## 3 Results and Discussion
### Mechanical Behaviour  
Figure 5 shows the mechanical properties values of the mortars, with different binders with siliceous sand, at 28, 90, and 180 days.  
The results presented in Figure 5 indicate that all mortar compositions show relatively low mechanical strengths and moderate values of modulus of elasticity. Nevertheless, the strengths and the modulus of elasticity, in general, increase with the curing age, which is attributed to the progression of the carbonation reaction. However, in A+NS5% mortar, a noticeable increase in the compressive strength values could be observed at early curing times, and then a steep decrease was recorded at longer curing times.  
Figure 4: SEM images of the mortars’ microstructure at low magnification, with a working distance (WD) of 15.0 mm. It is possible to observe the compact microstructure of the A (slaked air lime) mortar (**a**) and A+Q30% (slaked air lime with 30% of quicklime) mortar (**b**); the general microcracking on the A+NS5% (slaked air lime with addition of 5% of nanosilica) mortar is shown (**c**); as well as the heterogeneous microstructure of the NHL (natural hydraulic lime) mortar with few unconnected sub-rounded macropores (**d**).  
Contrary to that observed in other studies with pozzolanic lime mortars [21, 26], the additions of NS, namely with 5% wt, lead to a decrease in strength at latter ages as a result of drying shrinkage (Figure 6). This can be due to the pozzolanic reaction that absorbs a significant amount of water, associated with a dry curing, which negatively induces mortars microcracking, as shown in SEM images (Figure 7a), affecting its physical-mechanical properties [22, 51].  
Figure 5: Mechanical properties of the mortars with siliceous sand.  
Figure 6: Cracking in lime paste applied on porous brick due to the effect of nanosilica.  
Mortar A is the most deformable; however, it has compressive strength values of the same order of magnitude as A+Q and A+NS mortars. Furthermore, if a small amount of cement is added (25%, in volume), the compressive strength of the mortars increases; however, the flexural strength is similar to that of lime mortar (A). The highest mechanical behaviour was obtained with the NHL mortar, and its strength is achieved at earlier ages compared to the air lime mortars. However, from 90 to 180 days, the A+Q30% mortar displays a significant evolution compared to other mortars compositions. This good behaviour of the A+Q30% mortar may confirm the literature argument [18] that quicklime absorbs the excess water from mixing in air lime mortars, thus reducing the drying shrinkage and consequently the microcracking during the drying process (Figure 7b), thus increasing the mechanical strength.  
In what concerns the mechanical performances of the prismatic specimens with substitution of part of the siliceous sand by limestone aggregate (Figure 8), it is observed that the substitution is beneficial to the A+Q mortars; however, for the C:A mortar, the incremental improvement of the mechanical performance is only observed with 30% of sand substitution. As seen in previous studies [52; 53], lime mortars prepared with limestone aggregate showed an increase in their mechanical strengths due to the crystallographic continuity between the limestone aggregate and the lime matrix [52].