woodsmith
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The first thing to note is that with modern cements with added plasticisers there is no need to add lime to a cement based mortar mix. You see lots of information how lime makes mortar breath but that is only if it has no cement in it. As soon as you add cement it negates any benefit from the lime, lime was historically added to cement based mortar to make it more workable.
Sharp sand and cement makes concrete, adding soft sand will make it more workable but unless the stone is very hard its likely to make a harder mortar than the stone, which is not normally a good idea.
Lime mortar has no cement in it, you can buy it ready to mix in bags or you can make your own. And this is made from sharp sand as the lime keeps the mortar permanently flexible.
First thing is to look at what mortar has been used for the wall. If it's softish, coarse, a bit friable and often with flecks of white in it it is almost certainly lime mortar. If it's hard and needs a hammer to break it into powder it's cement.
I've mostly used pre mixed lime mortar as its consistent and you can get it in various colours to match the existing mortar as lime mortar is otherwise very white. If you are going the cement based route I would experiment around a 1:5 mix cement to soft sand and only replace some soft sand for sharp sand if you find the mortar slumps.
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