## 2 Materials and Methods  
As a binder material, CEM I 42.5R cement that followed the EN 197-1 standard requirements was utilized. The material conformed to the requirements of the EN 197-1 standard [24]. Biomass fly ash (BFA) was acquired from the local combined heat and power plant, which uses fluidized bed combustion (750 \({}^{\circ}\)C) for firing woody material. Chemical oxide compositions of the fly ash are presented in Table 1. The main oxides in fly ash compositions were calcium oxide, potassium oxide, and silica oxide. The minerals detected by the XRD study were quartz, calcite, calcium oxide, portlandite, calcium sulfite hydrate (CaSO\({}_{3}\)-0.5H\({}_{2}\)O), iron hydrogen oxide (Fe\({}_{1.78}\)H\({}_{0.66}\)O\({}_{3}\)), calcium silicate hydrate (Ca\({}_{2}\)SiO\({}_{4}\)-0.5H\({}_{2}\)O), calcium silicate, hematite, arcanite (Figure 1). The fineness and loss-on-ignition values of the fly ash are presented in Table 2. Both Strength Activity Indices (SAIs) of the biomass fly ash required by EN 450-1 prior to its activation were below 75% for 28-day SAI and 85% for 90-day SAI [17]. Table 3 presents the grain size distribution of initial untreated fly ash. Particles with a diameter above 45 \(\upmu\)m were 38.2% of the particle size mix. Standard sand that followed EN 196-1 requirements was used. Tap water was used during mixing. \begin{table}
\begin{tabular}{c c c} \hline
**Oxide** & **Biomass Fly Ash** & **CEM I 42.5R** \\ \hline SiO\({}_{2}\) & 9.6\% & 17.4\% \\ Al\({}_{2}\)O\({}_{3}\) & 2.4\% & 4.2\% \\ P\({}_{2}\)O\({}_{5}\) & 0.0\% & 0.3\% \\ Na\({}_{2}\)O & 0.0\% & 0.3\% \\ K\({}_{2}\)O & 14.7\% & 0.7\% \\ MgO & 0.0\% & 0.7\% \\ CaO & 56.1\% & 67.9\% \\ MnO & 3.1\% & 0.1\% \\ Fe\({}_{2}\)O\({}_{3}\) & 7.9\% & 3.6\% \\ CuO & 0.0\% & 0.0\% \\ ZnO & 0.5\% & 0.1\% \\ As\({}_{2}\)O\({}_{3}\) & 0.0\% & 0.0\% \\ SO\({}_{3}\) & 4.3\% & 3.8\% \\ SrO & 0.0\% & 0.2\% \\ TiO\({}_{2}\) & 0.6\% & 0.5\% \\ Cl & 0.7\% & 0.2\% \\ BaO & 0.1\% & 0.0\% \\ \hline \end{tabular}
\end{table} 
Table 1: Chemical oxide composition of biomass fly ash before activation and CEM I 42.5 R cement used in the study. \begin{table}
\begin{tabular}{c c c} \hline
**Property** & **Biomass Fly Ash** & **EN 450-1 Requirements** \\ \hline Fineness & 43.1\% & \(\leq\)40\% (N category) \\ Specific surface (Blaine method) & 2559 cm\({}^{2}\)/g & - \\ Volume-specific surface (dynamic light scattering) & 5738 cm\({}^{2}\)/cm\({}^{3}\) & - \\ \hline Loss on ignition & 7.8\% & \(\leq\)9\% (C category) \\ Strength Activity Index after 28 days & 64.3\% & \(\geq\)75\% \\ Strength Activity Index after 90 days & 66.1\% & \(\geq\)85\% \\ \hline \end{tabular}
\end{table} 
Table 2: Physical properties of biomass fly ash used in the study before activation. Figure 1: XRD analysis of biomass fly ash used in the study before activation. Three types of biomass fly ash were used: untreated fly ash (FA(B)N), fly ash that was sieved with a 125 \(\upmu\)m sieve (FA(B)125), and fly ash that was sieved with a 63 \(\upmu\)m sieve (FA(B)63). For the basis of the calculation for the mixes, the EN 196-1 standard mortar mix (450 g of cement, 1350 g of sand, and 225 g of water) was used [25]. Totals of 5%, 15%, or 25% of cement binder in mortars were replaced with fly ash and expressed as an experimental factor: the mass of fly ash to the whole binder mass ratio (fa/b). Based on those assumptions, mix proportions of cement mortars were prepared and are presented in Table 4.  
For each series, twelve 40 mm \(\times\) 40 mm \(\times\) 160 mm mortar specimens were prepared in accordance to EN 196-1 guidance [25]. Specimens were cured in tap water up to the test day. Consistency tests were performed on fresh mortar mixtures according to EN 1015-3 [26]. For each series, mechanical properties tests (compressive and tensile strength tests) were performed after 7, 28, and 90 days of curing. Water absorption and density tests were performed after 28 days of water curing.