## 1 Introduction  
In 2019, 28.4 million cubic meters of concrete were produced in Italy [1]. According to the ERMCO Report [2] and [PERSON] et al. [3], in European industrialized countries, approximately 2-3% of the concrete produced returns to the batching plants at the end of the day. Based on this estimate, the returned concrete produced in Italy consists of approximately 710,000 cubic meters per year. Considering that aggregates constitute approximately 80 percent of the total volume of concrete [4; 5], about 1.6 million tons of recycled aggregates could come from the recovery of returned concrete. The use of recycled materials in the construction sector can lead to several environmental benefits, including sustainable natural resources conservation, less waste disposal in landfills and, finally, the obtaining of new alternative materials capable of totally or partially replacing virgin raw materials [6; 7]. In particular, there are many recycled materials that can be used in construction (e.g., construction and demolition waste, supplementary cementitious materials, geopolymers, steel slags and much more). The recycled aggregates obtained from the treatment of construction and demolition wastes, especially those deriving from concrete fractions, are valid substitutes for natural aggregates in concrete mixtures and in engineering works [8; 9; 10]. However, it is necessary to take some precautions, as, compared to natural aggregates, recycled aggregates show lower physical-mechanical performance [11; 12]. In particular, as stated by [PERSON] et al. [9], contaminants and adhered mortar in the recycled aggregates should be minimized and their use kept to a limited percentage (30%). This limit, however, can be increased (i.e., 50% and 100%) if the mix design, batching methodology and the moisture conditions of the recycled concrete aggregates are properly managed. Moreover, the use of supplementary cementitious materials (SCMs), which have been shown to improve the quality of recycled concrete aggregate concrete, is also recommended. As stated by [PERSON] et al. [13] and [PERSON] and [PERSON] [14], SCMs are commonly used in concrete mixtures to replace a portion of cement in concrete. This practice is industry-friendly, resulting in concrete with lower cost, lower environmental impact, higher long-term strength, and improved long-term durability. This is mainly due to the fact that the production of cement involves environmental pollution caused by dust and high energy consumption [15]. So, in order to reduce CO\({}_{2}\) in the cement industry, high volumes of SCMs, such as ground granulated blast-furnace slag (GGBS), fly ash (FA), and/or silica fume (SF), are commonly blended with clinker to make Portland cement. At the same time, to minimize CO\({}_{2}\) emissions, several studies evaluated the possibility of using geopolymers as alternative materials [16; 17; 18; 19]. To date, the management of returned concrete is quite difficult for the concrete manufacturing industry. This is mainly due to the fact that the returned concrete is generally used on site to produce concrete blocks, whose deposit limits storage spaces of the batching plants. Therefore, companies sometimes need to transport part of the returned concrete to landfill to provide adequate storage spaces [20]. Sometimes the returned concrete can be also discharged into the batching plant for subsequent processing. The hardened discharged concrete is crushed and reused as a base for pavement or backfilling operations [21]. Regarding the potential recovery of returned concrete and its components, the literature presents several possibilities. In particular, depending on the type of treatment adopted, aggregates, washing water and cement sludge can potentially be recovered. [PERSON] et al. [22] converted the returned concrete in a granular material that can be used as recycled aggregate for concrete production.