**1. Introduction**

Recycling is currently one of the main ways of managing concrete rubble. The huge consumption of concrete in the world and the fact that its manufacturing consumes a large amount of non-renewable natural resources and other materials, e.g., aggregates (80% of concrete mass), Portland cement (10%), supplementary cementitious materials (3%) water (7%), and its production is responsible for 5% of anthropogenic worldwide CO2 emissions, encourage a responsible approach to searching for methods and possibilities of its effective recycling [1]. The global aggregate production is currently estimated at 40 billion tons, which is leading to depletion of natural resources and high energy consumption and has negative impact on the environment [2].

The use of recycled aggregates (RA) from construction and demolition waste (CDW) in manufacturing concrete and mortar is a viable way to reduce the unsustainable level of consumption of natural aggregates worldwide and avoid landfilling CDW [3,4].

Research on the recycling of concrete is mainly devoted to finding the most effective way to obtain recycled aggregates of the best quality, which usually means removing the impurities from the surface of the natural aggregate grains. The recycled aggregate's quality is closely related to the adhered cement paste properties, since the bond between the natural aggregates and the cement paste is usually weak in the interfacial transition zone [5–8]. It is widely accepted that the presence of cement paste in recycling concrete aggregate causes its worse physical, mechanical and chemical properties compared to natural aggregates. During the process of crushing concrete, even 30–60% of its mass is a fine fraction (<4 mm) containing mainly cement mortar [9]. In order to improve the quality of coarse recycled aggregate, many refining methods (gravity concentration method, heating and rubbing method, mechanical grinding, etc.) have been developed for separating mortar from the surface of its grains [10]. As a result of these treatments, 35% of high-quality coarse aggregate and a total of 65% of the fine fraction containing mainly cement mortar are obtained [11]. An alternative method of treating recycling aggregates was proposed by Tam et al. [12]. They have studied three pre-soaking treatment methods—ReMortarHCl, ReMortarH2SO4 and ReMortarH3PO4—aiming at reducing the old cement mortar attached onto the RA. Experimental results show that the water absorption of the pre-treated RA has been significantly reduced with improvement of mechanical properties of the recycled aggregate concrete. A similar approach to construction waste recycling was presented by Robayo-Salazar et al. [13]. The results obtained in their investigation demonstrate the viability of reusing red clay brick waste, concrete waste and glass waste to produce alkali-activated cements that can be used to fabricate blocks, pavers and tiles. The alkaline activators used were solutions of either NaOH or NaOH and water glass.

According to other test results [14,15], the mortar content in recycled aggregates may be as high as 41% of the volume of the concrete rubble. Many studies indicate that this material cannot be used as a fine aggregate, as it significantly worsens the properties of concretes prepared with its use [16]. Therefore, other methods of using it in cement composites should be sought, and such attempts have been made by researchers for several years.

Gastaldi et al. [17] and Schoon et al. [15] used up to 30% of fine recycled material (grain diameter <63 μm) mainly for the production of Portland clinker, obtaining favorable results in terms of C2S content in the clinker. Considering the significant amount of non-hydrated cement in the fine fraction of recycled concrete, Bordy et al. [18] presented the results of studies of composite in which part of the cement was replaced with finely ground (to diameter below 80 μm) powder made by crushing and milling of cement paste made in laboratory conditions. It was observed that there was about 24% of active clinker in the cement paste that could be rehydrated. Zhao et al. [16] observed the effect of water saturation of fine fraction from recycled aggregates, used as a partial cement replacement, on the properties of fresh mortar and mechanical properties of hardened new material and the microstructure of interfacial transition zone. Test results showed that mortars made of recycled dry fine aggregate were characterized by higher compressive strength than mortars with saturated aggregate due to the reduced interfacial transition zone.

Some researchers [19,20] studied the rehydration reactivity of fine fraction from recycled concrete after its heating at different temperatures. The results confirm that cement paste heated at a sufficiently high temperature is dehydrated. Particularly as a result of portlandite decomposition, reactive lime is formed. As a result of re-contact with water, it regains the ability to rehydrate. Ahmari et al. [21] proposed the production of a new geopolymeric binder from ground waste concrete powder mixed with fly ash, which can then be used with recycled concrete aggregates to produce new concrete. Tests carried out by some researchers [22,23] have proved that recycled mortar contains non-hydrated cement, calcium hydroxide (CH) and dicalcium silicate (C2S), which are capable of hydration and creation of rehydration products. Some research has shown, however, that the mortar remaining on the surface of the recycled aggregates stored outside for a longer time does not show any rehydration reactivity [24]. Nevertheless, a study carried out by Vegas et al. [25] of the mix proportions and characteristics of mortars made with recycled concrete aggregate showed that up to 25% recycled aggregate can be used in cement-based masonry mortars with no significant decline in performance and no new admixtures or higher cement content required. Braga et al. [26] have analyzed the behavior of cement mortars using fine recycled fractions as a substitute for natural sand. In this study, 15% of the required natural sand was replaced by recycled aggregate. An increase in compressive strength with a simultaneous decrease in modulus of elasticity and an increase in water absorbability in comparison to traditional cement mortars were observed.

Considering the need to manage whole concrete rubble, the authors have developed a method for comprehensive recycling of concrete. The method allows obtaining high-quality secondary aggregate and a fine fraction that can be used as a partial cement replacement. Research on the effect of recycled aggregate on new concrete properties has been described in [8].

The aim of the now presented research work was to determine the effect of thermal and mechanical treatment of concrete rubble on the properties of the fine fraction and to assess the possibilities of using waste cement mortar as cementitious supplementary material. For this purpose, two variables have been analyzed: calcination temperature and time of thermal treatment of concrete rubble. The objects of the study were the composites in which part of the Portland cement had been replaced with recycled cement mortar (RCM) after thermal treatment. The RCM applicability as a reactive supplementary cementitious material was assessed based on such composite properties as compressive strength, flexural strength and water absorbability. X-ray diffractometry (XRD), differential thermal analysis (DTA), thermogravimetry analysis (TG) and scanning electron microscopy were used to characterize the microstructure of RCM and also to explain its rehydration reactivity. The conducted research partially resulted in issuance of a patent (PAT 229887 [27]).
