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The Natural Philosopher

wrote:
IME, if you can't protect ordinary lime mortar from the weather, shallow
applications such as pointing will be washed away by the first rainfall.
Hanging damp sacks all over the place doesn't seem that practical to me.

Do you mean heavy rain washing out new pointing? The initial set of at
least the surface should take a few hours. After that I would be
surprised if it washed out to a significant degree with normal
rainfall. I have heard at (at a limes seminar) of a heavy section of
new/repair lathe and plaster ceiling detaching - as a consequence of
poor mix and poor application. Potentially quite dangerous - it does
take time (weeks/months) for a thick area of lime mortar/plaster to
carbonate right through and gain it's full strength.

The other issue is the degree to which modern additives have improved
the properties of cement. At a basic level I'm sure we're all familiar
with how pva reduces cracking and doubtless there are more sophisticated
products around.

I don't think you can get away from the fact that cement pointing will
trap moisture and lead to soft bricks spalling. It also acts as a
sacraficial material around limestone, so it's the mortar that slowly
erodes and not the stones.

My impression is that lime pointing does it worse actually.

Old bricks are soft and porous and crumbly, they need a soft crumbly
porous mortar like lime. Modern bricks are not like that.