Dan,

As this question gets more attention, I suspect the reasoning will evolve, but 
here's the way I see it:

1. Original service life of portland-lime mortar seems to be on the order of 
about 75 years. As carbonation proceeds, the mortar becomes more brittle, and 
eventually it either delaminates or cracks. Natural cement doesn't seem to do 
that. I have seen lots of 100 to 150-year-old natural cement mortar in 
excellent condition, and the pattern of damage tends to be localized, e.g., a 
corner washes out below a hole in the roof that went unrepaired for some 
extended period of time.

2. Repointing work does not seem to last as long as original mortar. I suspect 
that a lot of that has as much to do with the poor quality of the work and 
poor match to original materials, but there seems to be a 25-year cycle for 
portland mortars on many projects. What little we have seen so far with 
natural cement repointing seems to hold out the promise of much longer 
durability. Perhaps an extreme and only anecdotal example, but the repointing 
done by Thomas Jefferson at Monticello using natural cement has performed for 
180 years. The key may be the long-term retention of flexibility.

> > 4. If portland mortars require repointing more frequently 
> > than natural cement mortars, then portland represents a less 
> > sustainable approach, as every round of repointing induces 
> > damage and loss of fabric to the original structure.
> 
> You might want to comment on why portland requires repointing more
> frequently....
> 
> Dan
> 
>

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