## 3 Results and Discussion
### SEM and TG Analysis  
TG-DTG analyses occurred in four sections [43]. The first section occurs from room temperature to around 105 \({}^{\circ}\)C, when the loss of free water occurs [43]. The dehydration of C-S-H gels and ettrinigite took place within the range from 105 to 200 \({}^{\circ}\)C [43; 44; 45]. The third section of the curve is the weight loss occurring due to thermal degradation around 400-550 \({}^{\circ}\)C [43; 44; 45; 46]. This section corresponds to the decomposition of portlandite that was created during hydration reactions [43; 44; 45]. The fourth section corresponds to the decomposition of calcium carbonate at 550-740 \({}^{\circ}\)C [43; 44; 45]. Qualitatively, mortars that had different types of sieved fly ash in their composition did not differ from each other--similar DTG peaks were observed as in the control specimen. Some changes in the DTG curves were observed in the scope of 20-200 \({}^{\circ}\)C, which were consistent with the addition of 15% (Figure 9) and 25% (Figure 10) of FA(B)63 fly ash. In specimens with fly ash sieved through the 63 \(\upmu\)m sieve, the peak at around 125 \({}^{\circ}\)C was around 12 p.p. bigger than for specimens with the respective amount of FA(B)N fly ash. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{2}{c}{**Series Codes**} & \multicolumn{1}{c}{**Water**} & \multicolumn{1}{c}{**Bulk Density**} \\ \cline{2-5}
**Nr** & **Type of Fly Ash (-)** & **Amount of Fly Ash (-)** & **Absorption (\%)** & **(g/cm\({}^{3}\))** \\ \hline
1. & FA(B)63 & fa/b = 0.05 & 8.3 & 2.10 \\
2. & FA(B)63 & fa/b = 0.15 & 8.8 & 2.07 \\
3. & FA(B)63 & fa/b = 0.25 & 9.2 & 2.03 \\
4. & FA(B)125 & fa/b = 0.05 & 8.4 & 2.09 \\
5. & FA(B)125 & fa/b = 0.15 & 8.6 & 2.08 \\
6. & FA(B)125 & fa/b = 0.25 & 8.9 & 2.06 \\
7. & FA(B)N & fa/b = 0.05 & 8.5 & 2.06 \\
8. & FA(B)N & fa/b = 0.15 & 8.6 & 2.07 \\
9. & FA(B)N & fa/b = 0.25 & 9.1 & 2.01 \\
10. & K & & 8.7 & 2.02 \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Water absorption and bulk density of cement mortars with the addition of sieved fly ash (FA(B)63 and FA(B)125) compared with untreated fly ash (FA(B)N). The difference indicates a higher amount of C-S-H gel phase and ettringite in specimens with sieved fly ash. When they were analyzed separately, no clear influence of sieved fly ash on the third (450-500 \({}^{\circ}\)C) and fourth (550-800 \({}^{\circ}\)C) sections of the DTG curves was observed (Figure 11). However, the sum of portlandite and carbonate phases tends to be higher with sieved fly ash than in the control specimens. Figure 10: Derivative thermogravimetric (DTG) curves of mortars with a 25% addition of sieved fly ash (FA(B)63 or FA(B)125) compared with the results of mortars with a 25% addition of untreated fly ash (FA(B)N). Figure 9: Derivative thermogravimetric (DTG) curves of mortars with a 15% addition of sieved fly ash (FA(B)63 or FA(B)125) compared with the results of mortars with a 15% addition of untreated fly ash (FA(B)N). SEM analysis confirmed the conclusions from the TG/DTG research. In terms of quality, the microstructure of cement paste was similar between series with the addition of fly ash. The characteristic structures of the C-S-H gel, tetringite crystals, and portlandite crystals were observed in all specimens with the addition of biomass fly ash. Nonetheless, specimens with the addition of FA(B)63 fly ash had a noticeably smaller number of portlandite crystals and a denser structure of C-S-H gel (Figure 12a) than the specimens with the addition of untreated FA(B)N fly ash (Figure 12b). The difference in the Ca(OH)\({}_{2}\) content in the cement matrix could be associated with the higher hydration rate and formation of the C-S-H gel in those specimens. Noticeably, the microstructure of tetringite crystals was different between specimens with FA(B)63 and FA(B)N fly ash.