## 1 Introduction  
The effect of seismic actions on masonry structures is problematic in a significant number of worldwide urban areas. The devastating effects of these actions are highlighted by several authors for the cases of infill and structural masonry [1; 2; 3]. The development and characterization of masonry overlay strengthening systems based on the use of composite materials has been recently showing promising results. Depending on the type of matrix, these composite materials can be divided into two main groups; cementitious [4; 5; 6; 7; 8] and polymeric [9]. Both show clear advantages when applied to infill and structural masonry elements that are vulnerable to seismic actions. Their main contributions are the increase in the load carrying capacity and energy dissipation ability, without significantly altering their original mass. Additionally, the strengthened elements develop a much higher ultimate deformation and residual strength [10; 11; 12; 13]. Consequently, after reinforcement, masonry elements are better prepared for supporting extreme events without brittle failure occurrence. This is particularly important when working with older buildings, mostly composed by sound ceramic brick or natural stone masonry elements but also when dealing with more recent infill ceramic brick masonry. The improvement in the properties of mortars by means of the addition of natural fibres is well known and has been employed empirically for centuries [14]. Recently, various fibre-reinforced mortars (FRMs) have been developed by adding natural [15; 16; 17; 18] or man-made fibres, showing very attractive mechanical properties [19; 20; 21]. These FRMs can be used to upgrade the in-plane and the out-of-plane loading and deformation capacity of the original elements. Additionally, when used as external overlays they constitute a protection system to the reinforced walls from weathering agents. However, the FRMs developed in the recent past were frequently composed of Portland cement binder, which has been shown to lead to compatibility problems with old masonry elements, namely by the crystallization of salts that damage natural stone. Additionally, the low flexibility of the Portland cement-based mortars may cause structural problems in older ceramic brick or natural stone masonry when the building is subject to movements [22], and delamination of the strengthening layers in the case of infill masonry [11]. The use of natural hydraulic lime (NHL) binders are believed to show better compatibility with masonry in terms of strength and hygroscopic behaviour [23; 24; 25; 26]. Furthermore, the choice for NHL mortars was found to be more sustainable when compared to the use of Portland cement (CEM) [27]. The flexural strength (\(f_{t,\theta}\)) vs. compressive strength (\(f_{c}\)) correspondence of FRMs developed recently, composed of different types of binders and fibres, is shown in Figure 1. The mechanical performance of the mortars can be clearly divided into two groups. The first group, labelled as \"Geopolymer and CEM-FRM\", is composed mainly of Portland cement or Geopolymer (Geo)-based mortars that show compressive and flexural strengths higher than 20 and 5 MPa, respectively [28; 29; 30; 31; 32; 33]. The second group, \"Lime and NHL-FRM\", integrates mainly fibre-reinforced lime mortars (NHL-FRM), showing lower mechanical properties [34; 35; 36; 37; 38; 39; 40; 41]. Desirably, the mechanical properties of the FRM should not be much different from the substrates, especially the elastic modulus and the compressive strength [4]. The area marked and identified as \"target values\" in Figure 1 represents the desirable range of mechanical properties of FRM when applied as strengthening overlay of infill or structural masonry elements. The compressive strength values were determined based in previous research developed by [PERSON] et al. [11; 42] and research from other authors that characterised the mechanical behaviour of a broad range of masonry [43; 44].