Roman concrete could which of the following?

• A. Could harden underwater and was waterproof
• B. Could not be used underwater
• C. Could only dry in sun
• D. Was always produced in marble

Answer: (A)

Roman concrete stands out for its hydraulic set: by mixing lime with volcanic ash (pozzolana), the material could harden even when submerged in water. The pozzolanic reaction forms cementitious compounds that cure underwater, so builders could pour harbors, piers, and underwater foundations directly in the sea or tidal zones. This also makes the resulting concrete dense and relatively resistant to water penetration, giving it a kind of waterproof durability in marine environments. The idea that it required sun drying or was made from marble isn’t accurate—the underwater hardening and durability come from the lime-pozzolana blend, not drying conditions or marble.