Why Use Lime Mortar and Lime Plaster?
Many old buildings are constructed from materials such as brick, stone and cob which are relatively porous and often low in strength. Lime products, such as plasters and mortars, were normally used for bedding these materials and used for the internal and external coatings of buildings.
Before this century, building techniques were very different from those employed today. Traditional properties need to “breathe” to allow moisture inherent in solid walls to evaporate from the external walls.
Mixes made with lime are hygroscopic and also highly vapour permeable; allowing moisture to evaporate at sufficient rates to help buildings manage dampness.
Lime mortar is a relatively soft mortar and is therefore able to withstand a certain amount of movement (without cracking) that comes with settlement and seasonal changes.
Which Type of Lime Should I Use?
Lime mortars and lime plasters made form traditional lime putty cure are rich, fatty and perfect for plasters and renders and for replicating historic mortar mixes.
Mixes made from a lime putty binder have a soft appearance. The are highly breathable and flexible and therefore work sympathetically with all buildings both old and new.
Natural Hydraulic Lime
Natural hydraulic lime (NHL) was introduced in the 18th century by processing limestone containing natural impurities which resulted in a harder type of lime that sets with water.
Hydraulic lime offers a potentially cheaper option for renders, plasters and mortars. Give careful consideration to their selection due to their higher strength, reduced flexibility and reduced breathability compared to lime putty.
NHL also comes in different strengths denoted by a number: 2, 3.5 or 5.
Note that this is a minimum strength value after lab test at 28 days. Different manufacturers strengths can vary widely even within these grades.
As a guide, NHL5 is suitable for limecrete floors and high strength works, NHL3.5 is used for building and pointing strong masonry or for work on solid, strong buildings, and NHL2 is used for backing coats of plasters and sheltered or internal work.
If in doubt, ask for guidance from our technical team before selecting the lime for your project.
More details can be found at: www.naturalhydrauliclime.net
Time of year
Care should be taken not to apply lime mortars when there is a risk of frost due to the potential for damage and failure.
Carbonation takes approximately a month for each millimetre of thickness. Therefore it may take over 20 months before lime mortar has carbonated to a depth of 20 mm. It may be necessary to increase the amount of Pozzolan added based on the time of year and exposure.
Lime work should be protected to ensure complete weather protection at all times of the year.