## 4 Conclusions  
The conducted research showed that sieving changed the properties of biomass fly ash. Those changes influenced the properties of cement mortar with the addition of activated materials and enabled the use of more fly ash in the binder composition without detrimental effects on mechanical properties:  
1. Sieving increased the amount of CaO and decreased the SiO\({}_{2}\) content in the oxide composition of fly ash. In the case of fly ash sieved with a 63 \(\upmu\)m sieve, the amount of lime increased by 9.3 p.p. compared with untreated fly ash. SEM/EDS analyses showed that most CaO is in the form of small particles. 2. Sieving through a 125 \(\upmu\)m sieve reduced the amount of used initial material to 77%, while sieving through a 63 \(\upmu\)m sieve left for usage around 63% of the untreated fly ash. 3. Sieving substantially increased the specific surface of fly ash. In the case of fly ash sieved with a 63 \(\upmu\)m sieve, the increase was twofold, and in the case of fly ash sieved with a 125 \(\upmu\)m sieve, the increase was 31.9%. The grain size distribution of fly ash sieved with a 63 \(\upmu\)m sieve was smoother, which increased the packing effect of the additive. 4. The 90-day Strength Activity Index of both activated fly ash did not achieve the 85% threshold. Nonetheless, the 28-day and 90-day Strength Activity Indices of both activated biomass fly ashes increased by 15-20 p.p. 5. The consistency of fresh mortar decreased when sieved fly ash was used in the cement mortar mix. However, contrary to untreated fly ash and fly ash sieved through a 125 \(\upmu\)m sieve, only fly ash activated through sieving with a 63 \(\upmu\)m sieve decreased consistency values with the increase in the amount of additive in the mix. 6. The utilization of sieved fly ash in the cement mortar mix increased the early tensile and compressive strength of the hardened material. The 25% b.m. additive of activated fly ash resulted in similar mechanical properties as those of the control specimens. The 2-day compressive and tensile strength results of untreated fly ash were about 35% smaller than those of the control specimens. 7. The 90-day compressive strength results of specimens with 15% b.m. of activated fly ash were similar to those of the control specimens. Only 5% b.m. of untreated fly ash could be used without a detrimental effect on the late compressive strength of cement mortar. 8. Water absorption and bulk density were only slightly and not explicitly affected by the size of the sieve used during activation. 9. TG analyses showed an increase in the amount of C-S-H and ettrinigite phases for mortars with a 15% and 25% b.m. addition of fly ash sieved with a 63 \(\upmu\)m sieve, which could explain its influence on the increase in the early and late mechanical properties of cement mortar. Activation through the sieving of biomass fly ash enabled an increase in the utilization rate of the additive from 5% of binder mass to 15% without a detrimental effect on the mechanical properties of cement mortar. The results of the consistency tests show that biomass fly ash, as with any other concrete additive, requires checking before its usage in a concrete mix. The high alkali content in fly ash requires an additional checking of alkali-silica reactivity, and the high content of lime requires an additional checking of soundness. **Author Contributions:** Conceptualization, J.P. and M.L. ; methodology, J.P. and M.L. ; software, J.P.; validation, M.L. ; formal analysis, J.P.; investigation, J.P.; resources, M.L. ; data curation, J.P.; writing--original draft preparation, J.P.; writing--review and editing, M.L. ; visualization, J.P.; supervision, M.L. ; project administration, J.P.; funding acquisition, M.L. All authors have read and agreed to the published version of the manuscript. **Funding:** The study was performed as part of a research project conducted at the Bialystok University of Technology and was funded by the Polish Ministry of Education and Science. **Institutional Review Board Statement:** Not applicable.