## 5 Results and Discussion
### Compressive Strength  
Figure 8 shows the results of the average compressive strength of the concrete samples with a different content of fly ash taken from the thermal treatment of sewage sludge. The test was performed over three periods of concrete maturation, after 28, 56 and 720 days. The samples were stored in a concrete care bath in water at room temperature \(\pm\)20 \({}^{\circ}\)C.  
Figure 7: Concrete mix consistency. Considering the pozzolanic properties and the physico-chemical composition of the used fly ashes from the thermal treatment of sewage sludge, it was observed that the higher concentration of SiO\({}_{2}\), Al\({}_{2}\)O\({}_{3}\) and Fe\({}_{2}\)O\({}_{3}\) and lower P\({}_{2}\)O\({}_{5}\), and CaO has a positive effect on the increased compressive strength of the concretes produced. The lowest compressive strength equal to 41.51 MPa after 28 days of maturation was achieved in the concrete samples FA5%; while the highest one, equal to 43.55 MPa, was seen in concrete samples in which 10% of the cement was exchanged for fly ash. Compared to the reference concrete samples, which reached the value of 42.65 MPa, the decrease in strength was 2.7% and the increase was 2.1%. The highest compressive strength after 56 days of maturation, equal to 45.64 MPa, was achieved in the concrete samples FA10%; while the lowest strength, equal to 42.56 MPa, was seen in the concrete samples in which 20% of the cement was replaced with ash taken from sewage sludge. Taking into account the longest maturation period (720 days); the highest compressive strength equal to 51.23 MPa was achieved in the concrete samples FA10%, and the lowest strength equal to 44.33 MPa for the FA20% samples--similar to 56 days of maturing. Compared to the strength determined after the first maturation period, the strength of the FA10% samples increased by 17.6%, and the other samples only by 3.9%. The addition of fly ash from the thermal transformation of sediments increases the strength of the concrete even after the period of standard maturation, i.e., 28 days. It has been proven in the research that the addition of up to 10% (inclusive) of ash has a positive effect on the increase in strength over time. When increasing the amount of the additive, the increases in strength over time are not as intense. The results of these studies were also confirmed by the authors in previous publications concerning fly ash from the thermal transformation of sludge taken from other wastewater treatment plants [35, 60]. Figure 9 shows the increase in compressive strength over time. Based on the obtained results, it was found that the greatest increase of compressive strength, at 17.5% after 720 days of maturation, was achieved by concrete FA10%; while concrete FA20% obtained the lowest one, equal to 4.2%. According to the information presented in other works, the optimal amount of fly ash from thermal treatment of sewage sludge in cement composites ranges from 5% to 20%. The presence of ash in an amount exceeding 15% by weight of the cement mass delays the setting process of the slurry and slows the growth of the compressive strength of concretes compared to composites made using only Portland cement. However, by extending the maturation time, the strength required for structural concretes can be obtained [11, 35]. Figure 8: Compressive strength after 28, 56 and 720 days of maturation. Other authors, as well as our previous research into the possibility of using fly ash from the thermal transformation of sewage sludge to improve compressive strength, has confirmed its positive influence. Combustion ashes show pozzolanic activity after a longer period, which makes them eligible for active mineral additives. The maximum exchange of the binder--cement to ash--is 5-20% [11; 21; 24; 31; 60]. The use of SSA ashes with a high CaO content, while maintaining the appropriate W/C ratio (0.5), means obtaining mortars and concretes with mechanical properties similar to the reference samples [70].