## 1 Introduction  
Located on the cliffs of Painogo beach, in Lourinha municipality, Portugal, Painogo's Fort was classified as a Building of Public Interest since 1957. Designed to defend the north coastline of Lisbon from Cascais to Peniche, it was built in 1674, by decision of the 3 rd Count of Cantanhede, and belongs to a large set of fortifications of the 2 nd defensive line of Tagus River [1]. This historical construction, which was abandoned after the Liberal Wars (1832-1834), has been subjected to a very aggressive environment due to the proximity of the sea: salty water spray, high humidity, intense sun radiation, thermal shock, and strong winds. Mortar renders, whose main function is the protection of walls, are particularly exposed to such actions that make them especially susceptible to degradation over time (Figure 1). Promoted by Lourinha Municipality, the \"Coast Memory Fort of Painogo\" European Economic Area (EEA) Grants project aims to safeguard and revitalize Painogo's Fort. For this purpose, an analysis of the original mortars and an assessment of the conservation condition of the Fort was carried out. Similar to the other Portuguese coastal fortificationsfrom the 17 th and 18 th centuries [2; 3; 4; 5], it is proven that this Fort is very well constructed, with well-selected materials and craftsmanship. Samples collected in situ clearly show that the original mortars used for masonry and renders were mechanically resistant and durable: they are based from calcitic air lime and mainly siliceous sand; shells were also found in their compositions, possibly from the local beach or fluvial sand deposits available near the fortification. Moreover, it was identified that the aggregate is free of clay. The binder/aggregate ratio varies depending on the type of mortar, ranging between 1:0.5:1 and 1:1:3 (limecalcareous sand:siliceous sand, mass proportions). Considering the mortars performance, all compositions present high mechanical strengths (ranging between 2.5 and 5 MPa in compression) and moderate rate of water absorption (capillary water absorption coefficient between 0.4 and 1.7 kg/m\({}^{2}\)-min\({}^{1/2}\)) [6; 7]. However, in the intervention carried out in 2006, most of the original renders and plasters have been removed and replaced by new ones, which today present erosion and loss of cohesion. Thus, new repair mortars need to be formulated considering the physical-mechanical and chemical characteristics of the original mortars and of the masonry of the Fort, as well as the severe environmental conditions to which it is subjected; otherwise, there is a risk of accelerating degradation. For this reason, the formulation of replacement mortars is a complex process that must consider compatibility requirements, without neglecting the original appearance as well as adequate performance and durability. In general, the repair mortars for this type of construction must have relatively high mechanical strength to resist erosion of the ocean wind and salt crystallization pressure, moderate elasticity modulus to accommodate deformations due to thermal variations, moderate capillary coefficient and high water vapour permeability to retard the entrance of water and allow a quick drying, as well as keeping compatibility characteristics. In addition, they should not have high contents of salts, to avoid the increase in salt contamination into the walls [8; 9]. Several studies have pointed out the use of lime-based mortars as an appropriate and compatible solution for the rehabilitation of historical buildings [10; 11; 12; 13; 14]: lime-based formulations are chemically, physically, and mechanically compatible with the ancient mortars due to their similar composition, high porosity, and ability to accommodate movements from masonry structures.