## 2 Methodology  
The Characterization of Raw Materials and the Mixing Composition of Alkali Activated Biomass Bottom Ash Pastes and Concretes  
In this study, the precursor was made from biomass bottom ash (BBA) which was obtained from the combustion plant located in Lithuania. First, this BBA was dried at 100 \({}^{\circ}\)C temperature for 24 h and then it was milled in ball mill. The PG used in this work had \(\alpha\) hemihydrate type. It is the waste product of orthophosphoric acid production. PG is formed by the reaction of sulfuric acid from natural apatite according this Equation (1) [19]:  
\[\text{Ca}_{5}(\text{PO}_{4})_{3}\text{ F}+5\text{H}_{2}\text{SO}_{4}+2.5\text{H}_{2} \text{O}=3\text{H}_{3}\text{PO}_{4}+5\text{CaSO}_{4}\text{-}0.5\text{H}_{2} \text{O}+\text{HF}; \tag{1}\]  
The powder of PG was taken from the conveyor belt of waste removal and dried at \(100\pm 5\,\mathrm{\SIUnitSymbolCelsius}\) temperature. According to SEM analysis the BBA particles have irregular shape with angular morphologies (Figure 1a). Microscopic analysis showed that semihydrate phosphogypsum crystals are of dense structure and irregularly shaped parallelepipeds (Figure 1b). The particle size distribution for BBA is presented in Figure 2a. The particles size is in wide range from 0.9 \(\upmu\)m to 460 \(\upmu\)m. The PG particles are finer (Figure 2b) and they are in the range from 0.8 \(\upmu\)m to 38 \(\upmu\)m. The specific surfaces areas according to Blaine for PG is 201 \(\mathrm{m^{2}/kg}\) and for BBA is 396 \(\mathrm{m^{2}/kg}\). The mean diameter of PG particles is 74.7 \(\upmu\)m and for the particles of BBA is 58.6 \(\upmu\)m.  
Figure 1: Microstructure of (**a**) biomass bottom ash and (**b**) phosphogypsum. Figure 2: Granulometric composition of biomass bottom ash (**a**) and phosphogypsum (**b**). The XRD analysis showed that in BBA dominated quartz, anorthoclase, gehlenite and calcium hydroxide with small amounts of calcium carbonate calcium oxide and magnesium oxide (Figure 3a). BBA has semi-amorphous semi-crystalline structure with a broad peak close to silica [20]. In the mineral composition of PG, according to XRD analysis (Figure 3b), dominated bassanite (CaSO\({}_{4}\)-0.5H\({}_{2}\)O) and a small amount of brushite (CaPO\({}_{3}\)(OH)\(\cdot\)2H\({}_{2}\)O). PG exhibit well definite crystalline structure [21]. This byproduct of biomass combustion in power plants (BBA) has the relatively high calcium, silicon and alkali contents (Table 1). According to the chemical composition of the PG, CaO and SO\({}_{3}\) are the major components of this material (Table 1). There is some amount of acidic impurities such as P\({}_{2}\)O\({}_{5}\)-0.81%; including water-soluble--0.10% and F--0.14%, which make PG difficult to reuse. Loss on ignition was 6.4%. The pH of the water suspension-4.7. Two types of alkali activators were used (Table 2). The first type was sodium hydroxide solution made with commercial NaOH pellets (analytical grades). The second one was made from the mixture of NaOH solution and the sodium silicate hydrate (WG) solution (silicate modulus 3.0, concentration 36%). \begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline \hline  & **CaO** & **SiO\({}_{2}\)** & **Na\({}_{2}\)O** & **Al\({}_{2}\)O\({}_{3}\)** & **MnO** & **MgO** & **K\({}_{2}\)O** & **Fe\({}_{2}\)O\({}_{3}\)** & **P\({}_{2}\)O\({}_{5}\)** & **TiO\({}_{2}\)** & **SO\({}_{3}\)** & **F** & **Other** \\ \hline BBA & 49.0 & 22.4 & 0.28 & 2.51 & 0.30 & 8.29 & 8.69 & 2.18 & 5.05 & 0.33 & 0.58 & – & 0.39 \\ PG & 38.60 & 0.37 & – & 0.13 & – & 0.04 & – & 0.03 & 0.81 & – & 53.48 & 0.14 & 6.4 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Chemical composition of initial materials, wt%. Figure 3: The mineral composition (XRD analysis) of (**a**) biomass bottom ash and (**b**) phosphogypsum.