## 3 Results and Discussion
### Compressive Strength  
The compressive strength test of cement mortars was conducted after 7 and 28 days of curing. The results are presented in Figure 7. For mortars modified with fly ash (MSWI-FA), compressive strength measured after 7 days of curing showed a decrease in all series compared to the control sample (51.2 MPa). The smallest decrease (22.9%) was observed for MSWI-FA-20, suggesting that an initial increase in MSWI-FA content has a beneficial effect shortly after curing. However, after 28 days, the most stable performance was recorded for the MSWI-FA-10 series, which used 10% waste as a sand replacement, with a reduction of 13.7%. In contrast, the strength of the MSWI-FA-20 series decreased by 28.9%, demonstrating that higher MSWI-FA content worsens long-term strength parameters. The MSWI-FA-15 series showed reductions of 27.8% and 26.2% for the 7- and 28-day periods, respectively, indicating a deterioration of properties in both curing intervals. For mortars modified with cement kiln dust (CKD), the initial reductions were more uniform, ranging from 24.4% to 22.1% for 10%, 15%, and 20% additions. After 28 days, mortars modified with CKD achieved more stable parameters than those modified with MSWI-FA. The smallest reduction was observed in CKD-10 (20.5%), with similar results for CKD-15 and CKD-20 (22.1% and 20.7%, respectively). This indicates that CKD may be a more effective additive for modifying mortar, particularly over longer curing times. In mortars combining MSWI-FA and CKD, the largest strength reductions were recorded in the MSWI-FA-CKD/17-17 series, with a change of 39.6% after 28 days, making it the least effective modification. The best results in this group were achieved by the MSWI-FA-CKD/3-3 series, with reductions of 24.4% after 7 days and 20.9% after 28 days. These results demonstrate that using minimal amounts of both additives may yield more favorable outcomes, although the combined effect is generally detrimental at higher proportions.  
[PERSON] et al. [20] studied the effect of CKD on mortar-concrete mixtures to partially replace Portland cement in concrete. The results showed that the overall compressive strength compared to the control mix decreased with increasing bypass dust content in the tested materials. However, the researchers came to the conclusion that the replacement of as little as 5% of dust with cement did not negatively affect the tested strength parameter. Similarly, [PERSON] and [PERSON] [22] investigated the effect of adding cement kiln dust (CKD) as a partial replacement for cement in amounts of 10%, 30%, and 50% of the cement weight. The obtained results showed a significant decrease in the strength of concrete modified with CKD. After 28 days of curing, the compressive strength was 28 MPa, 25 MPa, and 22 MPa for 10%, 30%, and 50% CKD content, respectively. In comparison, the reference mix (without CKD) achieved a compressive strength of 35 MPa. [PERSON] et al. [21] explored the possibility of using cement kiln dust (CKD) as a partial replacement for cement in the production of mortar and concrete [43]. Various mixtures were developed, containing CKD in proportions ranging from 0% to 45%. In some formulations, CKD was supplemented with fly ash and blast furnace slag. The conducted tests included evaluations of compressive strength, flexural strength, and durability. The results revealed that CKD-OPC blends exhibited limited strength due to the absence of calcium silicates and the low fineness of CKD. Nevertheless, it was demonstrated that CKD can be effectively combined with pozzolanic materials such as blast furnace slag or fly ash.