## 1 Introduction  
Self-compacting concrete (SCC) is a type of concrete with high-performance fresh behavior [1]. Its high flowability and low viscosity allows free flowing and proper formwork filling without compaction, which in turn saves energy [2]. This optimal fresh behavior makes it the preferred type of concrete when there are large pumping heights [3], when the reinforcement of a structural element is very heavy [4], and in the precast-concrete industry, since the reduction of energy consumption due to vibration allows for cost savings [5]. This high flowability is achieved with a specific composition, which is based on a high proportion of fine aggregate (size less than 0.063 mm) [6], an adjusted content of coarse aggregate [7], and the use of superplasticizers [8]. SCC composition has major importance, since any modification to it alters SCC fresh behavior [6, 7], which may even cause the concrete mix to lose its self-compactability [9]. Thus, the impact of any alternative raw material must be examined in great detail:  
* First, the so-called alternative or sustainable aggregates can be mentioned. Their usually irregular shape causes them to exhibit greater friction with other SCC components when dragged by the cement paste [4, 10]. The cement paste of SCC therefore needs to be denser and more compact in order to be able to achieve proper flowability [11]. They also affect the granular skeleton and, in turn, the packing level when fresh [12, 13]. Furthermore, each of them has its own peculiarity. For example, recycled concrete aggregate shows high water-absorption levels [14], which, if not compensated for in the mix design, will make SCC insufficiently flowable [15]. Another example is electric arc furnace slag, an aggregate with a high density [16] that requires SCC to have a cement paste with a high proportion of aggregate particles of less than 0.50 mm to reach a sufficient high dragging capacity [17]. * Second, there are alternative binders. They enable the cement content of SCC to be reduced, thus improving its sustainability [18; 19]. However, they have different characteristics than ordinary Portland cement, which alter the cement-paste dragging capacity [18]. Some of the most studied examples are ground granulated blast-furnace slag and silica fume. Ground granulated blast-furnace slag has a higher ground fineness than conventional cement (higher specific surface area) [20], which decreases the compactness of the cement paste [21]. This in turn means that a SCC with good performance in the fresh state can only be achieved if the coarse-aggregate content is adjusted, commonly reducing it [22]. Silica fume tends to agglomerate [23], hindering the creation of a workable cement paste [24]. In this case, a proper mixing process that ensures its adequate dispersion is key to achieve self-compactability [25]. * Fibers are the last concrete component to be highlighted. These elements act as a three-dimensional reinforcement in hardened concrete, which increases flexural strength, reduces shrinkage and improves durability by limiting the formation and propagation of cracks [26; 27]. However, their impact on the fresh state of the SCC is negative, as they interfere with the movement of the rest of the SCC components, mainly hindering the flow of the coarse aggregate [28]. A sufficiently compact cement paste in the fresh state by a proper amount of fine aggregate and adding an adequate proportion of fibers is key for SCC manufacturing when adding fibers [9; 28]. The blades from dismantled wind turbines currently cause a great environmental impact, as the recycling of their constituent materials is not easy [29; 30]. These constituents are basically glass fiber-reinforced polymer (GFRP), balsa wood, and different types of polymers, which are usually glued together [31]. Furthermore, the need to find a solution for the recycling of such wind-turbine components is becoming increasingly pressing. In Europe alone, it is estimated that 77,000 wind-turbine blades will need to be recycled over the next 20 years [32].