## 3 Results
### Mechanical Properties  
In this section, the mechanical properties obtained during the characterization of the developed mortars are presented. Firstly, Table 9 shows the results for surface hardness and bond strength. As can be seen in Table 9, there are no significant differences between the surface hardness of mortars made with natural sand and mortars made with RCA. Likewise, the cement/aggregate ratio used is not a discriminating factor for this property. However, a slight decrease in Shore D hardness is observed in those mortars that incorporate natural fibers in their composition (approximately 6-7% on average). A similar behavior was experienced by [PERSON] et al., in their research with mortar made with the addition of recycled mineral wool fibers [57]. On the other hand, in terms of bond strength, greater differences were observed between the reference mortar and the mortar made with RCA. For both cement/aggregate ratios, the adhesion of the traditional mortars was superior to that of the RCA samples, with better adhesion for the samples with higher cement content (1:3 cement/aggregate ratio). This effect was already observed by [PERSON] in his doctoral thesis, which showed a non-linear relationship between the increase in adhesion and the cement content of the mortars [92]. On the other hand, a slight decrease in the adhesion of mortar on ceramic walls is observed when natural reinforcement fibers are incorporated. This effect may be due to the water retention capacity of these natural fibers, as well as to the weakening of the mortar's internal microstructure at the cement-aggregate interface. Of all the mortar types analyzed, those with the incorporation of toquilla straw fiber were the least resistant to adhesion. In any case, it is worth noting that, during the test, the breakage of the samples with fibers occurred in all cases at the mortar-ceramic piece interface. Figures 7 and 8 show the results obtained for the flexural and compressive strength tests of the mortars. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Sample** & **Superficial Hardness** & **Bonding Strength** & **Sample** & **Superficial Hardness** & **Bonding Strength** \\  & **(Shore D Units)** & **(MPa)** & **(Shore D Units)** & **(MPa)** \\ \hline NA-1:3 & 84.1 \(\pm\) 0.7 & 0.53 \(\pm\) 0.05 & NA-1:4 & 83.7 \(\pm\) 0.6 & 0.42 \(\pm\) 0.02 \\ RCA-1:3 & 83.8 \(\pm\) 0.4 & 0.4  
Figure 7 shows that mortars with a higher cement content (1:3 cement/aggregate mass ratio) have a higher flexural strength. Also, in accordance with the results obtained in other investigations, the total substitution of the natural aggregate by RCA results in a decrease in the mechanical flexural strength [93]. For this reason, the incorporation of reinforcement fibers has become a recurrent solution to improve the mechanical strength of these construction materials produced under circular economy criteria. Thus, it is observed that in all cases, the addition of natural fibers led to an improvement with respect to the flexural strength values obtained for recycled mortars without fibers. In addition, it can be seen that coconut and abaca fibers showed a similar behavior, being the ones that increased the flexural strength to the greatest extent. These results are in agreement with others analyzed in the literature review, where natural fibers increased the deformation capacity and reduced cracking in cement mortars [27,33]. Regarding to mortar compressive strength, shown in Figure 8, it is also observed that the traditional mortars surpassed the mortars made with recycled aggregate. In this sense, the dosages with a 1:4 cement/aggregate mass ratio show a lower strength compared to their counterparts with a higher amount of cement. This effect on the compressive strength of recycled aggregates has already been observed by other researchers [94], who highlight this characteristic as one of the main limitations to their widespread use in the construction sector.