Pozzolana

Pozzolana was the first material that Romans used to make roofs waterproof. Pozzolanic ash was first discovered and dug in Italy, at Pozzuoli, in the region around Vesuvius.
It reacts with calcium hydroxide in the presence of water, creating compounds possessing cementitious properties at room temperature which have the ability to set underwater. It transformed the possibilities for making concrete structures, although it took the Romans some time to discover its full potential.
Modern Pozzolana blend may be stronger than Portland cement, due to its lower porosity, which also makes it more resistant to water absorption and spalling.

Pantheon dome made of Pozzolana

@B Read more about Pantheon dome curiosities

@ Explore pozzolana.it (ITA)