Very interesting research has been done recently on the "self-healing" concrete recipe developed by the ancient Romans. It helps explain the longevity of so many of those ancient Roman structures, but also carries broad implications for the future... in construction, environmental concerns, etc. 

The Secret to Making Concrete That Lasts 1,000 Years | WIRED

I don't think they discovered anything new.  My grandfather was a mason and I worked with him almost 40 years ago.  We did a lot of repair work on very old houses - some from the 1600's.  We never did repairs with modern mortar, we would use lime mortar because modern mortar is too hard and any water freezing between the new and old will push out the old.

We mixed the mortar formula ourselves using "slake lime" or "quick lime".  Sounds exactly like what the article is describing.  By heating crushed lime, some magic chemical change happens and it makes good brick mortar.  If it gets tiny cracks, the lime will move into the crack and repair itself.

I have an old farmhouse. Probably built in the 1870's.  The sandstone basement walls were parged inside with this old lime concrete mix.  I know because 150 years of these walls being wet - dry - wet - dry has caused the cement to move.  I have thousands of what look like curled up snail shells on the walls.  The lime mortar gets damp, the lime softens and flows out from the wall a thousandth of an inch.  The next time - same thing.  Eventually, you get these curled up pieces of cement that have peeled away and makes a scroll shape.

I think the ancient Romans had the same mortar we had 100 years ago.  Mystery solved!

1 hour ago, silverado427 said:

He sleeps with the fishes. 

Construction nerd here.  Love it!