Kalkmortel

Kalkmortel, or lime mortar, is a traditional binding material used in masonry. It consists primarily of lime as binder, combined with aggregate such as sand and water. The lime binder can be produced as lime putty from slaked lime (air lime) or as hydraulic lime containing natural impurities that give a hydraulic set. In modern practice, lime mortars may also use hydraulic lime products to balance workability with earlier-set strength, while preserving breathability.

There are two main families: air-lime mortars and hydraulic lime mortars. Air-lime mortars cure by carbonation, are highly permeable, and remain relatively flexible, making them ideal for historic brick and stone structures. Hydraulic lime mortars gain strength through hydraulic set and perform better in damp conditions; they still breathe and accommodate moisture movement, reducing cracking.

Properties and usage: Kalkmortel is vapor-permeable and more forgiving to substrate movement than cement-based mortars. This makes it compatible with historic materials and finishes, such as lime plasters and porous bricks. Mortar mixes are typically lighter than cementitious mortars and are chosen to match the masonry's strength and porosity. Poor compatibility, such as using cement-rich mortars, can trap moisture and damage historic masonry. Repointing and repairs should use lime mortars to maintain structure integrity.

History and contemporary use: Kalkmortel has been used since Roman times and remained standard in Europe through the pre-industrial era. With the rise of Portland cement, lime mortars declined, but are now widely used in restoration and new builds that aim for sustainability and compatibility with traditional materials. Lime mortars are valued for longevity, reversibility, and environmental advantages when sourced responsibly.