The mystery has been cracked. Structural experts have always been amazed at the quality and longevity of Roman era concrete as witnessed by standing viaducts, the Coliseum, the Pantheon and many other architectural marvels.
Turns out, it’s structure has a long held secret that provides a self-healing property:
Hot mixing, they thought, would ultimately produce a cement that wasn’t completely smooth and mixed, but instead contained small calcium-rich rocks. Those little rocks, ubiquitous in the walls of the Romans’ concrete buildings, might be the key to why those structures have withstood the ravages of time. …
“In every sample we have seen of ancient Roman concrete, you can find these white inclusions,” Masic says, … “The researchers’ chemical analyses of such rocks embedded in the walls at the archaeological site of Privernum in Italy indicated that the inclusions were very calcium-rich.” … "A ready supply of calcium was already on hand: It would dissolve, seep into the cracks and re-crystallize. Voila! Scar healed.
But could the team observe this in action? …"