Traditional Lime Plaster
Heritage houses and building restoration, typically results in the property having lime plaster or pointing within the construction.
The types of lime
Natural Hydraulic Lime or NHL
Hydraulic Lime, normally comes in bag form, and should not be confused with Hydrated Lime as it contains other materials like cement, blast furnace slag, limestone filler and other materials that react to harden the mortar, NHL2 is normally used for internally plastering.
NHL’s lime sets much quicker than a Lime Putty based mortar, but are less flexible and not as vapour permeable ‘breathable’ as a Lime Putty Mortar, these attributes are reduced as the NHLs get stronger.
Mixing Natural Hydraulic Lime
NHL is mixed with a good quality sharp washed sand, the mixing ratios vary depending on use generally a 3:1 mix for most plastering jobs, but for floor screeds you would use a 2:1 mix.
- When mixing wear protective goggles, dust mask and waterproof gloves must be worn, to prevent injury.
- One of the most essential points is that the lime is uniformly dispersed and that any fine agglomerations are broken down, the mixing will be controlled by the efficiency of the mixer.
- Introduce half of the sand and add all of the lime, mix well until a uniform colour is achieved normally for 2 to 5 minutes.
- Stop the mixer and scrape down any material adhering to the back, add the remaining sand and mix again to get uniform dispersion.
- Continue mixing adding water slowly giving plenty of time for water to be fully incorporated, the mortar should be like a dough the less water added to achieve this, the better the mortar performance.
- The longer the final mixing time the more workable (fatter) the mortar will be. Workability will also be improved by allowing the mixed mortar to stand for 15 minutes or longer before re-mixing for a further 5 minutes.
- In hot weather do not over-mix as water will be lost through evaporation.
Formulated lime (FL): Formulated lime consists of hydrated lime and/or natural hydraulic lime with added hydraulic or pozzolanic material. It is a legal requirement that the inclusion of any cement or cement clinker must be declared and a limited percentage.
Lime Putty is the product that has historically been used in heritage houses as the binder in lime mortars and is sometimes call Non Hydraulic Lime, Fat Lime or Air Dried Lime.
Lime Putty is made by a different process than Hydraulic lime and doesn’t contain impurities, quicklime is slaked with enough water to make a liquid and then left to mature for 3 months plus to form into a cottage cheese like consistency.
Lime Putty cures by carbonation with the air once dry, this is why it can be kept for years in the air tight containers it’s delivered in unless opened. The Lime Putty naturally forces water to the surface when stored in the container so it forms, its own air tight barrier.
The Lime putty gets better/ more workable as it ages as it has had time to fully slake and force out all of the excess moisture that it has gained during the slaking process.
Lime Putty based mortars take longer to set than Natural Hydraulic Limes and cement, this is because it carbonates at around 1mm a month, dependent on the environmental conditions.
|Hydrated Lime sometimes confused with natural hydraulic lime due to similarity of name and also being a powder but is not used for the same applications.|
Hydrated Lime is made when quicklime is added to just enough water to slake it into a powder. It is a lesser form of lime putty, it is normally used as an additive in cement as a plasticiser, it should never be used as a binder in its own right as it is not strong enough, and will fail.
Calcium oxide/ quicklime, is also known as lump lime, this is the raw material that is used to make lime putty, quicklime is made by burning limestone or chalk in a kiln. This drives carbon dioxide into the atmosphere, leaving a very reactive material, calcium oxide. It is made in a range of sizes from lumps down to very fine powder, which can be mixed to form a lime wash, and used a vapour permeable paint, to be applied to the lime plaster.