How is it that Roman buildings “heal” themselves?

Question

Saw an article or documentary and it said something to the effect of Roman buildings heal themselves after being damaged or weather damage.

How can that be

In: Engineering

Answer 1

One of the key ingredients in cement is lime. Lime is easy to break into a fine powder – the romans used a rough lime with larger granules on purpose. If a large granule broke years later, the lime would release and “heal” that area.