Why use lime?
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 stonework or render. Many old buildings are constructed from materials such as brick, cob and stone which are relatively porous and often of lower strength. Lime putty mortars were normally used for bedding and plastering. Lime mortar is a relatively softer mortar and therefore it is able to withstand a certain amount of movement (without cracking) that comes with settlement and seasonal changes in ground conditions. Lime mortar is porous and allows moisture to evaporate, helping to keep a building free of dampness.
What lime do I use? A rough guide
Pure lime putty mortars and plasters dry by suction and evaporation and then they carbonate. This is the oldest, most flexible and breathable form of lime. The Romans discovered that adding volcanic ash from Pozzuoli to a lime putty mortar created a chemical set making the lime mortar hydraulic i.e. it enabled the mortar to set in the presence of water and allowed the Romans to build structures such as aquaducts. Substitute materials have long since been used instead of volcanic ash, hence any burnt clay that reacts with pure limes to create a hydraulic lime set is referred to as a Pozzolan after the original discovery. If lime putty is kept moist and free from Pozzolans, it will last indefinitely and actually improve with age reducing wastage.
So the original hydraulic lime was in fact a combination of pure lime putty and a Pozzolan that can still be replicated and is usually used these days in the UK to render and point externally or internally on to walls that are inherently damp and require the chemical set.
A dry powdered Natural Hydraulic Lime was introduced much later when limestone was discovered with natural impurities that when burnt mimicked the Pozzolans used by the Romans. This is perhaps more like a weaker, early form of OPC (Ordinary Portland Cement) as it needs to be dry stored and a chemical set begins once water is absorbed into it. 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:
Please note that great care should be taken not to apply lime mortars when there is a risk of frost damage due to ice crystals forming within the lime mortar. It is important to prevent frost crystals forming within the mortar soon after application. Carbonation takes at least a month for each millimetre of thickness, therefore it may take over 20 months before lime mortar has carbonated to a depth of 20mm. It may be necessary to increase the amount of Pozzolan added. It is ideal to ensure complete weather protection of the work at all times of the year.