## 1 Introduction
Another important issue is that BFA characteristics differ from plant to plant due to the large variability of fuel blends used, added additives, combustion temperature [17; 20], installed combustion technology [4; 19; 20; 21] and used air pollution control devices [19; 20; 21]. Even in the same plant BFA can show temporal variation in their characteristics [3] mainly due to fluctuations in the daily biomass blends used. In terms of management practices, this variability constitutes a problem because is almost impossible to establish a general application [22]. The use of BFA, as OPC substitute on mortars, has been studied by several research groups. [PERSON] et al., 2009 [4] replaced, in cement pastes and mortars, OPC Type I (42.5 R) by 10, 20 and 30 wt.% of BFA resulting from the combustion of forest residues (mainly eucalyptus wastes resulting from logging and wood processing activities). Dried (60 \({}^{\circ}\)C) and sieved (75 \(\upmu\)m) BFA was used, and the prepared samples were cured for 28 days. For 20 wt.% of OPC substitution the mechanical strength of the specimens was around 75% of the reference mortar and was taken as acceptable. A strong strength deterioration was noticed for larger replacement amounts. Additionally, the relative high contents of carbon, chlorides, and sulphates in BFA limit the substitution rate. The presence of chlorides and sulphates in cement-based materials can deteriorate the microstructure and, consequently, the durability of products. The presence of organic matter interferes with the setting time due to the tendency to adsorb water. In a later work, [PERSON] et al., 2015 [23], starting from the same formulations used in the aforementioned work [4], evaluated the fresh and hardened (for shorter and longer curing times) state properties of the cement pastes and mortars. The increase of the OPC substitution by BFA promotes a rise in water demand and setting time and a decrease in the hydration temperature of the pastes. Comparing with the standard mortar composition, the substitution of OPC by BFA decreased the flexural and compressive strength of the samples cured for 7, 28, and 90 days. However, for longer curing times (90 days) the difference was shortened. [PERSON] et al. [3] studied the partial OPC (CEM I 42.5 R) replacement (10, 15 and 20 wt.%) by a dried (70 \({}^{\circ}\)C) milled mixture of BFA and bottom bed ash, from wood chips combustion, in mortars that were cured for 28 days. The milling process provided a fineness similar to that of OPC. All the prepared mortars show good functional properties. The addition of 10 wt.% BFA improved the flexural and compressive strength of the mortars, for the other two additions it was observed a slight decrease in both properties. Leaching tests were performed on the mortars and it was verified the presence of only trace amounts of chlorides, nitrates, and sulphates. [PERSON] et al. [6] prepared mortars with 20 and 40 wt.% of OPC (CEM I 42.5N) substituted for three different types BFA. The technical performance of the mortars was influenced by the type and percentage of BFA used, and it was verified that increasing the OPC replacement rates the strength decreased. Substituting 20 wt.% of OPC by BFA from fluidized bed combustion of Ca-rich biomass, provide mortars with compressive strength compliant with the requirement for structural concrete application of 42.5 MPa after 28 days of curing. For the same substitution rate, the use of BFA from the combustion of wood pellets in a pulverized fuel installation provide mortars with the lower compressive strength, but that complies with the requirement for non-structural applications (32.5 MPa). In a recent work [18], [PERSON] et al. used as-received BFA generated from the combustion of wood chips in a fluidized bed furnace as a substitute of OPC (CEM I 42.5 R) in a mortar formulation. The substitution rate varied from 10 to 70 wt.%. Up to 30 wt.% of OPC replacement by BFA the mortars obtained after 28 days of curing kept their functional properties.