## 1 Introduction
In particular, a superabsorbant polymer (SAP) is directly inserted into the truck mixer in a variable quantity (0.4-0.8 kg SAP/m\({}^{3}\) of concrete, depending on water to cement ratio, type and dosage of cement and climate conditions) and, in a few minutes, the concrete is transformed into a granular material composed of an internal core, given by the original natural aggregate, covered by sand and hydrated cement paste. The properties of these new aggregates are related to the technical performance of the returned concrete, and, obviously, the mortar layer that surrounds the core of the new aggregates greatly influences their characteristics both in terms of size and other physical properties, such as lower density and higher water absorption values. Despite this, the hardened granular material can be used for the production of concrete, as partial substitute for the natural coarse aggregate. The results obtained by the authors indicated that concrete made with a replacement rate of 30% guarantees mechanical performance similar to that of traditional concrete. [PERSON] et al. [23] with a similar procedure investigated the use of a two-component powder product based on polymers and inorganic compounds that can be mechanically mixed in the fresh returned concrete to separate fine and coarse aggregates. The aggregates obtained were used in concrete mixtures at replacement levels of 25%, 50%, 75% and 100%. The results demonstrated that the use of both granulometric fractions (fine and coarse aggregate), regardless of the replacement ratio, do not affect the workability of the fresh concrete but lead to a reduction in compressive strength. In particular, the reduction in compressive strengths achieved at low levels of replacement is quite small (i.e., 25% replacement showed a compressive strength loss of less than 10%). When the flexural strength is considered, the authors recorded an increase in the flexural strength of the concrete made, using coarse recycled aggregates and a very slight decrease in concrete made with the sand fraction. Other studies evaluated the possibility of using washing water (composed of water, cement, and the thinnest part of aggregates) as mixing water for concrete production [24; 25]. In this case, it is important to consider the quantity of fine solids that are introduced in the conglomerate, which can influence the mechanical performance of the final product [26]. In particular, [PERSON] et al. [27] studied the rheological and mechanical properties of concretes manufactured with washing water as a partial substitute of drinking water. Specifically, three different washing waters with solid residue amount in the range 0.13-5.5% by mass were used. The results showed that the content of fine solids in washing water does not cause anomalous air entrapping inside the concrete or significant modifications in the concrete specific weight. Conversely, the presence of fine particles drastically affects the workability of concrete. In particular, a significant loss of workability was recorded in concretes prepared with washing water, having a higher content of suspended solids, especially after 60 min. This can lead to undesirable jobsite water additions. In terms of compressive strength, the results highlighted that the lower content of fine particles guarantees mechanical performance similar to that of a traditional concrete. On the other hand, concretes prepared with waters with an abundant content of fine particles exhibited slightly higher compressive strength values at early ages. This can be ascribed to the improvement of the cement paste-aggregate interface. Same results were also obtained by other authors, such as [PERSON] et al. [28] and [PERSON] and [PERSON] [29]. As far as the cement sludge is concerned, [PERSON] et al. [30] evaluated the feasibility of using this component for the production of mortars with a cement replacement rate of 10% and 20% and as an additive for soil stabilization.