comfort driving, minimal noise, optimal skid resistances, and straightforward maintenance, have rendered them a pervasive pavement material [
1]. In Europe, the total road mileage is over 5.51 million kilometers, with over 90% of these roads paved with asphalt. These asphalt pavements support 80% of passenger transport and 70% of inland freight transport [
2,
3]. In China, over 90% of high-grade pavements are constructed with asphalt materials to complete the pavements [
4]. The necessity of the construction of new roads to meet transport demand is increasing, while the task of maintaining existing roads is becoming increasingly onerous. In the period between 2000 and 2021, the total investment in road construction in Europe was approximately EUR 1880 billion, while EUR 1920 billion was spent on road maintenance [
5]. The increasing engineering of new road construction and maintenance work on existing pavements directly results in a notable surge in the consumption of natural resources, including aggregates and asphalt binders [
6,
7]. Consequently, the advancement of greener and more sustainable materials for pavement construction and maintenance has emerged as a pivotal objective.
The utilization of bulk solid waste in road construction is currently regarded as a highly promising avenue for reducing the exploitation of natural resources and achieving green and sustainable development. Steel slag is a typical bulk solid waste, which is a by-product of the steelmaking process. The annual production of steel slag exceeds 120 million tons, representing 12% of crude steel production [
8,
9]. However, because of the poor stability and abrasiveness, it is a kind of solid waste that is difficult to be used in metallurgical slag, and its comprehensive utilization rate is less than 30% in China [
10,
11]. Although there are many studies and cases proving that steel slag can be used in asphalt mixtures, there are still some problems. Some previous studies have reported that steel slag has the advantages of high hardness, good angularity, and wear resistance [
12,
13,
14]. These properties also give asphalt mixtures containing steel slag aggregates excellent slip resistance, high temperature stability, and fatigue durability [
15,
16,
17]. However, some studies have reached different conclusions regarding its resistance to moisture. The results of some laboratory and field studies have shown that steel slag materials are sensitive to moisture damage, especially in powder form [
18,
19]. It was discovered that f-CaO in steel slag can be converted to CaCO
3 by reacting with water and carbon dioxide (CO
2) under moisture erosion. These enriched calcium carbonate deposits can lead to debonding of the steel slag–asphalt interface and, consequently, to damage, contributing to steel slag’s moisture sensitivity [
20,
21]. The specific surface area of powdered steel slag is greater, thereby increasing the likelihood of the swellable component being exposed to the moist environment, which, in turn, leads to damage [
22,
23]. Xiao et al. discovered that steel slag powders can be employed as a partial replacement for limestone filler, and the optimal substitution amount is 25% of the total volume of the filler [
24]. Therefore, it is necessary to develop a filler material that can work with steel slag to avoid the defect of moisture sensitivity.
In consideration of the frequent occurrence of moisture damage to asphalt pavements, the most commonly used treatment method is adding agents that can prevent stripping. Liquid antistripping agents, such as the chemical surfactants of amines, have been demonstrated to have a good antistripping effect; however, they are susceptible to decomposition at high temperatures during the mixing process of over 180 °C, and their durability is highly questionable [
25,
26,
27,
28]. Inorganic antistripping agents, such as cement and lime, are widely used because of their good antistripping effect, heat resistance, and the easy availability of raw materials. Nevertheless, the considerable carbon footprint of materials such as cement has prompted the exploration of alternative materials as a promising avenue of research [
29]. Blast furnace slag is a porous, amorphous silicate melt byproduct of the blast furnace ironmaking process. It has long been used as a supplementary cementitious material in cement concrete due to its similar chemical activity to that of cement, which also allows it to be used as a filler in asphalt concrete and to provide antispalling effects [
30,
31,
32]. Concurrently, the annual output of blast furnace slag is in excess of 300 million tons, representing 35% of pig iron production, which is sufficient to meet the demands of engineering construction [
33,
34]. Because of its favorable gelling properties, blast furnace slag powder is frequently employed as a filler in cold mix asphalt mixtures. Its hydration reaction results in the formation of C-S-H gel, which has the potential to markedly enhance the engineering performance [
35,
36]. Lu et al. and Dulaimi et al. mixed blast furnace slag powder, fly ash, calcium carbide slag, and cement and found that this mixing system can enhance the interfacial adhesion performance, making it more suitable for pavements that are in harsh environments in terms of moisture and temperature [
37,
38]. Shaygan et al. explored the potential of blast furnace slag powder as a filler in microsurfacing mixtures, and their findings deemed acceptable the use of blast furnace slag powder as a substitute for up to 10% filler in a microsurfacing [
39]. Huang et al. conducted an investigation into the potential use of blast furnace slag powder as a substitute for limestone filler in hot mix asphalt mixtures. Their findings indicate that the powder enhanced the elastic recovery and rutting resistance of the asphalt mastic, as determined through rheological tests and viscosity tests [
40]. Ou et al. evaluated the possibility of replacing blast furnace slag powder as a filler using rheological and surface free energy theories. The results demonstrated that the addition of blast furnace slag powder improved the fatigue performance of asphalt mastic under strain loading, increased its fatigue life, and maintained its good performance under long-term loading [
41]. Amin et al. employed it as a filler for dense-graded asphalt concrete, and the results of tests such as the wheel tracking test and four-point bending test demonstrated its suitability for use in asphalt pavements [
42]. The above studies demonstrate that blast furnace slag powder has the potential to be employed as a filler, while its hydration and cementation products enhance the interfacial strength in harsh moisture environments, thereby improving the performance of asphalt mixtures below moisture erosion.
The incorporation of steel slag and blast furnace slag as fillers in asphalt concrete not only represents a resourceful use of solid waste and a reduction in the consumption of natural resources but also serves as an inorganic antistripping agent, thereby enhancing the resistance of asphalt pavements to water damage and facilitating their high-value utilization. This research proposed the use of compounded metallurgical slag powders as fillers in asphalt mixtures to improve the moisture resistance, and the characteristics of the metallurgical slags powder, adhesive properties of the asphalt mastics, and water resistance of the asphalt concrete were characterized. This proposition expands the utilization of metallurgical slags and eliminates the potential adverse effects of metallurgical slag on asphalt concrete.
4. Conclusions
The improvements in the moisture resistances of the asphalt mixtures with compounded recycled metallurgical slag powders were investigated in this research. The above analysis and discussion led to the following conclusions:
Steel slag powder is characterized by a high specific surface area, which is a consequence of its porous structure and uniform particle size. In contrast, blast furnace slag powder exhibits a lower specific surface area but a larger average particle size. The adhesion property of steel slag powder is enhanced by its larger specific surface area, which is over 30% larger than blast furnace slag powders while that of blast furnace slag powders is enhanced by the gelling activity.
Both steel slag powder and blast furnace slag powder have the capacity to enhance the adhesion performance of asphalt mastics. However, the effect of steel slag powder is more pronounced, the maximum force difference of which exceeds 200 N. The antagonistic effect of steel slag powder and blast furnace slag powder on the ability of the adhesion interface to resist moisture damage was observed through the trend of the moisture resistance first decreasing and then increasing.
The invasion of moisture will result in the deterioration of the mechanical properties of the asphalt mixture, especially for the samples with LF, some indicators of which dropped by more than 20%. The incorporation of blast furnace slag powder serves to enhance the resistance of the asphalt mixture to such invasion. Concurrently, the antagonistic effect of steel slag powder and blast furnace slag powder on water is also evident in the scale of the asphalt mixture. The damage caused to asphalt mixtures by dynamic moisture is more pronounced. As the circulation time increases, the damage trend will tend to become more stable, and with the damage level gradually approaching a steady state, the difference in the indexes over time will be less than 5%.
In conclusion, the incorporation of steel slag powder has been demonstrated to enhance the intrinsic mechanical properties of asphalt mixtures. The incorporation of blast furnace slag powder was shown to enhance the resistance of asphalt mixtures to moisture damage, although this may result in an antagonistic effect with steel slag powder. Therefore, it is essential to exercise caution when controlling the dosage in practical engineering applications. The combined addition of steel slag powder and blast furnace slag powder can result in a synergistic effect on asphalt mixture, leading to enhanced mechanical properties and heightened moisture resistance.