I, among many other people and organisations, bang on about the need to use lime products on older buildings, but what do we mean by lime?
There are many different types of lime:
1. Hydrated lime
2. Lime Putty (soft and hard)
3. Hydraulic Lime (different strengths available)
4. Cement (different types available)
1. Hydrated lime is not really a lime in the sense that it can be used to create a set mortar or render. Instead it is added to a cement mix to help ease workability. So do not use hydrated lime to make up a ‘lime’ mortar or render, you will require a cement to allow it to set.
2. Lime putty is the older form of lime. It sets by carbonation (so it takes back in carbon dioxide from the air in order to set) and is the softest and most flexible form of usable lime for mortars and renders. It is also the most breathable. So for these reasons it is often the best option for conservation work. However it should be used with the right aggregate. We recommend it being used with carbon rich aggregates like limestone, but it can also be successfully used with insulators like hemp. It should not be used with sand or similar materials.
Lime putty is created by heating up calcium carbonate (chalk or limestone) and then slaking it with water. It is then covered with water and the longer it is left the better it becomes. It comes in a putty format and so mixing it with aggregate requires a flat bed mixer rather than a conventional mixer as it needs to be smeared over the aggregate.
The difference between chalk and limestone sources is marked. So if you are going to use the lime in a highly damp area we would strongly recommend using limestone derived putty. The chalk one will not last.
For the external use of lime we would recommend using lime putty as being the most breathable form of lime it will draw out any water from the main walls to the outside.
It is worth noting that the lime needs to be applied correctly. Ideally it is pumped onto the surface not troweled on. Troweling putty on can draw the putty away from the substrate and hence not make as good a bond. Also note that if rendering the various layers of render need to be applied when the lower coat is still ‘green’. If the underlaying coat is dry then you can get a poor bond and layering can occur.
3. Hydraulic lime sets through hydrolysis (if needs water to set) and so can set underwater. Hydraulic limes come in different strengths (normally NHL 2, 3.5 and 5) NHL 2 is the softest and 5 the hardest. These are less breathable than lime putty and are commonly used with sand. They come in bags of powder similar to cement. This is because hydraulics are essentially softer versions of cement. Personally I think that great care is required when choosing hydraulic lime and I would also recommend using a breathable aggregate rather than sand.
No doubt hydraulics are easier to use than putty and that this has meant that they are much more common (builders use them in the same fashion as cement). They are more expensive than putty to buy, but the labour costs and skills associated with them are less.
4. Cements are the most common binder used in building today. They are cheap, easy to use and are regarded as being the hardest, strongest and hence the best (I think that we do live with a male dominated construction industry). However their strength is also their downfall. They are brittle, cannot expand over any great distances without cracking and are not breathable. So they can easily allow water into a structure and then lock it in. This makes walls damp, inefficient (decreases energy efficiency by approx. 30%) and also destroys the underpinning structure if used on an old building.
If using on a modern cement based building then their use is fine, but ideally I would always recommend using a real lime product if possible. By ‘real’ lime I mean putty.
I hope that this helps you with understanding the ‘lime’ world. We have found that there are frighteningly few people who truly understand lime, even CADW struggles! So if you want some guidance on contractors please contact the Eco Home Centre.