Why Can Traditional Mortars Damage Historic Masonry?

Historic brick and stone walls are not merely remnants of old structures—they are part of our cultural heritage. Their durability depends on proper conservation and the use of compatible materials. Although standard mortars perform well in modern construction, they can cause serious damage to historic structures. So why are conventional lime and cement mortars unsuitable for the renovation of historic walls?

Problematic Properties of Traditional Mortars

1. Low durability of air lime mortars
Traditional mortars based on air lime have low mechanical strength and poor resistance to weathering. In urban environments, where moisture and air pollution (SO₂, CO₂) are prevalent, harmful salts can form, leading to efflorescence, erosion, and crumbling of joints.

2. Excessive impermeability of cement mortars
Cement-based mortars trap moisture inside the wall, promoting salt crystallization at the interface between mortar and brick. This leads to microcracks, detachment of bricks, and gradual structural degradation. Additionally, thermal expansion differences between cement and brick cause internal stress and mechanical damage.

3. Material incompatibility
Using mortars with different physical properties than the original materials disrupts moisture transfer and stress distribution. This accelerates deterioration, especially in masonry built with traditional methods, where the mortar joints also serve a breathable function.

To preserve the integrity of historic walls, it is essential to use restoration mortars that are chemically and physically compatible with the original materials.