Which lime mortar?

[Lindyboo]

I want to re-point a brick chimney breast. I’ve seen online ready-mixed lime mortar in 20 or 25kg buckets/bags. Is this the stuff to go for? I’ve also read that if kept sealed it will keep for months, which would be useful for projects to come in the spring. How long will it keep?

Also, am I correct that hydrated lime is the stuff that will dry in the wet and non-hydrated has to be kept dry to set?

Finally, anyone know of suppliers in Lincolnshire, from where I could collect, or which otherwise would cost less to deliver than from far flung places?

Thanks

 

[Gervase]

Easier to mix your own - and considerably cheaper.
Hydrated lime is also known as 'bag lime', slaked lime or just plain lime. It can be mixed 1:3 with sharp sand to make a perfectly serviceable mortar. It will keep for ages if covered, and will just need knocking up again to make a nice plastic mortar - and is actually better if allowed to mature a bit. It sets by absorbing carbon dioxide from the air.
Hydraulic lime comes in various 'flavours' depending on how hydraulic it is - that is, how fast it will set chemically. This is the stuff that will not keep beyond an overnight park in the mixer drum. The standard strength from most builders' merchants is NHL3.5 (standing for Natural Hydraulic Lime).
As for which to use, I would advise using hydrated lime for a chimney breast. Hydraulic lime is for exposed external work or for work which will be subject to continued damp (below ground, for example).
Get a bag of lime from your local builders' merchant (it'll cost about £9) and mix it 1:3 with sharp sand to make a sticky mortar. Don't add too much water, as it will get sloppier the more it is mixed.
There are some downloadable instructions on repointing here. They describe repointing stone, but the basic technique for brick is similar, and the work is much quicker.

 

[MdB]

Mixed lime putty or mixed lime mortar (made from hydrated lime) will keep forever in a sealed bucket / tub as long as it stays damp.