**Katarzyna Kalinowska-Wichrowska, Marta Kosior-Kazberuk \*and Edyta Pawluczuk**

Faculty of Civil and Environmental Engineering, Bialystok University of Technology, 15-351 Bialystok, Poland; k.kalinowska@pb.edu.pl (K.K.-W.); e.pawluczuk@pb.edu.pl (E.P.)

**\*** Correspondence: m.kosior@pb.edu.pl; Tel.: +48-797-995-935

Received: 21 November 2019; Accepted: 18 December 2019; Published: 21 December 2019

**Abstract:** The process of recycling concrete rubble is accompanied by the formation of a large amount of fine fraction, which cannot be reused as aggregate. The results of research on the possibility of using recycled cement mortar (RCM), obtained during concrete recycling, as a cementitious supplementary material, are presented. The experimental research was carried out on the basis of two variables determining the recycling process: *X*1—temperature (range of variation 288–712 ◦C) and *X*2—time (range of variation 30–90 min) of thermal treatment of concrete rubble. The experiment included 10 series of new composites made with RCMs subjected to different variants of thermal treatment, and two additional control series. The best treatment parameters were determined based on the assessment of selected physical and mechanical properties of the new cement composites, as well as the analysis of characteristics and microstructure of RCM. The test results showed that proper thermal treatment of concrete rubble makes it possible to obtain a high-quality fine fraction, which has the properties of an active addition and can be used as a partial replacement for cement in mortars and concretes.

**Keywords:** recycled cementitious supplementary material; comprehensive concrete recycling; recycled fine fraction; rehydration reactivity
