Roman concrete made the Colosseum possible. Inside the Colosseum, visitors have ample space to picture the roaring crowds of more than 50,000 that once thronged the arena for events ranging from bloody gladiatorial battles to opulent processions and chariot races. Also known as the Flavian Amphitheatre, the venue’s grand opening in 80 AD featured 100 straight days of games and gore. At four stories tall, and 188m across at its widest point, the oval structure is still the largest amphitheatre in today’s world.

     Roman concrete isn’t that obvious in the Colosseum, but it has played an integral role in the arena’s survival. The most prominent material of the structure’s construction is travertine limestone, but concrete is what holds the amphitheatre’s many iconic archways aloft. Impressive as this is, perhaps the most significant contribution of Roman concrete to the Colosseum’s longevity is out of sight. The reason the Colosseum is still standing is because of its incredibly robust concrete foundation. That concrete foundation is packed with dense, heavy lava rock aggregate and is a full 12m thick. Without such a strong, long-lasting material at its foundation, the Colosseum would have been reduced entirely to rubble by the region’s earthquakes.

     No visit to Rome would be complete without a visit to the Colosseum, but for anyone seeking the pinnacle of concrete construction in the Roman world, the Pantheon’s unreinforced dome is a must.

     The Pantheon, constructed around 40 years after the Colosseum, houses a dome that spans 43m of air, and culminates in a pupil-like circular window at its apex, known as the oculus. The oculus floods the interior with natural light. Despite the ravages of time, the iconic half-sphere remains intact, and is still the world’s largest unreinforced concrete dome. When trying to appreciate the Pantheon’s dome, “unreinforced” is really the key word. If an architect tried to build the Pantheon today, the plans would be denied because without reinforcement, such as the steel bars commonly used in modern concrete structures, the dome would violate modern civil engineering code.

     Roman concrete harbour structures are also fascinating because of their impressive longevity. Battered by waves for over 2,000 years, they are still standing, whereas our modern-day concrete will last a few decades under such pounding, if we are lucky. Roman cement will even cure underwater. In fact, it will cure quicker in seawater. Modern cement will not cure in water, let alone seawater.

     Why, then, is 2,000-year-old Roman concrete so much better than what we produce today?

     Scientists have now uncovered the chemistry behind this mystery, but have still not unlocked its long-lost recipe. Roman concrete is not only more durable than what we can make today, but it actually gets stronger over time.

     Modern cement is typically made with Portland cement, which is a mixture of silica sand, limestone, clay, chalk and other ingredients melted together at blistering temperatures. In concrete, this paste binds ‘aggregate’ – chunks of rock and sand. The resulting aggregate has to be inert, because any unwanted chemical reaction can cause cracks in the concrete, leading to erosion and crumbling of the structures.

     That’s not how Roman concrete worked. Theirs was created with volcanic ash, lime and seawater, taking advantage of a chemical reaction Romans may have observed in naturally cemented volcanic ash deposits called tuff rocks. Mixed in with the volcanic ash mortar was more volcanic rock acting as aggregate. In contrast to the aggregates used in modern concrete, the volcanic materials used by the Romans are highly reactive, and the resulting concrete remains chemically active for centuries after it first hardens.           

     If scientists can crack the Roman recipe, modern engineers could tap into the potential of a material that doesn’t need steel reinforcements and can last for centuries.

     In today’s world of environmental concerns, Roman concrete makes even more sense. That’s because the production of Portland cement is responsible for at least 8% of global carbon emissions. The lime-based binder in Roman concrete only needs to be heated to around 900C, while Portland cement needs to be fired at close to 2,000C. This alone means that Roman concrete has the potential to offer massive reductions in the carbon footprint of concrete production.

     The U.S. Department of Energy is currently working on a project to develop a Roman-like concrete with the goals of potentially reducing the emissions associated with concrete production and installation by 85%, while quadrupling the lifespan of that made with Portland cement. One of the scientists involved said, “We don’t need to copy what the Romans did exactly, but when it comes to making concrete more durable and more sustainable, they clearly have some things to teach us.” An understatement if ever there was one!

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