I recently did some outside brick wall patching and tuckpointing. Since the old mortar was whitish, I didn't use the gray ready mix from the Home Depot but I mixed my own, the same what I used for my exposed indoor brick wall restoration: 1 part masonry lime (white powder), 2.5 parts masonry sand, plus water. I also used the same mix for some indoor plaster patches (house is old and has plaster over brick).
But I noticed that that recipe doesn't hold very well on the outside, crumbles and dissipates, unlike on the indoor walls where it's been rock solid and beautiful several years now.
Is lime based mortar not weatherproof and should there always be cement for outdoor applications?