**4. Conclusions**

The spent catalyst from the catalytic cracking in the petrol refinery was used as a component of the high volume fly ash binder. Both of the waste materials were mechanically activated prior to the preparation of the new binder and the ground materials had a PSD comparable to the PSD of commercial cement. It was found that the binder that contained 21 mass % of the sFCCC had setting times comparable to the setting times of cement pastes. The use of the sFCCC as the binder component also had a positive effect on the early (2 days) compressive strength of the binder. However, the compressive strength of the binder prepared without sFCCC was higher at later ages. Structural analyses revealed that the main reaction products of the new binders were calcium aluminate hydrate, calcium silicate hydrate and ettringite, while the microstructure of the binders was heterogeneous and relatively porous. Differences in the observed properties of the binders prepared with and without sFCCC were attributed to the high reactivity of the mechanically activated waste materials.

**Author Contributions:** Conceptualization, Z.B.; methodology, Z.B. and J.R.; validation, J.R. and Z.B.; formal analysis, J.R.; investigation, J.R.; resources, Z.B. and J.R.; data curation, Z.B. and J.R.; writing—original draft preparation, J.R. and Z.B.; writing—review and editing, Z.B.; supervision, Z.B.; project administration, Z.B.; funding acquisition, Z.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research work was funded by the Ministry of Education, Science and Technological Development, Republic of Serbia, contract number 451-03-68/2022-14/200053 and by the funding of the EUREKA project E! 9980 INBYCON.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author. The data are not publicly available due to legal restrictions.

**Acknowledgments:** The authors are thankful to Sabina Kovaˇc for performing XRD analyses. The publication cost of this paper was covered with funds from the Polish National Agency for Academic Exchange (NAWA): "MATBUD'2023-Developing international scientific cooperation in the field of building materials engineering" BPI/WTP/2021/1/00002, MATBUD'2023.

**Conflicts of Interest:** The authors declare no conflict of interest.
