The Pantheon in Rome is a marvel of engineering. It’s an unreinforced concrete dome that’s stood tall and proud for nearly two millennia. The aqueducts that once carried water across the empire still work across modern landscapes, and some of them are still in use. Even ancient harbors built with Roman concrete have weathered centuries of relentless waves.
How did the Romans make such strong concrete, and could we start using it ourselves?
The question has important climate consequences. Concrete is the backbone of civilization, the most-used manufactured material in the world. But it comes at a steep environmental price. The production of modern cement, the key ingredient in concrete, is a voracious consumer of energy and a prodigious source of pollution, responsible for roughly 8% of global carbon dioxide emissions. In the urgent race to decarbonize, researchers wondered whether Roman concrete was not only stronger but also greener.
The answer is, unfortunately, “it’s complicated.”
A Surprising Carbon Footprint
Both Roman and modern cement have limestone as the core ingredient. When heated to very high temperatures, limestone decomposes and can be combined with other minerals to form a paste that binds together into a lasting material.
Instead of modern Portland cement, which is made by heating limestone and clays to a searing 1,450°C, Roman builders used a lime-based mortar. They heated limestone to a lower temperature of around 900°C and mixed the resulting lime with water and pozzolan—a fine volcanic ash abundant in regions like Pozzuoli, near Naples, which gave the material its name. This pozzolanic reaction created a famously stable and resilient binder. Modern concrete, meanwhile, uses various types of sand and gravel.
To test the sustainability of this ancient method, Martinez and her colleagues did the math. They modeled the energy, water, and emissions required to produce various Roman concrete recipes using both ancient and modern techniques.
The results weren’t what the researchers expected. They found that without a doubt, it was more efficient to make Roman concrete with modern technology. But Roman concrete generated comparable, and often higher, CO2 emissions per cubic meter than its modern equivalent.
“Studying Roman concrete can teach us how to use materials in a way that can maximize the longevity of our structures, because sustainability goes hand-by-hand with durability,” says author and engineer Daniela Martinez of Universidad del Norte in Colombia.
“Contrary to our initial expectations, adopting Roman formulations with current technology may not yield substantial reductions in emissions or energy demand,” says Martinez. “Using biomass and other alternative fuels to fire kilns may prove more effective in decarbonizing modern cement production than implementing Roman concrete formulations.”
However, the study did find a silver lining. The Roman production method would lead to a sharp drop in air pollutants like nitrogen and sulfur oxides — harmful gases that contribute to respiratory disease but can be managed in the production phase. The researchers estimated these emissions could be cut 11% to as much as 98%, depending on the energy source used.
Adding Longevity into the Mix
But just because the production process isn’t greener doesn’t necessarily mean Roman concrete isn’t useful. The answer is in the sheer endurance of the final product. Modern concrete structures often need significant maintenance or replacement within decades. Roman concrete has lasted for centuries.
But there’s a catch.
Unlike modern reinforced concrete, the ancient Roman structures did not use steel bars to increase strength. “Corrosion of steel reinforcement is the main cause of concrete deterioration, so comparisons should be made with great care,” says author and engineer Paulo Monteiro of the University of California, Berkeley, USA. Also, Roman infrastructure hasn’t had the same type of pressure and use that modern infrastructure has.
When you do all the sustainability math, there is a breaking point at which the Roman way of making concrete becomes more climate friendly. For an application like a road, which requires frequent replacement, the Roman-style concrete would need to last between 29% and 97% longer to offset its initial emissions and break even. Given that Roman structures have lasted thousands of years, this seems possible, but it’s far from guaranteed.
But the team says there could be sweet spots where Roman concrete works even better. In some applications, under some production processes, it would definitely trigger fewer emissions.
Lessons from the Romans
“There’s a lot of lessons that we can draw from the Romans,” says Martinez. “If we can incorporate their strategies with our modern innovative ideas, we can create a more sustainable built environment.”
This doesn’t mean we should all start making more Roman concrete.
The fundamental lesson isn’t about resurrecting a lost recipe or falling for the myth of the ancient builders. It’s about embracing a philosophy of permanence. The study suggests that the most effective path to greener construction may not be a single ancient formula, but a hybrid approach that marries ancient principles of longevity with modern technological advances.
Journal Reference: iScience, Martinez et al., “How sustainable was Ancient Roman concrete?” https://www.cell.com/iscience/fulltext/S2589-0042(25)01313-6
