## 3 Results and Discussion  
### Characteristics of Activated Fly Ash  
Both sieving methods of the fly ash changed the grain size distribution of fly ash (Figure 2). Sieving decreased the maximum dimension of the particles of the material. The smoothness of the slope of the cumulative mass graph for 63 \(\upmu\)m shows that sieving through such a small sieve size eliminated clusters of particles. Sieving decreased the value of EN 450-1 fineness (understood as the amount of residue left on the 45 \(\upmu\)m sieve after wet-sieving or air-sieving). In the case of FA(B)63 fly ash, there was a change of 41.6 percentage points (p.p.) from 43.1% for FA(B)N to 1.5% (Table 5). Both sieved types of fly ash were characterized by higher than required by EN 450-1 28-day Strength Activity Index values. A 25 p.p. increase in the 28-day SAI value was observed in fly ash sieved through the 63 \(\upmu\)m sieve. With sieving, the specific surface of fly ash increased. A similar influence of sieving was observed in the loss-on-ignition values, indicating that a \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & \multicolumn{3}{c}{**Series Codes**} & \multicolumn{3}{c}{**Biomass Fly Ash**} & & & \\ \cline{2-9}
**Nr** & **Type of Fly** & **Amount of Fly** & **Sieved with 63 \(\upmu\)m** & **Sieved with** & **Untreated** & **(g)** & **Sand** & **Water** \\  & **Ash (\(\cdot\))** & **Sieve (g)** & **125 \(\upmu\)m sieve (g)** & **(g)** & **(g)** & **(g)** & **(g)** \\ \hline
1. & FA(B)63 & fa/b = 0.05 & 22.5 & 0.0 & 0.0 & 427.5 & 1350 & 225 \\
2. & FA(B)63 & fa/b = 0.15 & 67.5 & 0.0 & 0.0 & 382.5 & 1350 & 225 \\
3. & FA(B)63 & fa/b = 0.25 & 112.5 & 0.0 & 0.0 & 337.5 & 1350 & 225 \\
4. & FA(B)125 & fa/b = 0.05 & 0.0 & 22.5 & 0.0 & 427.5 & 1350 & 225 \\
5. & FA(B)125 & fa/b = 0.15 & 0.0 & 67.5 & 0.0 & 382.5 & 1350 & 225 \\
6. & FA(B)125 & fa/b = 0.25 & 0.0 & 112.5 & 0.0 & 337.5 & 1350 & 225 \\
7. & FA(B)N & fa/b = 0.05 & 0.0 & 0.0 & 22.5 & 427.5 & 1350 & 225 \\
8. & FA(B)N & fa/b = 0.15 & 0.0 & 0.0 & 67.5 & 382.5 & 1350 & 225 \\
9. & FA(B)N & fa/b = 0.25 & 0.0 & 0.0 & 112.5 & 337.5 & 1350 & 225 \\
10. & K & 0.0 & 0.0 & 0.0 & 450.0 & 1350 & 225 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Cement mortar mix proportions. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Material** & **\textgreater{}2.0 mm** & **2.0–1.0 mm** & **1.0–0.5 mm** & **0.50-0.25 mm** & **0.250-0.125 mm** & **0.125-0.063 mm** & **0.063-0.045 mm** & **0.045-0 mm** \\ \hline Biomass fly ash & 0.0 & 0.0 & 0.0 & 0.3 & 12.0 & 18.7 & 7.2 & 61.8 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Grain size distribution of biomass fly ash used in the study before activation. substantial amount of unburnt carbon is in the form of particles smaller than 125 \(\upmu\)m. After sieving through the 125 \(\upmu\)m sieve, about 77% of the initial material was used. After sieving through the 63 \(\upmu\)m sieve, around 63% of the untreated fly ash was used. Around 23-37% of initial untreated material would require further utilization as a by-product of activation. The morphology of the fly ash did not significantly change through activation. SEM photographs, presented in Figure 3, show that activated and untreated fly ash mostly consist of irregularly shaped particles. Sieving successfully separated bigger particles of quartz from entering the mix (Figure 3b). However, it does not completely eliminate irregular black particles of unburnt carbon. Sieving mainly changed the amount of two crucial oxides, SiO\({}_{2}\) and CaO, in the oxide composition of the activated material (Table 6). Sieving fly ash through 63 \(\upmu\)m increased the amount of lime in the oxide composition by 9.3 p.p. and decreased the amount of silica by 6.3 p.p. This suggests that a substantial amount of quartz in the fly ash was present as particles of the upper parts of its grain size distribution, whereas lime is present as smaller particles.