# Biomass Fly Ash Self-Hardened Adsorbent Monoliths for Methylene Blue Removal from Aqueous Solutions
## 3 Results and Discussion  
### Raw Materials Characterization  
The chemical composition of BFA and PLC, expressed in terms of oxides, is shown in Table 2. The main components of BFA are CaO (32.3 wt.%), SiO\({}_{2}\) (21.8 wt.%), K\({}_{2}\)O (9.28 wt.%) and Al\({}_{2}\)O\({}_{3}\) (8.74 wt.%), which are in accordance with the results reported by other authors [10; 19; 20]. However, the characteristics of BFA may vary with the properties of the biomass used as fuel and the operating conditions [10]. In the case of PLC, the five main constituents are CaO (73.1 wt.%), SiO\({}_{2}\) (13.0 wt. ), SO\({}_{3}\) (4.02 wt.%), Fe\({}_{2}\)O\({}_{3}\) (2.94 wt.%) and Al\({}_{2}\)O\({}_{3}\) (2.78 wt.%) which are in the usual concentrations reported for commercial PLC [19]. BFA has an LOI value of 10.48 wt.%. The chemical composition of the ash, namely the CaO content, is extremely important for its potential self-hardening. According to EN197-1:2011 [21], a hydraulic binder is defined as a material that, when mixed with water, sets and hardens, and can maintain its strength and stability even while submerged in water. In the same standard, it is also mentioned that for Portland cement clinker to be considered a hydraulic material, the CaO/SiO\({}_{2}\) ratio (by mass) should be higher than 2, while a granulated blast furnace slag possesses hydraulic properties when the (CaO + MgO)/SiO\({}_{2}\) ratio (by mass) surpasses 1.0. The actual BFA only complies with the last requirement (see Table 2). \begin{table}
\begin{tabular}{c c c} \hline \hline
**Compounds (wt.\%)** & **BFA** & **PLC** \\ \hline CaO & 32.3 & 73.1 \\ SiO\({}_{2}\) & 21.8 & 13.0 \\ K\({}_{2}\)O & 9.28 & 1.69 \\ Al\({}_{2}\)O\({}_{3}\) & 8.74 & 2.78 \\ Cl & 6.21 & 0.13 \\ Na\({}_{2}\)O & 6.04 & 0.35 \\ MgO & 4.40 & 1.53 \\ Fe\({}_{2}\)O\({}_{3}\) & 4.40 & 2.94 \\ SO\({}_{3}\) & 3.74 & 4.02 \\ P\({}_{2}\)O\({}_{5}\) & 1.30 & 0.18 \\ MnO & 0.91 & 0.04 \\ TiO\({}_{2}\) & 0.55 & 0.16 \\ CaO/SiO\({}_{2}\) & 1.48 & 5.62 \\ (CaO + MgO)/SiO\({}_{2}\) & 1.68 & 5.74 \\ LOI & 10.48 & 16.41 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Chemical composition and LOI of BFA and PLC. Figure 1 shows the XRD patterns of the PLC, BFA, and hardened samples (0C:0A and 5C:0A). The PLC has the expected crystalline phases: alite (C\({}_{3}\)S-Ca\({}_{3}\)SiO\({}_{5}\)), belite (C\({}_{2}\)S-Ca\({}_{2}\)SiO\({}_{4}\)), calcite (CaCO\({}_{3}\)) and gypsum (CaSO\({}_{4}\).2H\({}_{2}\)O). This is in agreement with that stated by EN 197:2011 [21] since the PLC used in this work (type II/B-L) is composed of clinker (65 to 79 wt.%) and limestone (21 to 35 wt.%), and with the values expressed in terms of oxides obtained by XRF for CaO (73.06 wt.%) and SiO\({}_{2}\) (12.98 wt.%). The XRD pattern of BFA shows that it is mostly constituted by quartz (SiO\({}_{2}\)), calcite (CaCO\({}_{3}\)), microcline (KAlSi\({}_{3}\)O\({}_{8}\)), sylvite (KCl), calcium hydroxide (Ca(OH)\({}_{2}\)), and muscovite (KAl\({}_{2}\)(Si\({}_{3}\)Al)O\({}_{10}\)(OH)\({}_{2}\)), thus, in agreement with the main components expressed in terms of oxides detected by XRF (SiO\({}_{2}\), CaO, K\({}_{2}\)O Al\({}_{2}\)O\({}_{3}\), and Cl). Quartz, calcite and microcline are the crystalline phases commonly found in BFA generated from fluidised bed combustion of forest biomass [2, 4, 10, 11]. Figure 2a-d, shows SEM images illustrating the morphology of BFA, PLC and AP, respectively. BFA (Figure 2a,b) and PLC (Figure 2c) are composed of irregularly shaped particles with a broad particle size range, with BFA presenting some acicular-shaped particles. In both materials, it is possible to observe the presence of agglomerates, which are larger in BFA. The particle's size distribution and shape, and the presence of large agglomerates play an important role in the compactness of the samples. The AP (Figure 2d) has a lamellar shape and a coarser particle size when compared to BFA and PLC. The particle size distribution of BFA and PLC is shown in Figure 3a and 3b, respectively.