## 1 Introduction
However, their development of mechanical strength is slower than for hydraulic mortars, since the hardening happens essentially by carbonation, which is a slow process and does not occur in saturated environments due to requiring diffusion of dissolved CO\({}_{2}\) through the mortar pore structure. On the other hand, it is well known that cement mortars must be avoided whenever possible in rehabilitations works because they favour salt damage due to their content of alkali and sulphate ions, and they have excessive mechanical strength and elastic modulus that could transmit high stresses to a weaker substrate due to restrained shrinkage [15]  
Figure 1: Paimogo’s Fort, in 2023.  
and the difficulty to follow the movements of the lime-based masonry. Moreover, cement mortars have lower water vapour permeability than lime-based materials, which limits the capacity to allow the evaporation of the capillary or adsorbed water in the wall [16], besides having a very distinct final appearance (in terms of colour and surface texture). However, in a similar case study, a lime-white cement mortar was applied in two thin coats of 1:1:6, as base coat, and 1:2:9, as the finishing coat (cement:lines:siliceous aggregate, by volume), with acceptable results for the last 30 years [17]. Other alternative innovative lime mortars solutions can be used for this case study, as suggested by [PERSON]; in the 17 th century, the author proposed the addition of quicklime to hydrated lime mix to increase its mechanical strength and provide volume stability of the lime mortars [18], since the excess water of the mix is then consumed by the quicklime instead of evaporated, avoiding macroporosity (typical of the lime mortars). However, a recent study by [PERSON] et al. [19] showed cracking and degradation problems in mortars with this type of mix. In the last decades, great attention was focused on enhancing the behaviour of lime mortars with the addition of nano products, such as nanosilica [20; 21; 22; 23; 24; 25; 26; 27]. An important effect of the use of nanosilica in a lime mortar is the fast pozzolanic reaction, and consequently, the increase in its strengths [20; 21; 22; 23]. Moreover, some studies report the positive impact of nanosilica on the mortars durability when subjected to laboratory weathering tests (temperature and relative humidity cycles, rain, UV light tests, and freezing resistance cycles) [24], as well as against sodium salts attack [25]. However, due to the very high specific surface of the nanosilica, it is expected to lead to high water/binder ratios, which will increase the coarse porosity and decrease the mortars' mechanical performance [25; 26]. Other industrial pozzolanic additions, such as metakaolin, could be also an alternative solution for these repair mortars [28]; however, in some literature reviews, it was observed that lime-metakaolin may lose strength in the long term [29; 30; 31], which was demonstrated in the results of a similar case study in a fortification near Lisbon that present high levels of degradation after 20 years of its application [32]. Therefore, choosing durable and sustainable new renders for these constructions is a challenging situation. In this investigation, several lime-based mortars compositions are proposed and characterised with the aim of selecting a compatible and sustainable mortar that is durable to the aggressive environment for the restoration of Painogo's Fort. Their main physical characteristics--capillary absorption and open porosity--and mechanical characteristics--flexural and compressive strength and dynamic elastic modulus--are analysed and compared at 28, 90, and 180 days. Moreover, to improve the mortar properties, a fine-tuning of the aggregate was carried out in two mortar compositions, which showed the best performance when compared to the other compositions.