Using volcanic rock in concrete was done to lighten vaults and domes, e.g. the Pantheon. This is distinct from using volcanic ash in the concrete mix which was

Using volcanic rock in concrete was done to lighten vaults and domes, e.g. the Pantheon. This is distinct from using volcanic ash in the concrete mix which was essential for the long lasting strength. Arguably Roman concrete was discovered by accident when someone mixed a batch of lime plaster using the volcanic ash around Pozzuoli instead of regular sand. https://en.m.wikipedia.org/wiki/Pozzolan https://en.m.wikipedia.org/wiki/Pozzolan