Even today, there are few spaces more impressive  

than the rotunda of Rome’s Pantheon. 43 meters in diameter, and 43 meters  

from floor to oculus – and everything besides  the columns and marble veneers – the whole  

structure of the massive walls and astonishing  dome – was made of the versatile and awesomely  

durable material that we call Roman concrete. Concrete was everywhere in imperial Rome,  

from the walls of tenements to the soaring  vaults of the great baths. But in late antiquity,  

after centuries of intensive use in cities across  the Empire, concrete virtually disappeared. 

We’ll explore how and why in this video,  which is sponsored by Squarespace. 

Roman concrete began with a stroke of geological  good luck. The city of Rome was located near  

deposits of the fine volcanic powder known as  pozzolana [Poe-tsa-lana]. By trial and error,  

Roman masons discovered that adding pozzolana to  lime and water created a remarkably strong mortar,  

which could be mixed with rubble to  create durable foundations or walls. 

Unlike modern concrete, Roman concrete was not  poured, but spread in courses. First, retaining  

walls of brick or stone were constructed as a sort  of form. A layer of coarse aggregate – usually  

chunks of limestone or broken tile – was laid down  between these walls, and concrete was troweled  

over the rubble bed, almost dry. The mixture  was then pounded firm with wooden mallets. 

For centuries, Roman concrete was primarily  a means of building walls and foundations  

quickly and cheaply. Ancient architects had no  way of mathematically modeling forces or stresses  

or other things likely to cause the  collapse of buildings and careers.  

They tended to be correspondingly conservative  in their use of building materials. 

harden underwater - in fact, thanks to chemical  reactions that the Romans knew nothing about,  

saltwater actually strengthened the material,  forming nearly unbreakable mineral bonds.  

So in the waning days of the Republic, the  Romans began to build breakwaters and piers  

from concrete. Perhaps the most famous example  is the artificial harbor of Caesarea Maritima,  

pictured here, which was built by Herod the Great  with pozzolana concrete imported from Italy. 

But it was only in the first century that  Roman concrete really came into its own.  

Building on earlier experiments, the architects  Severus and Celer created a dazzling series  

of concrete domes and vaults for Nero’s Golden  House. After this, the creative floodgates opened. 

The next century and a half witnessed  the golden age of Roman architecture,  

whose greatest achievements were made  possible by concrete. The Colosseum,  

built in a marshy valley, was prevented from  collapsing by gargantuan concrete foundations. 

The cathedral-like concrete halls of the great  Roman baths, filled with the sound of fountains  

and the glow of sunlight on glass mosaics,  were walled and vaulted with concrete. 

And the rotunda of the Pantheon, as we have  seen, is basically a gigantic concrete cylinder,  

crowned by the famous dome. It would be hard  to imagine a more spectacular demonstration  

of concrete’s possibilities. Yet within a  few centuries of the Pantheon’s completion,  

the Romans would virtually abandon  concrete as a monumental building material. 

Before we explore that process, and while we’re  still on the theme of building, I’d like to talk  

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Back to the show. So what happened to Roman  concrete? How could a building material that was  

so widespread and so useful just vanish? It should  be said, first, that the use of concrete was never  

widespread outside of Italy. This partly reflected  the fact that – although Roman concrete could be  

made with crushed terracotta or volcanic dust from  other sources – pozzolana made the best concrete,  

and pozzolana was only found in Italy. The reach  of concrete was also limited by the persistence of  

local building traditions in the provinces, which  left little room for new construction methods. 

The ultimate reason for the decline of concrete  in late antiquity, however, was straightforward:  

the buildings for which concrete was best  suited stopped being constructed. Sometimes,  

this reflected a decline or  reorientation of imperial patronage.  

In Rome, for example, the last truly spectacular  concrete building was the Basilica of Maxentius,  

pictured here, which was finished by Constantine.  Afterward, with the emperors based elsewhere,  

no benefactor had the means or motivation  to sponsor anything on a comparable scale. 

Monumental buildings were still  being constructed in late antiquity,  

but the most prestigious were now churches;  and especially in the Western Roman Empire,  

the default church design was the basilica.  Basilicas like Old St. Peter’s – shown  

here in cross-section – were long and  narrow, with window-filled masonry walls  

and wood-framed roofs. There was little  need for concrete in such buildings. 

Even in the Eastern Roman Empire, where  Justinian’s Hagia Sophia made domed churches  

popular, concrete was seldom used. Concrete had  never been popular in the culturally Greek cities  

of the eastern Mediterranean, where walls, vaults,  and domes were traditionally made of brick.  

In keeping with that practice, even the great dome  of Hagia Sophia itself was not made of concrete,  

but of brick bedded in thick layers of mortar. Roman concrete was still being used by  

the later emperors, at least in specialized  contexts. We know that Justinian, for example,  

used it to build a new harbor at Constantinople  in the mid-sixth century. But after Justinian,  

the political turmoil that had already destroyed  the Western Empire overtook the Eastern Empire  

as well, effectively ending large-scale  construction projects for two hundred years. 

Not all concrete buildings,  of course, were monumental.  

Even after the emperors stopped commissioning  large projects, concrete was still sometimes used,  

as it had been since the beginning, in the  walls and foundations of residential buildings.  

But the buildings that were best-suited  for cement – the big multi-unit insulae  

built by professional construction gangs – were  no longer in demand. With the general collapse  

of urban populations across the Roman Empire, in  fact, many cities now had more houses than people. 

During the early middle ages, in short,  when the few new buildings being constructed  

were almost invariably modest structures  with rubble walls, there was little demand  

for concrete. Concrete was not actually forgotten,  since – for the literate minority – descriptions  

and recipes were still available in the  texts of Pliny the Elder and Vitruvius.  

The seventh-century scholar  Isidore of Seville, likewise,  

mentioned Roman concrete in his widely circulated  etymological encyclopedia. By his time, however,  

concrete, like so many other aspects of the Roman  world, was little more a curiosity – intriguing,  

perhaps inspiring, but ultimately irrelevant. If you enjoyed this video, please consider  

supporting toldinstone on Patreon. You  might also be interested in my book,  

Naked Statues, Fat Gladiators, and  War Elephants. Thanks for watching.