# Biomass Fly Ash Self-Hardened Adsorbent Monoliths for Methylene Blue Removal from Aqueous Solutions
## 3 Results and Discussion
### Monoliths Characterization
However, the samples without AP also present the same large pores resulting from air entrainment during the mixing process. Interestingly, 0C:9A is the sample that presented the higher difference between the total (72.7%) and the open porosity (69.1%). Macroscopically, looking at Figure 4b, this sample seems to be the one with the largest pores, probably resulting from the coalescence of smaller ones. In sample 5C:9A, this effect is not as evident. Figure 4b also shows that there is anisotropy in the size and the distribution of the pores in all the monoliths. Figure 5 shows the microstructure of the prepared monoliths, obtained by optical (Figure 5a) and SEM (Figure 5b). In general, it is possible to observe that the porosity increases with the rise of AP content. The addition of PLC (5 wt.%) does not significantly change the mechanism of porosity generation. These observations agree with the values obtained for density and porosity (Figure 4a). In Figure 5a, it is possible to observe, in the samples with AP, the presence of macroscopic channels consisting of interconnected pores. By increasing the AP content (from 0.01 to 0.09 wt.%), the channels tend to be deeper. The released H\({}_{2}\) bubbles Chemical reaction (1) are trapped inside the sample but, during the self-hardening process, they have time to coalesce to some extent, thus generating this type of microstructure. By SEM it is possible to observe the appearance of dispersed rounded pores in the samples with  
Figure 5: Influence of PLC and AP content on the cured (14 days) monoliths microstructure (**a**) optical and (**b**) SEM micrographs. The red arrows indicate examples of the formed macroscopic channels. 0.01 wt.% of AP (0C:1A and 5C:1A). This is in line with the decrease in density and the increase in total porosity. However, when the AP content increases from 0.05 to 0.09 wt.%, a decrease in the number of dispersed round pores and the appearance of larger voids resulting from the coalescence/interconnection of smaller pores are observed. The influence of PLC and AP content on the monoliths' compressive strength is presented in Figure 6a, together with a scheme that exemplifies the PLC effect in the microstructure Figure 6b. The increase in AP content (from 0 to 0.09 wt.%) promotes a decrease in the compressive strength of the samples (from 1.96 to 0.03 MPa in the compositions without PLC) and from 2.70 to 0.23 MPa in the compositions with 5 wt.% of PLC. This behaviour results from the increase in the porosity of the samples. For the same AP amount, the introduction of PLC leads to an increase in the mechanical strength of the samples that results from the better hydraulic characteristics of PLC, in comparison to BFA. PLC promotes stronger bonds between particles and the number of large voids, such as the one seen in Figure 6b, decreased. Such voids tend to exhibit cracks in their inner wall, their occurrence being strongly attenuated by the presence of PLC. This also explains the mechanical resistance improvement in the samples with PLC. The compressive strength of the sample without PLC and AP stays within the range of values reported by [PERSON] et al. [5] and [PERSON] et al. [6] for self-hardened monoliths. However, the comparison between the results obtained here and in those works is only indicative since the preparation conditions are very different. In the two mentioned studies, the samples were kept for 28 days at room temperature in closed containers and in this study only 7 days, the water to total solids weight ratio (w/s) was higher (ranging from 0.48 to 1.50) and, although the used fly ash was also from a bubbling fluidised bed boiler, the burned fuel blends are different.