Lime mortar

Lime mortar, in Turkish kireç harcı, is made by mixing slaked lime with sand or a similar aggregate and water. Air-lime mortars harden slowly as the calcium hydroxide they contain reacts with carbon dioxide in the air and turns back into limestone, a process called carbonation. Hydraulic limes, made from limestone containing clay, can also set in damp conditions. Lime mortar was the bedding mortar, render and pointing material of most historic masonry buildings.

Why lime suits historic buildings

Lime mortar is relatively soft, porous and flexible. These qualities are not flaws; they are what keeps a masonry wall healthy:

• It lets water vapour pass, so moisture inside the wall can escape.
• It accommodates small settlements and thermal movement through fine cracking, so damage goes to the mortar rather than the stone or brick.
• It can be raked out and renewed without harming the masonry.

Lime mortar in restoration

Cement mortar is harder and less permeable than historic stone and brick. When the wall can no longer breathe through its joints, moisture and dissolved salts evaporate through the stone and brick instead; stone faces scale and bricks spall. Repair mortars are therefore designed from laboratory analysis of the original and never harder than it. Because aggregate colour and grain size govern how a joint looks, trial panels are made in advance. Lime sets slowly, so work on site needs control of moisture and temperature, shading and a patient curing period.