Kalkput

Kalkput, or lime putty, is a traditional lime-based binder used in plaster, render, and mortar in historic building practice. It is produced by slaking quicklime (calcined limestone) with water to create calcium hydroxide, which is then worked into a putty that can be stored and aged before use. When mixed with aggregates such as sand, it forms lime putty mortar or plaster.

Production and aging are important aspects of kalkput. Fresh lime putty is workable but becomes stronger and more workable with time as it matures. This aging process allows impurities to settle and the mix to develop a more stable consistency. Kalkput is typically applied in thick, breathable coats and is often used in conjunction with traditional lime-wash finishes.

Properties and performance of kalkput are characterized by breathability, flexibility, and a relatively gentle set. It hardens primarily through carbonation—the reaction with carbon dioxide in the air—rather than hydration, which makes it compatible with historic masonry and other lime-based materials. Lime putty mortars are generally more forgiving of movement and moisture than cement-based mortars, and they allow moisture to escape from the substrate, helping prevent trapped moisture damage. However, lime putty mortars are weaker and slower to gain strength than modern cementitious binders and require careful workmanship and curing conditions.

Applications today are mainly in restoration, conservation, and traditional construction. Kalkput is favored for repairing or recreating historic walls, where it matches the appearance, texture, and performance of original materials. In modern practice, it is often used with hydraulic limes or lime-putty plasters to balance workability with early strength and durability. Safety considerations include handling lime products, which can be caustic and irritant to skin and eyes.