On 29/07/2017 21:05, newshound wrote:
In any case, in a "breathable" mortar there's always going to be some
water (and hence oxygen) present by diffusion from the outside.

Water doesn't count. There have been cases where the stainless steel
keelbolts on boats have rusted through - and when the keel falls off
that kind of boat they'll be upside down in seconds.

Andy