**4. Summary**

Silica fly ash from the Opole power plant is a good alternative to cement. It can be used as an additive in cement mortars. Its share of 10% in the binder had a negligible effect on the compressive strength. It should be noted that a slight increase was even observed in compressive strength. It should be noted, however, that in the case studied, little effect of this additive on the change in consistency was observed.

The blast furnace slag used in this study, which comes from the Katowice steelworks, can be used as an additive in cement mortar. The 6% proportion of this additive to the binder does not significantly alter the compressive strength. An additional advantage is that the use of this additive does not change the consistency of the mortar.

Of the additives analysed, the addition of limestone powder had the greatest effect on compressive strength. Each cement replacement with this additive was associated with a decrease in compressive strength. At a limestone powder proportion of ten per cent, the decrease in Rc was 10 per cent.

The results obtained make it possible to realistically consider the possibility of using silica fly ash and ground granulated blast furnace slag in cement mortar. Such action is in line with the sustainable development strategy. It makes it possible to recover valuable post-industrial by-products and reduce the use of pure, highly emissive CEM I cement. The implementation of similar solutions on a global scale would reduce dust and greenhouse gas emissions.

**Author Contributions:** Conceptualization, W.B. and A.C.; methodology W.B.; software W.B.; validation, S.S. and K.Z.; formal analysis, M.K. and M.S.; investigation W.B. and A.C.; resources M.S. ˙ and S.S.; data curation K.Z. and M.K.; writing—original draft preparation A.C.; writing—review and ˙ editing W.B. and A.C.; visualization W.B. and K.Z.; supervision A.C.; project administration A.C.; ˙ funding acquisition. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** The study did not require ethical approval.

**Informed Consent Statement:** The study did not involve humans.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** 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.
