airlime

Airlime is a type of hydraulic lime mortar that has been used in construction for centuries, particularly in regions where traditional lime was readily available. The term "airlime" refers to lime that sets and hardens through carbonation when exposed to air, as opposed to hydraulic lime which sets through chemical reactions with water.

The material is produced by burning limestone or chalk at high temperatures to create quicklime, which is then slaked with water to produce lime putty or hydrated lime powder. When mixed with sand and water, airlime forms a workable mortar that gradually hardens as it absorbs carbon dioxide from the atmosphere, converting back to calcium carbonate.

Airlime mortars are characterized by their relatively slow setting time, which can range from several days to weeks depending on environmental conditions. This slow curing process allows the mortar to remain flexible and breathable, making it particularly suitable for historic building restoration and conservation work. The flexibility of airlime helps accommodate minor movements in masonry structures without cracking.

Compared to modern Portland cement mortars, airlime has lower compressive strength but offers superior compatibility with historic brick and stone masonry. Its vapor permeability allows moisture to evaporate from walls, reducing the risk of freeze-thaw damage and salt crystallization that can occur with less breathable mortars.

The use of airlime declined significantly during the 20th century with the widespread adoption of Portland cement, which sets faster and achieves higher early strength. However, airlime has experienced a revival in recent decades among conservation professionals and traditional building specialists who recognize its advantages for heritage restoration projects.

Airlime is available in various formulations, including feebly hydraulic varieties that offer improved water resistance while maintaining the essential characteristics of air-setting lime mortars.