## 2 Materials and Methods
### Research Methods  
A control mortar was prepared according to the standard [66] along with 14 series of modified mortars incorporating waste materials. Fly ash (MSWI-FA) was added in amounts of 10%, 15%, and 20% of the cement mass, with corresponding adjustments to the volume of standard sand. Cement kiln dust (CKD) was used as a replacement for cement in amounts of 10%, 15%, and 20% of the cement mass. These waste materials were dosed both individually and in combination in various proportions. The compositions of the individual mortars are presented in Table 2. MSWI-FA was used as a sand replacement because it does not possess binding properties (either hydraulic or pozzolanic) at a level that would allow it to replace cement. In contrast, CKD has binding properties and can replace cement, supporting hydration processes. Individual series of mortars were marked as follows: mortars modified with fly ash-MSWI-FA-% content; cement kiln dust-CKD-% content; and ash and dust MSWI-FA-CKD -% content of individual waste. Figure 3: TGA-DTA thermogram for the CKD. Figure 2: TGA-DTA thermogram for the MSWI-FA. For the fresh cement mortars, consistency was tested using a flow table in accordance with PN-EN 1015-3 [70]. The PN-EN 1015-3 standard specifies a method for testing the consistency of masonry mortar using a flow table. The consistency of the mortar indicates its ability to flow and determines how easily the mortar can be spread on a surface. For the hardened mortars, compressive and flexural strength tests were conducted, a key parameter for assessing their mechanical properties and ensuring the durability of building structures. The tests were performed using an MMC-3742 testing machine (FPR; ToniTechnik 2030, Berlin, Germany) in accordance with PN-EN 196-1 [67] on prisms measuring 40 \(\times\) 40 \(\times\) 160 mm. The compressive and flexural strength tests were carried out after 7 (so-called early strength) and 28 days of sample curing (full strength). The flexural strength test was conducted on three prisms by gradually increasing the load at a rate of 50 \(\pm\) 10 N/s until the samples broke. The compressive strength test was performed on six halves of the prisms, applying a uniform compressive force increment of 2400 \(\pm\) 200 N/s to ensure the precise evaluation of the samples' load resistance. Additionally, water absorption was tested using the soaking method in accordance with PN-B-04500:1985 [71], conducted after 28 days of curing. For this test, three prisms measuring 40 mm \(\times\) 40 mm \(\times\) 160 mm were prepared from mortars in each series. The bulk density of the dry cement mortar was also measured in accordance with PN-EN 1015-6:2000 [72]. Freeze-thaw resistance testing was conducted in accordance with PN-B-04500:1985 [71] using a Toropol K-010 chamber (Toropol, Warsaw, Poland). From each mortar series, 12 prisms measuring 40 mm \(\times\) 40 mm \(\times\) 160 mm were prepared. Six samples underwent 25 cycles of alternating freezing and thawing, while the remaining prisms served as control samples. Freezing took place in air at a temperature of \(-\)18 \(\pm\) 2 \({}^{\circ}\)C and lasted for at least less 4 h. Samples were defrosted in water at +18 \(\pm\) 2 \({}^{\circ}\)C-time 2 h-4 h.