# Performance Evaluation of Steel Slag Asphal Mixtures for Sustainable Road Placement Rehabilitation
## 1 Introduction  
The need to accelerate the convergence to the 2030 Agenda for Sustainable Development Goals (SDG), adopted by the United Nations Member States in 2015, is becoming increasingly urgent. Numerous technological innovations have emerged from research in recent years, contributing to a circular economy and more sustainable and resilient human activity. Moving from theory to practice is crucial. Simple and easy-to-implement technologies should be promoted. Transport infrastructures (e.g., roads, railways, airfields, pipelines) are essential for the development of society and must contribute to the SDG. To maintain the quality of infrastructures, every year, resources are consumed on a large scale worldwide in construction and maintenance actions. Using waste materials in transport infrastructure applications is a promising and efficient strategy [1; 2]. Waste generation is a global environmental problem, and the steel industry is one of the critical contributors to this negative impact. In addition to moving a large volume of raw materials and consuming energy, it also generates large amounts of solid waste, such as slags. The steel industry produces two main types of slag: Blast Furnace Slag (BFS) and Steel Furnace Slag (SFS). The total production of BFS from SFS in 2018 in Europe was 20.7 and 16.3 million tons, respectively, according to the latest production survey carried out by Euroslag, the European association representing producers and processors of metallurgical slag [3]. Nowadays, the disposal of this waste in landfills is not feasible due to the high volumes being generated, the high costs involved, and the environmental inconvenience it entails. Thus, several alternative solutions are being studied to enhance these by-products for other applications, promoting the environmental sustainability of this material. The construction and rehabilitation of road pavements is a significant consumer of aggregates, not only fornon-bonded sub-base and base layers but also for asphalt or hydraulic mixes. To promote more sustainable solutions, pavement engineering has been using new alternative materials to reduce the consumption of non-renewable natural resources. In the case of SFS, 70.6% of production in 2018 has already been used in road construction in Europe [3]. In the USA, 7.7 to 8.3 million tons of steel slag is used annually, mostly in granular bases or as a construction material [4]. Besides physical and mechanical properties, complementary studies related to expansibility and environmental evaluation should be addressed when dealing with non-traditional materials to prove their adequacy for construction purposes. In the case of steel slag, one of the main concerns is the propensity for expansion, which can cause detrimental damage to materials. In Portugal, electrical arc furnace steel slag is submitted to maturation periods to become an inert material. After this process, SSA acquires the CE marking as an aggregate for use in civil engineering work and road construction according to EN 13242 [5]. In this work, the SSA used in the experimental program was previously submitted to leaching tests, which confirmed that it behaves as an inert aggregate [6]. The feasibility of using steel slag aggregates (SSA) in asphalt mixtures, substituting natural aggregates, has been extensively conducted in the literature concerning mechanical and environmental aspects [7; 8; 9]. Two main perspectives for the use of SSA can be achieved: a perspective of promoting sustainability through the reduction of the consumption of natural resources (substitution of natural aggregates) [10; 11]; and a more ambitious perspective related to the promotion of multifunctional pavements by inducing self-healing or self-sensitive properties of the asphalt mixtures [12; 13; 14]. Several authors have confirmed that incorporating SSA into asphalt mixtures did not impair pavement performance.