Bluestone Repointing help needed

Hi,

I'm having a crack at repointing an old bluestone house. The house is 125 years old and it appears the old mortar is just beach sand and lime (or something similar). Its broken down such that its pretty much just crumbly sand.

Ive made a mix of 1 part lime, 1 part brighton lite and 8 parts brick sand. It dries to a colour that suits the job

Ive done a test patch after wetting down the walls and thought I had done a pretty good job, but as it dried it started developing hairline cracks - quite a few.

Any tips would be appreciated - especially re avoiding the cracking.

Thanks in advance

The problem is either the sand or cement mix. I'd try a sharper washed sand, the issue might be joint thickness V clay content. Can't comment on the mix, we used neat cement to bond the stone but on fine gaps, but that is not appropriate for building work needing to allow for movement.

thanks very much - I'd heard that the clay content in brick sand makes it prone to cracking. Is this true? if so, will it to continue to crack over a period of time or will it just develop a few hairline cracks as it sets and for the main part stay that way.

Also will the depth I fill have much bearing on the cracking - ie if I repoint to a depth of more than about 30 mm is it more likely to develop cracks than if I only fill say 10 mm.

What I was thinking is that the stone is very non-porus and the old mortar is extremely porus. Therefore even though I wet everything down the old mortar will surely suck more moisture from the new mortar than the stone resulting in uneven drying times and more cracking. Do you think it would help if I 1/2 filled with new mortar, effectively covering the old mortar with a layer of new that will not be seen (if it cracks it doesn't matter). Then when that is dry - or nearly dry repoint the remaining say 10 mm?