Why Use Lime for Pointing?
Although this is not either meant or going to be a full lesson on the reasons to use lime – I will try to give the basics as I see it.
Lime was traditionally used in the construction of old buildings and was mixed with whatever materials people had locally. It is breathable and as a result damp was able to evaporate from the pointing or the render harmlessly. The fact that the moisture was able to evaporate meant that the moisture left the building via the lime mix and not via the stone / brick. If moisture gets trapped in the stone or brick (as it does when you use cement) and you get a frost then the water will expand during the freezing process and generally it will damage the stone or brick – which is not good for the fabric of the building. Lime pointing is meant to be sacrificial and it is better that the pointing gets damaged rather than the substrate of the building.
** please note that I am not decrying the use of modern materials such as cement and gypsum plaster. They both have their place; they are much quicker to build with as they do not require as much skill to use them properly and as long as the buildings have a damp proof course installed they will act perfectly well. Working with lime products just takes a bit more time and can only be done during certain times of the year.
Types Of Lime
The are basically 2 types of lime that we need to concern ourselves with
1) Non-hydraulic Lime and
2) Hydraulic Lime
Non-hydraulic Lime is produced by burning limestone (Calcium Carbonate CaCO3). During this process CO2 is driven off leaving us with Calcium Oxide CaO – better known as quicklime.
When you mix this with water a violent reaction takes place (known as slaking) – The resulting slurry is known as Lime putty – (Calcium Hydroxide Ca(OH)2) which can then be stored in a moist state until it is used. The longer it is kept the better it will become – as long as it doesn’t freeze! because any unslaked particles will slake over time. When it is exposed to air it will set – not because it dries out but because it will absorb Carbon Dioxide CO2 from the atmosphere and turn back into Calcium Carbonate CaCO3.
This is called non hydraulic lime because it requires the Co2 in the air to carbonate and won’t set under water.
Hydrated Lime – This is the type of lime that is available from most builders merchants. It is made by the same process as above but slaking takes place with a precise amount of water which is driven off during the process – resulting in the dry powder – hydrated lime. It is not as good as lime putty for a number of reasons (it carbonates in the bag!) & in my opinion it is only used by builders that have been asked to do lime pointing and they add it to their cement mix – Pretty pointless really although it does make the cement easier to work with ! But it doesn’t make it breathable or flexable.
Hydraulic Lime is made in the same way but is produced from limestone that contains clay. There are 3 main strengths available in the UK NHL2; NHL3.5 & NHL5. (NHL stands for Natural Hydraulic Lime)
The higher the number the higher the clay content and the faster it will go off. (NHL2 – c. 12% clay / NHL3.5 – c. 12 to 18% clay / NHL5 – c.18 to 25% clay)
The curing process of Hydraulic lime – although still a carbonisation process can take place under water.
It has properties similar to those of cement and is therefore more widely used than the non hydraulic limes BUT people should still treat this differently to cement as you should not let it dry out whilst it is going off – it still needs to be looked after and tended properly.