## 3 Results and Discussion  
The compressive strength of alkali-activated BBA samples is shown in Figure 4. There are two types of samples: one type of hardened AAM pastes was alkali activated by using NaOH solutions and the mixture of NaOH and sodium silicate hydrate solutions was used in the second type of samples. When SiO\({}_{2}\)/Na\({}_{2}\)O molar ratio was 2 the samples containing 20% PG substitute had the highest compressive strength. In this case compressive strength reached 24.3 MPa and precursors were alkali activated with NaOH solutions (Figure 4a). Similar values of compressive strength (23.0 MPa) were obtained for that samples which were activated with the mixtures of NaOH and sodium silicate hydrate solutions. In this case the optimal content of PG substituting was 15% (by mass of BBA). In all investigated cases (Figure 4a) the substitution of BBA to PG had gains in compressive strength. This substitution is recommended not to exceed 25%. Similar compressive strength (25.83 MPa) had geopolymer samples formed with circulating fluidized bed combustion coal bottom ash according to [PERSON] et al. [27]. Figure 4b shows the compressive strength of the alkali activated BAA paste with SiO\({}_{2}\)/Na\({}_{2}\)O molar ratio 3. The positive effect was detected in this case. In the PG 15-3 samples with the alkali activator of NaOH solutions the highest compressive strength reached 30.0 MPa after 28 days. By using the same molar ratio but as alkali activator the mixture of NaOH and sodium silicate hydrate solutions was the compressive strength was reached 23.0 MPa after 7 days of hardening. After longer duration (28 days) of hardening, the reduction was observed of more than 3 times of compressive strength (8.0 MPa). This reduction of compressive strength may be explained by the fast alkali reactions resulted in quick strength gain after 2 days. This gain should be due the increase of gel like matrix. After 28 days the structure samples showed cracks on the surface which could be caused by the drying shrinkage (Figure 4b) [28]. In all investigated cases the use of calcium promoter such as PG which substituted BBA had positive effect to compressive strength gain. After 28 days the compressive strength was higher than compressive strength of reference samples. Similar results related with positive effect of calcium promoters in bottom ash geopolymer fine-grained concrete report Hanjitsuwanet al [29]. Figure 4: Compressive strength of alkali-activated biomass bottom ash pastes when SiO\({}_{2}\)/Na\({}_{2}\)O molar ratio is (**a**) 2 and (**b**) 3 (Table 2). The mineral composition of alkali activated biomass bottom ash is shown in Figure 5. The X-ray diffraction study is carried out only on the 8 pastes because they are the ones that have shown the highest compressive strength values. The reference compositions were investigated as well. In all X-ray diffraction patterns it is possible to detect quartz and calcium hydroxide which left unreacted from BBA. During alkali reactions calcium silicate hydrate, calcium aluminum oxide hydroxide hydrate, sodium aluminum silicate hydrate formed. When PG was incorporated in the system, additional mineral calcium aluminum hydroxide hydrate formed (PG 20-2, PGWG 15-2, PG 15-3 and PGWG 15-3). The crystal phases remained the same in all samples and it did not depend on the molar SiO2/Na2O ratios which were used in this work. By using lower SiO2/Na2O molar ratio (SiO2/Na2O = 2) the higher amount of alkali had an impact on the formation of Na2 CO3(H2O) (without PG) and Na2 SO4 (with PG). The formation of Na2 CO3(H2O) had negative affect on the development of compressive strength [30]. When PG was inserted in the alkali activated BBA, PG reacted with NaOH and this reaction products Na2 SO4 with Ca(OH)2 were (Figure 5a). During hydration process, Na2 SO4 is an effective activator for alkali activated binders [31]. Sodium sulfate could motivate the formation of calcium aluminum silicate hydrate and calcium silicate hydrate.