Lime mortar

Mortar is one of the oldest building materials, used to allow large structures to be constructed from small, easy-to-handle components such as bricks, blocks and stone. It is composed from a mixture of a fine aggregate, a binder and water. This combination creates a paste that is used in masonry construction as a bedding and adhesive to bind and fill the gaps between adjacent blocks.

In modern construction, the fine aggregate is typically sand, and the binder cement. This is known as a cement mortar. However, if lime is used as the binder, this is known as lime mortar. Rather confusingly, a small amount of lime may also be also be used in cement mortars, where it acts as a plasticiser, making the mortar slower to harden and more flexible.

Lime mortar has been used in construction since the time of the ancient Egyptians around 4,000 to 6,000 BC. It continued to be used until the 19th century, when the introduction of Portland cement led to the emergence of cement mortars, which were faster setting and had a higher compressive and flexural strength. However, lime mortar is still used today, particularly on older buildings, both as a mortar and a render.

Lime is manufactured from limestone or chalk (calcium carbonate) which is crushed and then heated in a kiln to around 1,000°C. This converts the calcium carbonate into calcium oxide (quicklime) which is reacted with water (slaked) to produce a fine powder.

The word lime is derived from the Old English lim, meaning 'sticky substance' and the root, lei relating to slime or slimy. It obtained this name because it has a very fine particle size, finer than cement, and so is able to penetrate smaller ‘holes’ in the materials it is binding. It then tends to bind them more ‘gently’, as it is more flexible and gives earlier adhesion, but then gains strength more slowly than cement mortars. Lime mortar also remains workable for longer than cement mortar, even when used with absorbent masonry, and so it can be easier to properly fill joints.

Where less flexible, dense mortars such as cement mortar are used to bind softer masonry, such as soft sandstones, they can cause local stresses leading to the deterioration of the masonry.

Lime mortar is also more ‘breathable’ than cement mortars, that is, it is more vapour permeable, and so is less likely to trap moisture within the masonry construction. It is also better able to accommodate moisture changes, reducing the potential of damage from salts.

Despite this, some poorly-informed contractors continue to use cement mortar on masonry that is better suited to lime mortar, leaving it at risk of long-term deterioration.

Damage caused by the use of cement mortar rather than lime mortar on a soft sandstone.

Lime mortar has lower embodied energy than cement mortar, and its binding properties mean it is more possible to re-use masonry. It has a ‘self healing’ capability, and it tends to fail under load before masonry, and is then easier to repair than the masonry would be if it failed.

Lime mortar can me made using hydraulic lime or non-hydraulic lime:

Hydraulic lime sets by hydration (the addition of water). For more information see: Hydraulic lime.
Non-hydraulic lime sets by carbonation (through exposure to the air).

Specifications for lime are set out in BS EN 459-1:2015 Building lime. Definitions, specifications and conformity criteria.

Lime mortar can take a long time to achieve its full strength, which depending on the conditions of moisture and temperature can be many months. Pozzolans can be added to achieve a harder, faster set, and other filler materials may be added to bulk up the mix.

If work is being undertaken to a historic structure, it is important to establish what sort of mortar was used in its original construction (or what sort of mortar predominates in the existing construction) as the use of the wrong mortar can lead to deterioration of the masonry, and the properties of lime mortar and cement mortar make them incompatible and so they should not be used together.