## 3 Results and Discussion  
### Physicochemical Properties of the Raw Materials  
The chemical composition of OPC and biomass fly ash (BFA), as received and after the applied pre-treatment processes, obtained by X-ray fluorescence (XRF), is presented in Table 2. For OPC, CaO, SiO\({}_{2}\), Al\({}_{2}\)O\({}_{3}\), SO\({}_{3}\) and Fe\({}_{2}\)O\({}_{3}\) are the main components (their total amount represents 89.53 wt.%) and the LOI value is 7.25 wt.%. BFA1 main components are SiO\({}_{2}\) (38.14 wt.%), CaO (26.97 wt.%), Al\({}_{2}\)O\({}_{3}\) (11.86 wt.%), and K\({}_{2}\)O (6.56 wt.%). BFA2 is also mainly composed by the same oxides but the amounts are different: 32.34 wt.% CaO, 21.79 wt.% SiO\({}_{2}\), 9.28 wt.% K\({}_{2}\)O and 8.74 wt.% Al\({}_{2}\)O\({}_{3}\). [PERSON] et al. [18] concluded that BFA from fluidized bed combustion of wood chips (composed mostly of softwoods, with minor amounts of hardwoods) contains SiO\({}_{2}\), CaO, and Al\({}_{2}\)O\({}_{3}\) as main oxides. The same observation was reported by [PERSON] et al. [4] for BFA from fluidized bed combustion of forest wastes, mostly eucalyptus. BFA1 chemical composition complies with both author's findings. BFA2 also presents SiO\({}_{2}\) and CaO as major oxides compounds, but the K\({}_{2}\)O content, although very close, is superior to Al\({}_{2}\)O\({}_{3}\). This fact can be due to differences in the biomass blends used as fuels and on combustion technologies as pointed by [PERSON] et al. [19]. BFA2 exhibits higher contents in Na\({}_{2}\)O (6.04 wt.%) and Cl (6.21 wt.%) than BFA1 (1.34 wt.% of Na\({}_{2}\)O and 1.12 wt.% of Cl). The SO\({}_{3}\) content is 3.74 wt.% for BFA2 and 2.41 wt.% for BFA1, and the LOI values are 10.48 wt.% and 6.05 wt.%, respectively, for BFA2 and BFA1. From the XRF results, it can also be noticed that there are some compositional differences between the as received, and the sieved (\(<\)63 \(\upmu\)m) BFA. The sieved materials (BFA1_S and BFA2_S) exhibit higher LOI and Cl amounts and a smaller SiO\({}_{2}\) content. The SO\({}_{3}\) amount is slightly higher for BFA1 and slightly smaller for BFA2. The larger particles existing in BFA result, essentially, from the contamination with bottom sand, which explains the smaller SiO\({}_{2}\) amount in the sieved fraction (\(<\)63 \(\upmu\)m). On the contrary, the unburned materials exhibit small dimensions, which explains their higher content in this fraction. The chemical composition of BFA1_GS and BFA2_GS is the same as their parent powders BFA1 and BFA2, respectively, because the material was ground until everything went through the sieve. Table 2 also present the sum of SiO\({}_{2}\), Al\({}_{2}\)O\({}_{3}\), and Fe\({}_{2}\)O\({}_{3}\) amounts, which value is commonly used to assess the applicability of siliceous fly ash (FA) produced by burning pulverized coal, with or without co-combustion materials, according to EN 450-1:2012 [32]. Because of the lack of proper classification for BFA, as guidance, the standard of coal FA for use in concrete was used. According to this normative, in a pozzolanic material, the sum of SiO\({}_{2}\) + Al\({}_{2}\)O\({}_{3}\) + Fe\({}_{2}\)O\({}_{3}\) contents must surpass 70 wt.% [11]. For BFA1 this value is equal to 55.46 wt.%, hence some pozzolanic activity is expected. Sieved BFA1 shows a lower amount (46.93 wt.%). BFA2 shows a lower value (34.92 wt.%), and sieving causes a further decrease (21.91 wt.% for BFA2_S). Therefore, no relevant pozzolanic activity is expected. The values of the (CaO + MgO)/SiO\({}_{2}\) ratio are presented in Table 2. BFA2 exhibits a value of 1.69, thus higher than 1.4, which is, as pointed by some authors [33; 34], the value that defines hydraulic activity of coal FA. The sieving promoted an enhancement of the hardening potential activity of BFA2. The hardening potential of BFA1 is smaller than that of BFA2. According to EN 197-1 [35], a material that has hydraulic activity must have a (CaO + MgO)/SiO\({}_{2}\) ratio superior to one, and this might explain the hardening potential of BFA1_S [11; 35].