## 3 Results and Discussion
### Screeds Fresh and Hardened State Properties
The samples that satisfy these requirements are the ones in which 17 wt.% of OPC was replaced by the as-received or the pre-treated BFA (sieved and grounded and sieved). Samples containing 50 wt.% BFA2_GS show flexural strength (3.6 MPa) satisfying the requirement for a commercial screed. The better compactness of these samples can explain this behaviour. Figure 6: Capillary water absorption of the mortars (cured for 28 days) prepared with: (**a**) BFA1 and (**b**) BFA2. The satisfactory results obtained in this work with 17 wt.% OPC substitution by as received BFA, are in agreement with other studies that pointed for 20 wt.% OPC replacement as the maximum recommended level [4; 6; 19; 23; 24]. [PERSON] et al. [18] suggested 30 wt.% as maximum OPC replacement level. It should be noted that [PERSON] et al. [4] used sieved BFA (<75 \(\upmu\)m) and [PERSON] et al. [24] used classified and ground BFA, to achieve such results. Figure 9 shows the effect of OPC content decrease, replaced by BFA, on compressive strength values. Predicted reduction for the standard formulation was also estimated (yellow cycles and dotted line), simply assuming the effect of binder reduction in the mixtures. This way we aim to detect if BFA has positive or negative effects on the mechanical strength of the mixes. Samples containing 17 wt.% BFA tend to show slightly higher compressive strengths than predicted from the OPC reduction, excepting the blend prepared with BFA1. This behaviour might reflect the poor compactness of this sample, being also a confirmation of the poor hardening potential of BFA1. Pre-treated ashes show suitable particle size distribution and samples are denser and more homogeneous. With higher replacement rates, the mentioned mixing difficulties and workability loss generate poorly compact samples. As a consequence, their mechanical strength values tend to be lower than the estimations based on the cement replacement. The only exception is the sample prepared  
Figure 8: Flexural strength of the mortars cured for 28 days, compositions prepared with BFA1 and BFA2. The red line represents the minimum values required for the target application. Figure 7: Compressive strength of the mortars cured for 28 days, compositions prepared with BFA1 and BFA2. The red line represents the minimum values required for the target application. with BFA2_GS (green open triangle), due to the narrow size distribution and suitable shape of ash particles, and its hardening potential (see Table 2). The amount of SiO2 + Al2O3 + Fe2O3 is equal for BFA1 and BFA1_GS (see Table 2), so it was expected same pozzolanic activity, which could not be confirmed by the obtained results. However, the milled ashes present a compressive strength slightly superior to the as-received BFA. A fact that can foresee a positive contribution of particle size reduction on their reactivity. It is possible that the contribution of BFA to the screed mortars strength improvement occurs for longer curing times, or, as suggested by [PERSON] et al. [38] that the pozzolanic activity is not directly proportional to the SiO2 + Al2O3 + Fe2O3 amount (determined by the chemical analysis) but to their amorphous content.