## 1 Introduction  
Construction and demolition waste constitute a very wide and varied range of materials. The fraction that contains concrete and bricks is the more suitable to be employed as a replacement for natural aggregates in several applications, including fillings, subgrades, foundation layers and bases of road pavements [1].  
The purpose of this paper is to define the optimal mix design of the mixtures to be used in road embankments, foundations and unbounded bases, according to the UE standards. Therefore, this study evaluates the use of recycled aggregates (construction and demolition waste (CDW) with eventual addition of electric arc furnaces (EAF) slag) as partial substitutes for natural aggregates to achieve environmentally sustainable mixtures.  
According to Eurostat 1, construction and demolition waste (CDW) constitutes, in absolute terms, the most significant flow of special waste produced in Europe. It has been estimated that in Europe 820 million tons of CDW are produced yearly and they represent about 46% of the total waste. For 2016, waste production in the EU was around 900 million tons (Mt), compared to 2014 (858 Mt). This value represents about 35% of the total EU special waste production of the same year [2]. For this reason, the European Commission has given priority importance to the management of CDW and has proposed to decrease the weight of CDW recycling by at least 70% by 2020 [3].  
One of the most popular and suitable applications of CDW deals with the production of recycled aggregates [4; 5; 6; 7] for road construction (embankments, pavement foundation layers and pavement unbounded bases) [8; 9; 10; 11; 12; 13; 14]. Their employment is gaining a growing interest due to their environmental and economic benefits [14; 15; 16].  
Several studies in literature have also highlighted that the substitution of natural aggregates with the artificial ones results in a decrease of the mechanical strength [17; 18; 19; 20; 21] while in other studies their employment improve the properties [22]. In addition to CDW and other several waste materials such as recycled glass [23], waste polyurethane foam [24] and used tyres [25], used in pavement layers, EAF slag is also employed in bituminous layers as natural aggregates substitute in subbase and base layers [26; 27; 28; 29; 30; 31; 32] in unbound layers. Another technique that recycles used aggregates for employment in road pavements is the reclaimed asphalt pavement (RAP) [33] and the soil-cement, which employs existing material on the layout of the road, preserving the quarries [34].  
The recycling process and the chemical composition of aggregates strongly affect their final properties; therefore, it is important to improve the recycling process that has several benefits including the demand reduction for new resources, the reduction of production energy costs, and the possibility to recycle waste which would otherwise be landfilled [35; 36]. It is well known that the granulated slag is not only a by-product, but also has an excellent resistance to fragmentation [37; 38]. Its use in this field allows one, therefore, to reduce the exploitation of limited natural resources (natural aggregates) and to reduce the potential quantities of waste.  
Considering that, in 2013, Italian steel production amounted to 27.3 million tons, of which about 17.9 million tons came from EAF, a production of over 3 million tons of slag per annum was recorded [39]. Data for 2010 show that the slag coming from the electric furnace cycle was used above all for the construction of road subgrades (38%) and cement concretes (28%), while the other part was employed for the construction of asphalt concretes (13%) and of road embankments (13%) [40].