## 4 Conclusions  
The conducted studies and analysis of the results demonstrated that the incorporation of fly ash (MSWI-FA) and cement kiln dust (CKD) as additives in cement mortars significantly influenced their mechanical properties. In mortars with MSWI-FA additions, a reduction in mechanical strength was observed, with the smallest decreases recorded for the lowest MSWI-FA content (10%), indicating an optimal level of addition for short-term mechanical stability. Cement mortar with CKD exhibited greater stability over longer curing periods, with less significant reductions in strength parameters compared to MSWI-FA, except for compressive strength for mortar containing 10% MSWI-FA.  
Combinations of MSWI-FA and CKD, although beneficial in reducing emissions and utilizing industrial waste, led to a significant reduction in mechanical properties of the mortars, particularly at higher proportions of both additives. The best results among the mixtures were achieved by the MSWI-FA-CKD/3-3 series (i.e., after 3% of the MSWI-FA and CKD waste), suggesting potential benefits from minimal amounts of these waste materials.  
The studies also indicated that mortars with CKD are more resistant to environmental factors, such as cyclic freezing and thawing, compared to those with MSWI-FA. The analysis also showed that mortars containing lower CKD content had comparable or slightly lower mass loss compared to mortars containing MSWI-FA, while the simultaneous use of both types of waste resulted in mostly lower mass loss of the samples compared to the control mortars. In summary, the use of MSWI-FA and CKD as cement replacements could thus be environmentally beneficial, but their impact on mechanical properties requires further investigation, especially regarding their synergy and optimal proportions. The results suggest that separate applications of MSWI-FA and CKD yield better outcomes than their combined use. These observations form a foundation for future work on more sustainable cement compositions.  
The incorporation of the discussed waste into the production of building materials reduces the amount of waste deposited in landfills and limits the exploitation of natural resources while simultaneously improving the mechanical properties (after frost resistance tests) and durability of the products. This strategy supports the circular economy, contributing to sustainable development and minimizing environmental impact.  
Conceptualization, M.U.; Methodology, A.P. and M.U.; Software, A.P.; Validation, A.P.; Formal analysis, A.P. and M.U.; Investigation, A.P.; Resources, M.U.; Writing--original draft, A.P.; Writing--review & editing, M.U.; Supervision, M.U. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research received no external funding.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.  
**Acknowledgments:** We would like to thank local companies that provided us with waste materials free of charge for testing.  
**Conflicts of Interest:** The authors declare no conflict of interest.