## 3 Results and Discussion
As seen in Figure 5a,b, the main peak of calcium silicate hydrate is more intensive in the samples where PG was incorporated. Figure 5: X-ray diffraction patterns of alkali activated biomass bottom ash when SiO2/Na2O molar ratio is (**a**) 2 and (**b**) 3. Notes: Q—quartz, SiO2 (83–2465); Ch—calcium hydroxide, Ca(OH)2 (84–1268); K—calcium silicate hydrate, Ca1,SiO3,5x H2O (33–306); T—thenardite Na2 SO4 (74–2036); Z—sodium aluminum silicate hydrate Na96 Al96 Si96O384216H2O (39–222); N—sodium carbonate hydrate Na2 CO3(H2O) (70–845); Hs—hydroxy–sodalite Na6(AlSiO4)6 8H2O (72–2329); H—calcium aluminum hydroxide hydrate Ca2 Al(OH)7.3H2O (33–255). In the samples with higher amount of SiO2 the molar SiO2/Na2O ratio was 3. The peaks of new formed hydrates appear more intensive (Figure 5b). This could be related with formation of higher amount of polymerization products in alkali activated system. The hydroxy-sodalite was detected in the sample PGWG 15-3 [32]. Figure 6 shows the morphology of alkali activated biomass bottom ash after 28 days of hardening. PG 15-3 and PGWG 15-3 samples exhibited different microstructures. In the microstructure PG 15-3 sample varied honeycomb-like C-S-H and honeycomb type amorphous gel structures (Figure 6a) [33]. It can be observed that in the PGWG 15-3 sample showed a higher degree of microcracking and unreacted the particle of BBA were detected as well (Figure 6b) [34]. This PGWG 15-3 sample had a more compact microstructure by comparing with the microstructure of PG 15-3 sample. This compact microstructure is closely related to the increased amount of hydration products which increased the amount of microcracks [35]. The compressive and flexural strength of alkali activated fine-grained concretes are shown in Table 4. As the aluminosilicate precursor the mixture of BBA with PG was used. The two types of alkali activator solutions were chosen: sodium hydroxide solution and the mixture of sodium hydroxide solution and sodium silicate hydrate. The proportions of PG and BBA were chosen according to the values of paste samples compressive strength (Figure 4). According to [PERSON] et al. [36] the compressive strength values of the alkali-activated pastes, fine-grained concretes and concretes with the same pastes were unequal. Fine-grained concretes had significantly lower compressive strength compared with the paste samples. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Samples**} & **Compressive** & **Flexural** & **Compressive** & **Flexural** & **Density (After** \\  & **Strength, MPa** & **Strength, MPa** & **Strength, MPa** & **28 Days)**, & **28 Days)**, \\ \cline{2-6}  & **After 7 Days of Hardening** & & **After 28 Days of Hardening** & **kg/m\({}^{3}\)** \\ \hline CPG 0-3 & 4.51 \(\pm\) 0.26 & 1.52 \(\pm\) 0.05 & 10.94 \(\pm\) 0.48 & 2.30 \(\pm\) 0.12 & 1965 \(\pm\) 14 \\ CPG 15-3 & 4.79 \(\pm\) 0.23 & 2.22 \(\pm\) 0.06 & 12.90 \(\pm\) 0.45 & 2.31 \(\pm\) 0.15 & 1971 \(\pm\) 14 \\ CPG 20-3 & 3.94 \(\pm\) 0.22 & 1.39 \(\pm\) 0.06 & 9.33 \(\pm\) 0.41 & 2.00 \(\pm\) 0.17 & 1853 \(\pm\) 15 \\ CPWG 0-3 & 6.42 \(\pm\) 0.28 & 1.32 \(\pm\) 0.05 & 10.73 \(\pm\) 0.42 & 1.84 \(\pm\) 0.16 & 1998 \(\pm\) 15 \\ CPWG 15-3 & 8.40 \(\pm\) 0.24 & 2.18 \(\pm\) 0.08 & 15.42 \(\pm\) 0.43 & 2.41 \(\pm\) 0.11 & 2019 \(\pm\) 14 \\ CPWG 20-3 & 6.20 \(\pm\) 0.27 & 1.20 \(\pm\) 0.07 & 11.94 \(\pm\) 0.31 & 1.54 \(\pm\) 0.13 & 1885 \(\pm\) 13 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Compressive strength, flexural strength and density of alkali activated fine-grained concretes samples. Figure 6: Microstructure of alkali activated biomass bottom ash samples (**a**) PG 15-3 and (**b**) PGWG 15-3. [PERSON] et al. [37] investigated and compared fly ash and bottom ash fine-grained concretes. The values of compressive strength are different for alkali activated fly ash and for bottom ash. Fly ash fine-grained concrete reached 35 MPa while bottom ash fine-grained concrete had compressive strength in the range of 10-18 MPa. Such a difference is explained by the degree of polymerization.