Lime is one of the oldest building materials: a binder made by burning limestone in a lime kiln. The burnt stone (quicklime) is then slaked with water to become slaked lime. Lime is used as a binder in masonry and plastering mortar and in limewash. It is vapour-open and more flexible than cement, which makes it especially suited to old and listed walls.
From limestone to binder
Limestone (calcium carbonate) is heated in the kiln to about 900 degrees. This produces quicklime (calcium oxide). Slaking it with water gives slaked lime (calcium hydroxide), the workable form. There are two main types:
- Air lime — hardens slowly by absorbing carbon dioxide from the air (carbonation), turning back into limestone. It hardens only in the presence of air.
- Hydraulic lime — contains substances that let it harden even without air, and even under water. It is a little stronger and sets faster.
Application
Lime is favoured in restoration and for laying soft, historic bricks, precisely because it is softer and more vapour-open than cement. Moisture can then escape from the wall and the hard binder does not damage the brick.
Points to note
- Do not work lime in frost, and let it dry slowly
- Air lime needs time and air to harden
Related terms
- Slaked lime
- Lime mortar
- Binder
- Masonry
Want to learn more about construction terms? Visit our knowledge base at fredsdiyplans.com.