## 6 Conclusions  
According to the regulations in force in Poland (Journal of Laws of 2016, item 108), which implement the Directive of the European Parliament and of the Council (EU/2010/75), the resulting fly ash from the incineration of sewage sludge, after meeting certain requirements, can be used as an additive in the preparation of concrete mixtures. The studies conducted so far indicate that the use of ashes from the thermal treatment of sewage sludge as an additive to concrete, allows for comparable properties to concrete produced based on fly ashes from coal combustion or co-combustion to be obtained. Research has shown that fly ash can perfectly replace cement, a hydraulic mineral binder obtained from mineral raw materials [82; 83]. By analyzing the results and the observations obtained during the conducted experimental studies, it was found that:  
1. The fly ashes obtained from the thermal transformation of sewage sludge produced in the sewage treatment plant and used for the preparation of ordinary concrete had a positive effect on its properties: compressive strength, depth of water penetration under pressure, and carbonation.
2. Concrete produced based on fly ash from sewage sludge combustion was characterized by a compressive strength comparable to that of the reference concrete without addition after 28, 56 and 720 maturation days. The increase in strength over time resulted in an increase in the class of concrete from C20/25 to C35/40.
3. No explosive spalling of the samples occurred during heating. This was likely due to drying the samples to a constant weight before heating them.
4. The compressive strength tests conducted after annealing showed that the addition of fly ash did not affect the strength. The obtained results were similar in the temperature range 300-700 \({}^{\circ}\)C. It has been shown that with the increase in heating temperature, the strength of the samples decrease, regardless of the amount of additive in the form of ash.
5. Fly ash from thermal sludge treatment does not meet the requirements of PN-EN 450-1:2012. However, there are no legal regulations regarding the physical and chemical properties of ash from sludge incineration; nor are there specifications concerning the possibility of their use in concrete technology. The highest percentage of the ash samples were oxides of silicon, calcium, phosphorus, and aluminum.  
The conducted experimental work confirmed the positive effect of fly ash from the thermal conversion of sewage sludge on selected properties of concretes produced with their participation. However, information regarding the testing of resistance of these concretes to high temperature is limited. The materials presented in this article constitute the basis for further research in this field. Due to the lack of regulations and standards for the use of fly ash obtained from the thermal treatment of sewage sludge, further detailed interpretation of the results requires supplementary studies.  
Author Contributions: Author Contributions: Conceptualization, G.R., P.O.; methodology, G.R., P.O.; software, G.R. and P.O.; validation, G.R. and P.O.; formal analysis, G.R. and P.O.; investigation, G.R. and P.O.; resources, G.R. and P.O.; data curation, G.R. and P.O.; writing--original draft preparation, G.R., P.O. and M.Z.; writing--review and editing, G.R., P.O. and M.Z.; visualization, M.K. ([PERSON]) and A.P.; supervision, M.K. ([PERSON]). All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
The data presented in this study are available on request from the corresponding author.  
The authors declare no conflict of interest.