# Biomass Fly Ash Self-Hardened Adsorbent Monoliths for Methylene Blue Removal from Aqueous Solutions
## 3 Results and Discussion
### Monoliths Characterization  
The monoliths were characterized for mineralogical composition, density, porosity (total and open), compressive strength and microstructure after 14 days of curing. The crystalline phases that promote self-hardening and mechanical strength development are related to the reaction of water with the initial phases present in BFA and PLC. The XRD patterns of 0C:0A and 5C:0A hardened samples (Figure 1) reveal the presence of a crystalline phase, which is a consequence of the presence of a crystalline phase. Figure 3: Particle size distribution of powder BFA (**a**) and PLC (**b**) samples. Figure 2: SEM images of BFA (**a**) and (**b**), PLC (**c**) and AP (**d**). ence of the crystalline phases already detected in BFA and a new phase--hydrocalumite (Ca\({}_{4}\)Al\({}_{2}\)(OH)\({}_{12}\)(Cl,CO\({}_{3}\),OH)\({}_{2}\)-4H\({}_{2}\)O). When qualitatively comparing the BFA diffractogram with that of the hardened samples (0C:0A and 5C:0A), it is possible to observe that the intensity of the peaks assigned to quartz (1), calcite (2), microcline (3), sylvite (4) and muscovite (7) decrease. In addition, hydrated aluminosilicate or calcium-rich phases could have also been formed but have a low degree of crystallinity or are present with a concentration below the detection limit. When comparing the diffractograms of the 5C:0A and 0C:0A samples (Figure 1), it is observed that the intensity of the peaks assigned to quartz (1), microcline (3) and sylvite (4) decreased significantly. The muscovite peak (7) disappears and the intensity of the peaks of calcite and hydrocalumite increases. This is due to the reaction of PLC (5 wt.% added) with water, which releases calcium hydroxide (Ca(OH)\({}_{2}\)) into the medium, which may have favoured the formation of a higher quantity of hydrocalumite. Calcite was brought to the system by the PLC addition. Figure 4a presents the influence of PLC and AP content on the density and porosity (total and open) of the cured samples. A photograph of the parallelepiped samples used in the MB adsorption tests is patent in Figure 4b. The true density (\(\rho_{th}\)) of the samples without PLC and AP (0C:0A) and with PLC only (5C:0A) is similar, being 2.63 g/cm\({}^{3}\) and 2.64 g/cm\({}^{3}\), respectively. The density of the samples decreases as the AP content rises due to the increment of the total porosity. For the samples without PLC, the density varies between 1.11 g/cm\({}^{3}\) (0C:0A sample) and 0.61 g/cm\({}^{3}\) (0C:9A sample). The introduction of 5 wt.% PLC slightly increase the density of the samples (1.17 g/cm\({}^{3}\) for 5C:0A and 0.71 g/cm\({}^{3}\) for 5C:9A). This increment was expected due to PLC characteristics, namely, higher true density, smaller particle size and extended hardening reactions when compared with BFA. Nevertheless, the samples without AP are already porous (\(\approx\)58%) due to the poor compaction of the BFA particles (presence of acicular-shaped particles and large agglomerates) and the fact that the hydration/hardening reactions of BFA are not intense. As expected, the porosity increases (\(\sim\)13%) with the increment of the porogenic agent. The addition of 0.09 wt.% AP increases the total porosity from 59.0 to 72.7% (samples without PLC) and from 59.6 to 72.0% (samples with 5 wt.% of PLC). For the samples without PLC, the open porosity varies from 58.1 to 69.1% and, for the samples with PLC, it ranges from 58.4 to 71.9%. For all samples, the porosity is almost completely open, which is excellent  
Figure 4: Density and porosity (total and open) of the samples produced with distinct PPLC and AP content (**a**). Parallelepiped samples used in the MB adsorption tests (**b**). for the target application (adsorption of toxic substances). Moreover, the open porosity of the samples, with or without PLC, is similar. In Figure 4b, it is possible to observe that the macroporosity of the samples increased with AP content.