When looking for lime products to use for plastering there can be some confusing terminology. Here we will try to clarify what’s what.
What are the benefits of using lime?
- Its relatively high flexibility and low stiffness (can withstand forces without cracking)
- There is less need for expansion (movement) joints.
- Lime mortar is weaker and breaks down more readily than the masonry, thus saving weaker stone such as sandstone and limestone from the harmful effects of temperature expansion and mortar freeze.
- It allows buildings to “breathe”, and does not trap moisture in the walls.
- It has a lower firing temperature than Portland cement, and thus a lower embodied energy.
- Stone and brickwork bonded with lime is easier to re-use.
- It is less dense than cement, thus less cold bridging.
- Lime re-absorbs the carbon dioxide (CO2) emitted by its calcination (firing in a kiln), thus partially offsetting the large amount emitted during its manufacture. The more hydraulic a lime, the less CO2 is reabsorbed during set, for example, 50% of CO2 is reabsorbed by NHL 3.5 during the set, compared to 100% of CO2 being reabsorbed by pure calcium hydroxide (fat lime putty).
There are two main sorts of lime available – hydraulic lime or non hydraulic lime.
What is Hydraulic lime?
Hydraulic limes (so called because they set under water) are made in the same way as non-hydraulic lime but using different limestone. This type of limestone contains naturally occurring impurities. These impurities speed up the setting time and increase the hardness of the resulting lime mortars, renders and plasters. They are sold as hydrated lime (powder) and have an initial set when water is added, followed by hardening while they absorb carbon dioxide. The more hydraulic a lime is the faster it sets and the higher it’s final strength, but this means that it is less breathable and flexible. NHL5 is the most hydraulic, then NHL3.5, and NHL2 the least hydraulic lime. The number indicates the compressive strength in N/mm2. Mortar should be softer than the masonry. This will absorb any movement in the walls and stop the masonry cracking. The stronger the NHL the less breathable and less flexible it is.
What does NHL stand for?
NHL Natural hydraulic lime. Made from naturally occurring limestone without additives. Natural hydraulic lime is produced by heating (calcining) limestone that naturally contains clay and other impurities: no materials may be added to create the hydraulicity. In the United States NHL may be called hydrated hydraulic lime (HHL) per ASTM C-141 Standard Specification for Hydrated Hydraulic Lime for Structural Purposes. In Europe NHL should follow European norms EN/BS 459, 1-2-3 ‘building lime’
What is Non hydraulic lime?
Non-hydraulic lime (CL or DL 70-90) is sold as either hydrated lime or putty lime; they set and harden through drying out and absorbing carbon dioxide from the air. This means they have a very slow set: CO2 is only absorbed when certain conditions are met. They are the softest, most breathable limes available. Confusingly the abbreviation NHL means the other type of lime, Natural Hydraulic Lime.
What is hydrated lime?
Hydrated lime Water is added to quicklime to make a powder that is more stable and safe to handle. This can be done to hydraulic lime or non-hydraulic lime. Sometimes in the construction industry people mean non-hydraulic hydrated lime when talking about hydrated lime, for example with S type lime commonly used in the USA.
Does ‘artificial’ hydraulic lime exist?
Yes. NHL-Z , FL or HL limes contain additions like cement and or pozzolans, to create hydraulic properties. These options are less suitable for ‘eco’ buildings.
What is S type lime ?
S type lime is the most common and widely available type of lime used in the USA mainly mixed in a mortar mixture to improve elasticity of a cement mortar.
What is lime putty?
Lime putty can be made from either type of lime. It is is made by adding an excess of water to quicklime. Hydraulic lime putty will set underwater within hours or days making them impractical, whereas non-hydraulic lime putty will remain plastic and improve with age. If kept in an airtight container and covered by a layer of water lime putty can keep for a very long time. So typically non-hydraulic lime is used for lime putty.
This type of lime is often used in the restoration work of historical buildings, as it is relatively soft, flexible and breathable. This prevents cracking and condensation issues in buildings with softer and more flexible structural building materials like wood, earth, limestone and brickwork.
Hydrated lime products are produced using lime putty as the binder and are usually supplied as a wet mix. They take a long time to cure, but are the most breathable and most flexible lime products available.
Lime putty is produced by adding quicklime to water. This process triggers an aggressive chemical reaction known as slaking. The putty is then set aside for a minimum of 4 months. This maturing process ensures that the slaking process is fully complete. Mature lime putty is mixed with different grades of aggregates to produce traditional lime mortars, renders, plasters and fillers. It is also mixed with water to create limewash.
Pozzolans are additions that may be added to achieve harder, faster sets to any sort of lime or cement. Pozzolans, when added, produce similar chemical reactions to those found in hydraulic limes, so they reduce breathability and flexibility in exactly the same way. The disadvantage is that you will never know how strong, breathable or flexible a pozzolan lime is beforehand, unless you have considerable experience or knowledge. Adding some types of pozzolans or even the smallest amounts of cement can be very damaging or produce poor performing lime mortars. We always recommend testing first.
Fat lime
Fat lime is another term used for lime putty, made from non hydraulic lime.
Limecretes, mortars, plasters and renders.
These can be made by mixing hydraulic or non-hydraulic lime with aggregate and water. tables with mixes coming soon
Should You Choose Hydraulic or Non-Hydraulic Lime?
This depends on the type of work to be done, the connecting surfaces, and their strength. Also it depends on the required final strength of the plaster or mortar and on the required breathability.
How exposed will the mortar or render be?
The more exposed to the elements a lime mortar or lime render is, the greater the need for a faster set and greater durability. Also it is possible to improve the durability of the plaster by treating it with soap. See our post about earth and lime plasters for a detailed description and how to video.
How much movement will the mortar have to cope with?
The less hydraulic a lime is, the more it will flex and move with a building. Timber structures therefore need a more flexible and breathable lime.
How much damp will the lime mortar have to cope with?
Lime putty or hemp lime mixes should be used with caution in houses where damp is a problem. (N.B. if a wall is permanently very damp, a putty mix may never set.) Low suction backgrounds (hard stone or blue bricks etc) and damp cool weather also make the use of lime putty very slow, consider using a hydraulic lime based mortar instead in these situations.
Lime putty is typically the first choice for internal plastering due to its plasticity and better vapor exchange: in thin layers it is easy for it to absorb CO2 and set. Limewash and lime paints are also typically made from non-hydraulic lime.
Does it really matter what type of lime I use?
Yes. Using a lime that is too strong can be very damaging for a structure. For example of a mortar that is too hard is used with very soft brick, it could cause cracking of the brickwork. Instead, the mortar should be able to ‘dampen’ forces on the building (for example expansion and compression through wind and temperature changes) much like softer collagen between harder bones.
Does hydraulic lime contains the same damaging chemicals as cement?
No. Natural Hydraulic Lime (NHL), doesn’t contain any damaging chemicals. Artificial Hydraulic Lime might, so check the source and ingredients.
Is it OK to add cement to a lime mixture to make it set more quickly ?
No. Even small amounts of cement in traditional mortars lead to inferior less durable mortars that can cause problems. If a mortar with a set is required then a hydraulic lime is more suitable.
About this article
this article is under construction: – pictures, infograph, tables with mixes coming soon
sources:
suppliers in the UK:
https://www.celticsustainables.co.uk
https://www.roundtowerlime.com/
https://www.lime-green.co.uk/knowledgebase
https://www.limestuff.co.uk/blog/hydraulic-or-non-hydraulic-lime
https://www.pozament.co.uk/products/hydrated-lime/
suppliers in and around Germany: