The appearance of efflorescence on external brick walls still remains a big issue for many contractors. The present study investigates the role of different mortar additives. Mortars were produced with a commercial cement (CEM II) and different type of additives (five commercial mortar air entrainers, one anti-frost and dishwashing liquid). Tests were performed on fresh mortar pastes, mortar prisms and walls associating mortars and bricks. Some additives and/or some concentrations induces a strong modification of the porosity of the mortar. This structural change is associated with a greater leaching of ions Ca²⁺ and SO₄²⁻. In some case, some air entrainers significantly increases the pore diameter increases (from 5 μm to about 80 μm). A linear relationship between the increase in average pore diameter, the total porosity and the amount of Ca²⁺ and SO₄²⁻ ions released is established. Above a total porosity of 40%, which corresponds to an average pore size of 0,2 μm and a median pore size of 2 μm, the amount of Ca²⁺ and SO₄²⁻ leached dramatically increase. On masonry, it was confirmed that these changes lead to an increase in formations of visible washable and non-washable efflorescence. FTIR analysis highlighted that these were mostly composed of calcite CaCO₃, gypsum CaSO₄ and syngenite K₂Ca(SO₄)₂. It is therefore essential to choose the right type and concentration of mortar additives in order to reduce the risk of efflorescence. This applies to mortars prepared on worksite but also to premix mortars.