**2. Overview of recycled cement production**

The first published work concerning the specific recovery of cementitious materials regarding cement recycling was probably presented by Splittgerber and Mueller [21]. The authors suggested exploring the inversion of cement hydration based on previous studies concerning the rehydration of hardened concrete subjected to fire temperatures. In fact, as mentioned, various studies have for long been carried out in this domain [16–18]. Basically, the thermal activation turns the cement hydration into a reversible process, obtaining dehydrated compounds with similar characteristics to those of the original clinker phases [21]. As better discussed in Section 3, the thermal activation explores the chemical transformations of the cement paste that occur at different temperatures, namely involving dehydration, dehydroxylation and decarbonation stages [22–26]. The production of recycled cement essentially involves three relevant steps, namely: the separation of the cement fraction from the other constituents of waste concrete; the comminution of the waste cement to an average particle size in the range of common ordinary Portland cements (OPC); thermoactivation of waste cement into RC. The closed circular economy involved in RC manufacture is illustrated in **Figure 1**.

One main obstacle that has hindered the production of recycled cement at an industrial scale is related to the individualization and separation of waste concrete constituents. This is not only a challenge for the production of RC, but also for obtaining high quality recycled aggregates. In fact, the contamination of aggregates by adhered cement paste increases their porosity and waste absorption [27], with repercussions on the durability, shrinkage and control of fresh concrete workability [28, 29]. For this reason, current recycled concrete waste is essentially reused as low-quality recycled aggregates for low grade concretes or as backfilling in road base layers and landscape recovery. Therefore, in order to enhance the CDW valorization and encourage a closed circular economy for concrete

#### **Figure 1.**

*Closed circular economy of thermoactivated recycled cement retrieved from hardened waste cement-based materials (adaptation from Carriço et al. [15]).*

production, the development of an efficient separation process is a priority goal. However, despite various attempts, essentially focused on the recovery of cleaner recycled aggregates [30–32], no effective solution has been achieved yet. Most suggested separation methods essentially include mechanical [33] and thermal processes [31]. These methods take advantage of the different physical properties of concrete constituents, such as the crushing strength and thermal expansion, to promote separation. However, these processes are usually high-energy intensive and high contamination levels are difficult to avoid [34]. Nevertheless, some studies have thermoactivated recycled concrete fines (RCF), up to 5 mm, obtained from these processes [33, 35, 36], but the level of contamination may be high, especially when non-siliceous aggregates are considered. Other methods based on microwave heating [37, 38] or high-voltage electrical pulse discharge [39] fail to be easily implementable at an industrial scale. The lack of an effective method for waste concrete separation explains why most studies regarding the characterization of RC have involved the consideration of laboratory produced cement pastes, avoiding the challenging stage of concrete separation. Recently Bogas et al. [40] have patented a novel easily implementable and cost-effective separation process that is reported to yield waste cement with less than 12% aggregate contamination, by volume, and high quality recycled sand with as low as 3% of adhered cement paste [41].

In a second stage of RC production, waste cement is subjected to gridding, usually by means of ball milling as done in the cement industry [42, 43]. Some authors opt to previously oven dry the waste cement before gridding, since it reduces the baling phenomenon and wall mill adhesion [36, 44, 45]. In other studies, the RC grinding was performed after thermal activation [42, 46–49], however this turns the thermal process less effective and may lead to less homogeneous RC.

The increase of RC fineness enhances its rehydration reactivity, which leads to denser microstructures [50, 51]. Moreover, reducing the particle size of porous RC particles decreases their absorption properties. Therefore, it is recommended to produce RC with at least the same fineness range of OPC. However, this goal is not easily achieved in laboratory mills, especially when large amounts of RC are required. For this reason, most studies have considered particle sizes up to 150 μm. Zhang et al. [43] suggested the intergrinding of waste cement with slag (of higher hardness) in order to prevent the waste cement agglomeration and improve the fineness level.

The RC production conditions adopted by different authors are summarized in **Table 1**. The thermal treatment typically follows a thermal curve initiated by a heating rate of 5–10°C/min, followed by a residence time at maximum temperature and the respective cooling. It is expected that a lower heating rate will favor a more effective dehydration, but optimization of this parameter has never been reported [26]. However, the maximum treatment temperature and respective residence time are the main factors affecting the complete dehydration process at a given stage [59].

The first studies in this domain considered a wide range of thermoactivation temperatures, from as low as 200°C to over 900°C [46, 49, 60]. Later research has been focused on a narrower range, between 500 and 800°C [19, 33, 35, 43, 47, 61, 62], in order to comprise the phases of C-S-H dehydration and CH dehydroxylation, without relevant decarbonation. Optimal treatment temperatures have been reported to be in the range of 600–700°C, ensuring high rehydration ability and low thermal energy consumption [2]. The residence time has ranged from 1 to 8 hours in literature, although 2–3 hours is most often adopted [44, 46, 48, 49, 54, 55, 60]. The influence of the residence time and treated temperature on the mechanical strength of mortars produced with 25% of RC from the cement fraction of waste
