Are all Limes Equal?

For those new to historic buildings, lime and its terminology can be intimidating. For owners of listed and historic properties, maintaining the structural and aesthetic integrity of their buildings is paramount and lime is integral to achieving this. It is a traditional binding material used in mortars, plasters, renders and limewashes and is valued for its flexibility, permeability, and compatibility with historic structures.

Not all lime however is created equal, and, in this article, we explore the range of limes used in building conservation and the appropriate usage for each.

TYPES OF LIME:
The production of all limes begins by the burning (calcining) of a calcium rich feed stock such as limestone, chalk or even seashells. The process of heating the feed stock produces calcium oxide, also known as quicklime. This is then combined with water in a process called ‘slaking’ to produce usable limes.

HYDRAULIC OR NON-HYDRAULIC?
Non-hydraulic lime ‘sets’ only by absorbing carbon dioxide from the atmosphere and is unable to set underwater. Hydraulic lime will set under water through chemical reaction.

1. Quicklime (Calcium Oxide, CaO):
What it is: Quicklime is produced by heating limestone (calcium carbonate, CaCO3) in a kiln to drive off carbon dioxide (CO2). This creates a highly reactive material in either lump, kibbled or powdered form.

Uses/Application: Typically slaked with water to produce lime putty or hydrated lime or slaked with aggregate and water to produce a hot mixed mortar. Quicklime needs careful handling due to its reactivity. It’s the basis of all limes.

2. Non-Hydraulic Lime (Calcium Hydroxide, Ca (OH)2, Fat Lime, Air Lime)
What it is: Non-hydraulic lime is produced by slaking high calcium quicklime with water. Slaking with an excess of water results in a creamy paste (lime putty) that cures slowly through the absorption of carbon dioxide (CO2) from the air - a process known as carbonation. Slaking quicklime with just enough water to convert to calcium hydroxide creates a dry power known as hydrated lime.

Properties: Stays workable for extended periods. Highly permeable and capillary active, allowing buildings to "breathe" and reducing the risk of trapped moisture. Soft and flexible, which is beneficial in accommodating slight movements in historic structures.

Uses/Application: Ideal for interior plasters, lime washes and finishing coats. Suitable for mortars in masonry work where flexibility and permeability are crucial. It is often also used for fine ashlar pointing and the rendering of soft substrates such as cob.

3. Lime Putty (Calcium Hydroxide (Ca (OH)2)
What it is: Lime putty is essentially non-hydraulic lime aged to improve workability. It’s produced by storing slaked lime in a moist environment. This aging process enhances its plasticity. In practice almost all lime putty is non-hydraulic. Highly versatile, ideal for fine decorative plaster work and pointing and bedding.

Uses/Application: Used in mortars and plasters where permeability is paramount.

4. Hydrated Lime (Calcium Hydroxide, Ca (OH)2):
What it is: Hydrated lime, or builders’ lime, is the result of adding just enough water to quicklime, to convert it to calcium hydroxide, but forming a dry powder rather than a putty. In practice all commercially available hydrated limes are non-hydraulic.

Uses/Application: Can be used in plasters, mortars, and as a stabiliser for soils and as an additive to improve the plasticity and workability of cement mixes. Not commonly used as a binder on its own. For internal plastering, pointing, and as part of lime and cement mortars.

5. Historic Hydraulic Lime (Lean limes, grey limes)
What it is: Hydraulic lime is made by slaking impure quicklimes made from burning limestone that contains clay and other reactive compounds. These compounds ‘set’ through a chemical reaction with water. Hydraulic lime is classified into three grades:

• Feebly Hydraulic Lime: Suitable for internal plasters and renders and mortars for soft substrates. Historic mortars were often feebly hydraulic.

• Moderately Hydraulic Lime: Often used for external work, including repointing and rendering. Provides a balance of strength and breathability. It sets faster and is more resistant to weathering than feebly hydraulic lime.

• Eminently Hydraulic Lime:  Used in areas subjected to harsh environmental conditions or significant structural stress, this lime sets quickly and has high compressive strength. It can be used in below ground and underwater work.

5.1 Modern Natural Lime (NHL)
Modern Natural Hydraulic Lime (NHL) is a modern hydraulic lime classified by its strength. It is available in different grades, NHL 2 (feebly hydraulic), NHL 3.5 (moderately hydraulic), and NHL 5 (eminently hydraulic). There has been recent research indicating that NHLs continue to gain strength over time, and this has caused concern that they may become less permeable. Properties: Faster ‘setting’ than non-hydraulic lime. Generally, less permeable but can provide a stronger and more durable finish. Sets both in air and underwater, allowing a wider range of potential uses.

Uses/Application: Commonly used for building, repointing and rendering. Used in areas exposed to the elements, such as exterior walls, roofs, and below ground. Suitable also for new builds offering a compromise between flexibility and strength.

6. Hot Mixed Mortar/Plaster
What it is: Hot mixed mortars involve the slaking of quicklime with sand and water, producing a highly reactive and hot mixture. This traditional technique creates a lime mortar that is rich in lime, offering good workability and adhesion. It isn’t a specific ‘type’ of lime and can be either hydraulic or non-hydraulic but in practice the vast majority of hot mixed mortars are non-hydraulic.

Uses/Application: Commonly used for bedding and pointing masonry and rendering. There is increasing evidence that hot mixed mortars may be more capillary active than other mortars, accelerating the drying of damp masonry. Ideal for use in historic buildings, particularly where there is a need to match original mortar compositions.

Properties: Provides high plasticity and stickiness, making it pleasant to work with. The exothermic nature of the mixing reaction has serious health and safety considerations and should only be undertaken by experienced operatives.

CHOOSING THE RIGHT LIME FOR YOUR PROJECT
Selecting the right type of lime is essential for the success of any project. Factors to consider include the building's location, the type of masonry, historic authenticity, required strength, the intended application, exposure to weather, and the aesthetic finish. Consulting with a conservation specialist or an experienced lime supplier can offer valuable guidance in choosing the most suitable lime for your property.

By understanding the differences between hydraulic and non-hydraulic lime listed property owners can make informed decisions that ensure the longevity and integrity of their historic buildings.

The general rule of thumb however should be that the softest material that will likely perform in any given situation should be used and that this should, where practicable, by sacrificial to the masonry units it is used with.

Rose of Jericho
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