## 1 Introduction  
The construction industry has put considerable effort over the past years into becoming more sustainable [1]. The sustainability efforts of the industry involve practices such as energy conservation, waste minimization, and recycling. These practices apply to all aspects of construction activity, including cement and concrete production, and one important sector of concrete production is ready-mixed concrete. It is estimated that ready-mixed concrete production accounts for 350 million tons per year in the EU in 2016 [2], and the waste associated with this production varies greatly depending on local construction practices and is estimated at about 0.5-2% of the total production [3]. The waste streams generated from the ready-mix concrete plants (RMC) also vary, depending on the waste management strategies employed. The literature indicates several waste management strategies for RMC plants: some strategies include the use of the washing-out process for reclaiming the aggregates from the fresh concrete returns [4], the direct use of the fresh concrete returns in downgraded concrete products [5], and water reclamation by pressuring the sedimentation tanks [6]. The main solid wastes generated from RMC are hardened concrete from trial mixes or concrete testing, returned fresh concrete, and sludge fines from the washing of trucks. All of these materials could be used, potentially, in new construction products as recycled aggregates [7; 8; 9]. The use of recycled aggregates in concrete is of paramount importance for the sustainability of the construction industry, either seen from the point of view of natural resource scarcity [10] or from the point of view of waste and landfilling minimization [11]. As in most recycling processes, it is not an easy task; recycled aggregates originate from various sources and their quality level is not guaranteed [12]. Typically, they have inferior properties compared to natural aggregates [13; 14] and, furthermore, they pose some technical difficulties (contaminants, higher absorption, etc.) [15; 16]. However, a great number of researchers have explored the possibilities of using recycled aggregates in concrete in a beneficial way, either by suggesting ways of upgrading the product [17; 18; 19; 20] or by categorizing and directing the proper quality of recycling aggregates to a suitable application [21; 22; 23]. The present report focuses on three main waste products from RMC, in the form of recycled aggregates from hardened good quality concrete (HR), recycled aggregates from hardened fresh concrete returns (CR), and recycled cement sludge fines from the washing of the concrete trucks (CSF). HR was produced by crushing test concrete specimens, typically used in the quality control of RMC concrete batches. In order to have a good-quality recycled aggregate, concrete specimens with an original compressive strength of 40 MPa or higher were selected for crushing, as it is shown from the literature that the original strength of the concrete influences the properties of the recycled aggregate and of the new concrete [24; 25]. CR was also produced from crushing hardened concrete; however, prior to unloading the trucks, it is common that a considerable amount of water is used to ease the placing process. Therefore, CR originated from a lower-strength concrete than HR and was expected to be of lower quality. [PERSON] et al. (2012) suggest that CR aggregates should be used in concrete mixtures with a w/c ratio as low as 0.35. HR and CR were tested for their suitability as concrete aggregates before substituting natural aggregates in test concrete mixtures. CSF has been found to consist mostly of hardened and unhydrated cement grain and fine aggregate [26]. It is a fine material which can be easily ground and sieved to a small size and has the potential to be used as a construction material [27]. In the present report, HR and CR were considered as recycled concrete aggregates (both coarse and fine) and were tested in concrete mixtures.