Kalkkalaastia

Kalkkalaastia is a lime-based plaster used in traditional Finnish and Nordic building practice. It is typically made from a lime binder (lime putty or hydraulic lime) mixed with aggregates such as sand, and it may include pozzolanic additives to influence setting and durability.

The plaster is usually applied in successive coats, including a scratch coat to provide a key, a brown coat, and a finishing coat. It is designed to be breathable, allowing moisture to pass through the wall, and it helps regulate interior humidity. The finish can be smooth or textured, and limewash or pigment finishes may be added for protection and appearance.

Historically widespread in Europe before the widespread use of cement-based plasters, kalkkalaastia remains important in heritage conservation and in modern sustainable renovations. It relies on carbonation to cure, which means it gains strength as it absorbs carbon dioxide from the air. Hydraulic lime variants cure faster and can perform better in damp conditions.

Maintenance and repairs should use lime-based plasters to ensure compatibility with older walls. Cement-based patches can trap moisture and cause damage. In contemporary use, kalkkalaastia is often chosen for restored historic buildings or for new constructions seeking breathable, low-embodied-energy finishes.