Blast furnace slag is a residue of steel production. It is a latent hydraulic binder and is normally used to improve the durability of concrete and mortars. Slag could be also used as rendering mortar for masonry and old buildings. Today, cement and hydraulic lime are the most popular hydraulic binders used to make plasters. They are characterised by a low durability when exposed to the action of chemical and physical agents. An attempt was made to compare the physical and mechanical properties of some renders made with ordinary Portland cement, hydraulic lime, or slag. Furthermore, mortar resistance to several aggressive conditions like acid attack, freezing and thawing cycles, abrasion, sulphate attacks, cycles in an ultraviolet screening device, and salt diffusion was analysed. The specimens, after chemical attack, were characterised by the BET method, XRD, and SEM. 16 refs.