Abstract Ladle furnace basic slag is a by-product of the steelmaking industry. This study examines the properties of masonry mortars made with ladle furnace basic slag and the other conventional components such as sand, cement, and admixtures. Eight different mixes were prepared and the main properties of the resulting mortars were analysed: density, strength, porosity, microstructure and permeability. The porosity of the hardened mixes was studied by means of two complementary techniques: mercury intrusion porosimetry and computerised X-ray tomography. Finally, tests were performed to analyse the behaviour of the mixes exposed to standard detrimental agents such as frost, moisture and sulphates, as well as other special environments, such as saline water or sulphidic atmospheres. The results show that the presence of ladle furnace slag does not damage and even contributes to increasing mortar durability. Its use also decreases or eliminates volumetric contraction during mortar ageing, thereby improving certain masonry applications.