Why Use Lime Mortar and Lime Plaster?
Before this century, building techniques and materials were very different from those employed today. Traditional properties need to “breathe” to allow moisture inherent in a solid wall construction without a damp proof course to evaporate from the external masonry or render.
Many old buildings are constructed from materials such as brick, stone and cob which are relatively porous and possibly lower in strength. Lime putty mortars were normally used for bedding these materials and lime plaster and lime render for the internal and internal coatings.
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 in ground conditions.
Lime mortar and lime plaster is also highly vapour permeable; allowing moisture to evaporate at sufficient rates to help keep buildings free from damp.
Which Lime Should I Use?
Lime mortars and lime plasters made form traditional lime putty cure by evaporation before further strengthening by carbonation.
Ancient Romans discovered that adding volcanic ash from Pozzuoli to a lime mortar created a chemical set making it slightly hydraulic (i.e. it set in the presence of water). This allowed the Romans to build structures such as aqueducts. Substitute materials have long since been used instead of volcanic ash, hence any material that reacts with pure limes to create a hydraulic set is referred to as a ‘Pozzolan’.
If lime putty is kept moist and free from Pozzolans it will last indefinitely and actually improve with age.
Natural Hydraulic Lime
The original form of hydraulic lime was a combination of pure lime putty and a Pozzolan. Dry powdered natural hydraulic lime (NHL) was introduced much later when processing limestone containing natural impurities mimicked the Pozzolans used by the Romans.
If stored dry and air tight, this form of hydraulic lime offers a potentially cheaper option for a bedding or pointing mortar. Care should be given to selecting the most appropriate Natural Hydraulic lime as they come in different strengths of NHL2, NHL3.5 and NHL5. Different manufacturers strengths can vary widely even within these grades. NHL5 is ideal for Limecrete floors, chimney flaunching, coping or ridge tiles. Obtain guidance from our technical advisors before selecting the lime for your project if in any doubt. Lots 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 from ice crystals forming within the lime mortar.
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.