I want to share this recent reply from Harry Francis (of www.limeonline.com)
to the question of mortar carbonization.

"Mortars (both lime and cement) cure by absorbing carbon dioxide from the
atmosphere.

Deterioration of properly cured mortar is due to environmental weathering -
the attack of acid rains soaking the mortar, and the lime content of the
mortar neutralizing these acids.

In Portland cement mortar, there is very little "free Lime" to carbonate

Over a long time, the carbonated free lime in lime mortar (and the cement
molecule in cement mortar) can be dissolved away by the acids formed in acid
rain.

The normal curing process (carbonization) of mortar starts from the exposed
area, and quite quickly forms a carbonate skin which, unless the mortar is
kept damp, will hinder the carbonization of the remaining joint.  It is
proper moisture curing that allows the dissolved carbon dioxide to migrate to
the hydroxide sites - converting them to carbonates.

Without proper moisture curing, the mortar cures on the face of the joint and
the remainder of the joint simply dries out.... leaving it uncured.

In the case of lime mortars, uncured mortar simply becomes powder, with a
little of the lime-sand particles somewhat glued together (wherever the
curing proceeded while damp), and the remainder simply physically separated
from the brick....

If properly damp-cured, the entire mass of mortar in the joint is formed into
calcium silicate-carbonates which strongly bonds the bricks together and
resists weathering

In the case of cement mortars, the cement does cure more quickly due to the
silicates formed in the cement manufacturing process creating calcium
silicate gels and, (with atmospheric CO2) the carbonate forms....Again if
sufficient
moisture is available during the curing process.

In either case, laying the brick dry in hot, dry climate is setting up a
situation for failure.  I suspect that when it sometimes may take several
years before failure,
the mortar was cured at the exposed surfaces but not between the bricks.
Years later, the mortar deep in the joint would slowly absorb carbon dioxide
on a particle by particle basis become carbonated ---not glued together..
crumbly mortar between the bricks---and, if analyzed, the material would
indeed show carbonation (due, not to weathering, but to improper curing).

Now, as the bulding ages, and the face of the brick and the mortar joint is
exposed to rain (acid rain for an industrial country), then slowly the thin
cured surface weathers away and the uncured, carbonated mortar is exposed....

Rake out a joint, and if the mortar 1/2 " back is soft, crumbly, etc, then,
in my opinion, it was not cured properly when constructed.  I expect many
brick structures share this fate.....

The buildings at Harvard --Cambridge University, and many in Philadelphia
have survivied many years without tuckpointing.....I think this is due to
proper placement of the mortar on damp bricks and proper damp cureing.....just
like frescoes in Italy---pure lime plaster --exposed today to outside
weathering,
yet still beautiful...

BTW, I think that the use of presure hydrated limes/aged limes  - with their
large surface area holds onto more surface moisture helping to assure proper
damp curing...whether in lime mortars or cement mortars...

Acid Rain:

Today we are faced with extremly high acid content in the rains in the
Eastern USA, with rain pH (normally 5.3 to 5.7) in the lower ranges of pH 4.1
to 4.5. This same high acid concentration extends from East of the
Mississippi over the entire Eastern seaboard....We call it smog...

This high acid concentrated rain is destroying statues, mortars, concrete
highways, bridges, etc. and providing an environment where many beneficial
organisms cannot survive.  When it is not raining, we are breathing these
fine particles into our lungs causing much discomfort, wheezing, etc. and
maybe even worse, over time.  This is ten times more acidic that normal!
When it rains, at least, these particles are washed out of the air we breath.

Most acids form insoluble compounds with lime.

Acid rain containing sulfates are not too much of a problem, as they do react
with lime/limestone, and form calcium sulphate molecules.  Calcium sulphate
is almost water insoluble, so once the sulfate surface forms, all weathering
stops.  On marble statues it looks bad, but the weathering does stop - or
almost so - at this point.
Ca(OH)2 + H2SO4 = CaSO4 + H2O...

Hydrofluoric acid (HF)  (fumes from glass making) forms calcium fluoride -
feldspar (water insoluble).

Hydrochloric acid (HCl), one of the few acids to form soluble salts with
lime, forms a soluble salt -calcium chloride.  Down wind from these fumes are
very destructive to lime/cement mortars, marble statures, etc. as the soluble
compounds formed continue to wash away.....

Unfortunately, burning of fossil fuesl --coal and oil , results in the
release of all of these acids, and at high temperatures, also form nitric
acids.

A mixture of nitric and hydrochloric acids forms "Royal Water" - Aqua Regia -
which will dissolve most anything (even gold).   That is what we are
producing in the form of acid rain today, and why we need to "scrub" these
gases , using lime to produce insoluble compounds, to clean the air from
harmful acids - both to preserve buildings and ourselves.

I.wrote.this.the.other.day.before.my.keyboard.went.blooey.

John.Mascaro