## 3 Results and Discussion
### Strength Characteristics  
Samples with the dimensions of 40 \(\times\) 40 \(\times\) 160 mm were used for the testing of flexural and compressive strengths of the experimental Formulas R1 and R3. The determination of the strength parameters was performed after 28 and 90 days of age of the test specimens, always on one set of the test specimens. Figures 5 and 6 show the results of average flexural strengths of the experimental formulas. The results of flexural strength test of Formula R1 with CEM I 52.5R, which are presented in Figure 5, show that the flexural strengths after 28 days are within the range of 7.30 to 8.13 MPa, with the highest value of 8.13 MPa being found for 50% replacement and the lowest value of 7.30 MPa for 75% replacement of pure mixing water with waste sludge water from the CP. The flexural strength after 90 days ranged from 7.35 to 8.85 MPa. The highest value of 8.95 MPa was achieved for 50% replacement and the lowest for 75% replacement. It is also apparent that after 90 days, there was an increase in flexural strength compared to the comparative value (i.e., 8.10 MPa) for 25%, 50%, and 100% replacement of pure mixing water with waste sludge water from the CP. This phenomenon is not evident for the flexural strengths determined after 28 days of the test samples. Figure 4: Exponential dependence of the initial and final setting time of Formulas R1 and R3. The results of the flexural strength tests after 28 and 90 days of the age of the test specimens prepared according to Formula R3 with CEM II/B-LL 32.5R (see Figure 6) show that the strengths after 28 days were within the range of 6.12-7.66 MPa and after 90 days within the interval of 6.19 to 7.57 MPa. In comparison with the flexural strength test of R1 formula, the strengths values are lower as a result of the use of different kind of cement with lower strength class. The highest flexural strength after 28 days (7.66 MPa) was found in case of 25% replacement and the lowest value (6.12 MPa) in case of 100% replacement. Further, Figure 6 shows the flexural strengths after 90 days showed the highest value (7.57 MPa) in case of 75% replacement and the lowest value (6.19 MPa) in case of 100% replacement. Figure 5: Flexural strength of Formula R1 with 25%, 50%, 75%, and 100% of mixing water replaced with waste sludge water from the CP. Figure 6: Flexural strength of Formula R3 with 25%, 50%, 75% and 100% of mixing water replaced with waste sludge water from the CP. The Figure 7 clearly shows that the replacement of pure mixing water in the amounts of 25%, 50%, 75%, and 100% with waste sludge water from the CP caused a decrease in the compressive strength after 28 days. The highest decrease in strength was found in case of 50% replacement of pure mixing water, i.e., by 8.57 MPa (12.5%). This trend of the decrease of compressive strength after 28 days is no longer apparent after 90 days. On the contrary, there was an increase in the strength compared to the comparative formula (replacement of 0%). The highest increase of 12.34 MPa (1.18 times) was recorded in case of 75% replacement of pure mixing water with waste sludge water from the CP. It can be concluded that when 25%, 50%, 75%, and 100% of pure mixing water is replaced with waste sludge water from the CP, it is necessary to take into account a decrease of the compressive strength after 28 days in the interval of 5.2% to 12.5%, depending on the percentage replacement. On the contrary, in the time interval of 28 to 90 days, there is an increase in compressive strength compared to the comparative formula (replacement of 0%) in the inerval of 9.2% to 18.0%, depending on the extent of percentual replacement. The following conclusions can be drawn on the basis of the results of the compressive strength tests of R3 formula, which are presented in Figure 8. After 28 days of the test samples, a 25% replacement of pure mixing water with waste sludge water from the CP caused an increase in strength of 2.66 MPa (4.8%).