## 1 Introduction  
Approximately 90% of construction and demolition wastes (CDW) are currently going to landfills even though they are potentially recyclable [1]. The use of this waste should be a priority to achieve the sustainable development objectives set by the European Commission, although this action is hindered due to lack of facilities and standards, lack of support from governments or lack of users' confidence [1, 2]. The use of CDW as aggregates in concrete production, mostly coarse recycled aggregates (CRA), not only means a saving of natural resources derived from the extraction of aggregate, but also economic savings. Analogously, concrete with electric arc furnace slags (EAFS) as aggregate is based on the use of waste (from the steel industry) that would otherwise be deposited in landfills. In this case, the reduction of CO\({}_{2}\) emissions in the processes without taking into account the transport and manufacturing of the materials can be as high as 35% [3]. On the other hand, CO\({}_{2}\) emissions induced by concrete crushing are not very different from those generated in the production of natural aggregates [4]. To improve the understanding of the paper, the acronyms used and their meaning are shown in Table 1.  
Most CRA are produced by crushing concrete that has ended its service life, i.e., they are composed mainly of natural stone and attached mortar. Typically, CRA are materials with lower density and porosity than natural aggregates (NA) because the attached mortar is less dense and more porous than the natural aggregate that it covers. The average density can be 8% lower and the average water absorption 5-6 times those of the natural aggregates [5; 6]. According to the current Spanish concrete standards, the aggregates' water absorption must be less than 5% to be used in structural concrete [7; 8]. According to [PERSON]  et al. [9], the shape index of NA is 25% and 28% for CRA produced in quarties, although its value depends on the crushing process. Typically, laboratory-produced CRA are made with a single crushing stage (usually a jaw crusher), whilst NA are produced with multiple crushing (primary, secondary and sometimes tertiary). [PERSON]  et al. [10] found that when CRA go through the same crushing process as NA their shape index is expected to be lower than that of NA. The water absorption and the shape index are vital for the calculation of the compensation water to determine the total water/cement (w/c) ratio [11]. There are many studies on the use of recycled aggregate (RA) in the production of structural concrete. Most of them only consider replacement of the coarse fraction of the aggregates, because the fine fraction has a great cohesion and water absorption that make it difficult to control the quality of the aggregates [12] and reduce the workability in the fresh state [13]. The risk of contamination of the finer fraction is also higher [14]. The use of CRA is more common also because it has less porosity and adhered paste. Typically, CRA concrete is around 4%-8% less dense [15; 16; 17], although this effect can be the opposite for high density CRA (e.g., CRA based on EAFS). The water absorption of these CRA concretes can be 500% higher than that of NA concrete (NAC) [18; 19], although it tends to decrease due to the crystallization of hydration products, depending on the crushing age of the source concrete and curing conditions [20; 21; 22]. The fresh workability with CRA is lower than that of NAC for equal w/c effective ratios, so it is important to correct the water content to achieve similar slump without the help of admixtures [16; 23; 24; 25; 26; 27; 28]. One of the main characteristics of this concrete is the presence of three interfacial transition zone (ITZ). One is between the original aggregate of CRA and the cement paste in the source concrete and it is formed by dense hydrates, and in the case of EAFS concrete is of higher quality than with NA [29].