Geopolymer Institute

The Coliseo, Rome, 2nd C. AD (left) The Pantheon, Rome, 2nd C. AD, inside (center) The Pantheon, Rome, 2nd C. AD, the concrete dome (right).

Concrete experts talk today about how to make concrete durable. Many ancient Roman concrete buildings are still in use after more than 2000 years. For these modern concrete experts, the Romans were fortunate builders in that they apparently simply used natural pozzolan deposits, which were found to be suitable for producing a hydraulic mortar. Contrary to this pronouncement, our recent linguistical study and new translation of Latin author Vitruvius’ book De Architectura (1st Century B.C.) states that the magnificent quality of Roman concrete resulted from the extensive use of artificial pozzolanic mortars and concretes. Two artificial pozzolans were intensively manufactured:

1. calcined kaolinitic clay, in Latin testa
2. calcined volcanic stones, in Latin carbunculus

See in #D The synthetic pozzolanic mortar by Vitruvius and #E Searching for Carbunculus .

In addition to these artificial reactive ingredients, the Romans used a natural reactive volcanic sand named harena fossicia wrongly translated as pit sand or simply sand by modern authors. The ingredients testa, carbunculus and harena fossicia were intensively used in Roman buildings. These reactive ingredients must not be confused with the traditional pozzolan whose name originates from the city of Puzzuoli, near Napoli (Mt Vesuvio). According to Vitruvius Book V, 12, the traditional pozzolan was exclusively used for making piers into the sea or foundations for bridges, whereas harena fossicia, carbunculus and testa produced the concrete for buildings on land.

Roman concrete technology was more efficient than traditional building with hewn stone. The Table compares the construction time for the domes of most famous world monuments.

Construction time for dome structures made of concrete and hewn stone