various things....

["william m woodard" <stoneturning@zzzzzzz>]

"Here in Central Texas, there are quite a
few old style lime mortared buildings still around--
.  In those, the lime
mortared joints are protected with a surface pointing of hard
portland mortar  which does crack and crumble and periodically
needs to be reapplied or the exposed lime mortar begins a
journey to the foot of the wall.  However, even without a good
maintenence program (most are totally ignored), these buildings
without benefit of concrete foundations, still stand proud and
true. The legacy of real stone masons. "
JVC

   OK!  I have to come in on this.  I've spent too much time taking OFF
Portland mixtures that will only prevent these living breathing building
'live' a long life, to let that pass.

As I understand it, for the same reason that the builders of these great
structures put a pitched surface  on face stones (to vastly increase the
suface area) evaperation has to happen. Portland is a barrier to this.
Trapping moisture behind cement mortor is the leading cause of deteriation
in a huge number of buildings.  Stone (softer), under the surface is exposed
to moisture, in and out, and gets its color that way(alabaster). If one
prevents this cycle then that's when it will begin to deteriate.
Buildings with a polished stone and a modern sealent on it - V - a pitched
stone with nothing on it.
Looking at the buildings in London as of late, even with acid rain and the
such, Limestone wins.
I, with others at Historic Santa Fe Foundation are in the process of taking
off all Porland based plasters/coverings on one of their main buildings to
restore and show the public what a non breathing cover on Adobe will do.

   Stone buildings move around? you bet they do. Rockhound.