In a muddle about lime

[vicky whitworth]

I know this makes me seem like a rookie (which I am) but I have gotten myself all confused about the different types of lime. One of my neighbours is starting restoration work on a garden wall. It's a long length of wall (probably 18th century and currently mortared in lime)) running alongside the churchyard with a public footpath hard up against it on one side. It's in a dreadful state - fallen in many places and a danger to the public. He's doing the work for the landowner - who's helping him. They have been told to use hydrated lime in bags by one of the well known lime suppliers and sharp sand. I can understand that being a garden wall it needs to be strong but is this good advice? I've lost the plot on what is hydrated lime versus NHL (what does that sand for?) and what each is used for. Please could some give me a few sentences to remind me. He's keen to learn, and keeps asking me questions - and now I look a right plonker. The bit they have done in the hydrated lime looks good but has gone off very quickly. I guess one of the reasons they may be using it is they want to work on the wall over the winter? many thanks, Vicky

 

[Schoolmarm]

The bag lime is fine. It is hydrated(dried) stuff. When water is added without sand, it makes lime putty.

If it has gone off quick, it is because they did not keep it damp whilst it cured/set. This will weaken the end result.


NHL stands for naturally hydraulic lime.

Hydraulic and hydrate are nothing to do with each other and are a trick set of names to confuse people.

 

[Flyfisher]

Also, my understanding is that lime (any lime) work is not recommended in cold weather, unless measures are taken to protect the work from temperatures below about 5C. So working on the wall over winter might not be advisable. Hopefully a real expert will confirm or correct this.

 

[Schoolmarm]

Oh and further to my earlier non expert bit :lol: The hydrated lime in the bag is the softest version.

 

[Carol]

I have similar troubles with this, chemistry was not my strong point. My lovely new limey wall is growing bit by bit and I have been assured by the stonemason that the temperature will be fine for lime for the next few weeks and a worse enemy is heavy rain. Anything below 5 degrees is not considered very good for building but apparently pointing is probably ok.

In the soft southern lowlands we should be safe from freezing for another month or two! But covers are all ready for my wall for the downpours expected at the weekend.

It's so exciting to watch the wall and the mixing, I wonder if I am being a nuisance, the cups of tea are only an excuse to watch and pick his brains!

 

[vicky whitworth]

Thank you for the advice. I have tried looking it up in The Old House Handbook published by SPAB, and there they seem to be advising against hydrated lime because it is inconsistent and "the risk of failure is not worth the small cost saving." Since it's not my wall being rebuilt, I think I might back out of this and leave it to others. I can understand why people would be put off using lime as it seems complicated. My builder used bagged naturally hydraulic lime NHL4 to build our new extension (modern wall construction) and it looks great. Thanks, Vicky

 

[Ushy M]

Vicky, I found this description from an old post by Nemesis most helpful in describing the difference between lime hydrate and NHL lime:

Both Hydrated and Hydraulic limes come in bagged powder form. Though Hydrated lime is the one generally found at your local builders merchants.
It can be mixed with water to form lime putty or used staight from the bag.
Natural Hydraulic lime (NHL) when mixed with water will set within a few hours. The number after NHL (eg NHL5, NHL3.5, NHL2) relates to the amount of natually occuring impurities within the limestone prior to manufacture and so during the manufacture these impurities get burt and then act as a pozzolan ready mixed within the lime, with 5 being the most impure and strongest and 2 being the least impure and weakest (The "Strength" or Weakness" refers to the set and is not an indication of quality or of one being better than the other, it's just different qualities for different applications). They are also refered to as Eminently (5), Moderately (3.5) and Feebly (2) hydraulic and although not so readily available they are fairly easy to get hold of.