In article , jacob
writes
Proper traditional lime mortar/render is not cement mortar plus bagged
lime as referred to earlier, but is slaked lime putty - available wet
and ready to use in various grades for rough work through to fine
mouldings etc. No cement content at all. Is available all over the
country e.g. in Derbyshire from Bleaklow Products. There are all sorts
of local variations -it is not consistent in quality like portland
cement.
If you are really keen you can buy quicklime and slake it yourself.
After slaking the longer you leave it the better it be. You can buy it
slaked as 'one year', '4 year', etc
Try browsing 'lime' 'lime putty' etc or try SPAB site (Soc. for
Protection of Ancient Buildings).
It is being used more often as it has been found that portland cement
mortar is incompatible with many sandstones . There is no doubt about
it and I have seen its effects often in this area - Derbyshire, with
crumbling stonework where pointed with cement mortar.

I'd agree with what Jacob has said, if it was lime mortar originally
avoid putting in cement it will cause more problems that it will cure.
The same goes for lime based rendering, replace like with like and don't
be tempted into a cement based stronger mix.
--
David