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Concerning strengthening techniques for existing masonry walls, the trend is toward the use of Fibre Reinforced Mortar (FRM) with both a significant increase of the mortar strength, and a drastic thickness reduction of the added external layers. In the present analysis, the focus is posed on some new lime mortar types which possess a very high strength in tension, although the elastic modulus remains in a normal range, leading so to a considerable toughness increase. The G-FRM system is composed with a glass fibre mesh which must possess a tensile resistance larger than the one of the mortar area including it, and this allows introducing some ductility in the composite. A total of 14 diagonal compression tests of masonry walls reinforced with G-FRM have been completed at LISG (Structural Engineering and Geotechnics Laboratory, University of Bologna), encompassing different FRM combinations. More precisely, the walls were reinforced with three different lime mortar compounds with layer thicknesses of 12, 15 and 30 mm, and reinforced with two different glass fibre meshes and two different arrangements of steel micro-wire strips. The performed tests showed that the interpretation of the observed behaviour needs a sound theoretical basis of the experimental setup, avoiding the simple analysis reported in standards and codes. A finite element model of the experimental setup was prepared, able to identify the features of the observed behaviour. Following this suggestion, a new theoretical model based on Mohr-Coulomb plasticity was defined and applied to the interpretation of a large database of experimental tests with a very good agreement.