Media release

From: Cell Press

Is ancient Roman concrete more sustainable than modern concrete?

Ancient Roman concrete, which was used to build aqueducts, bridges, and buildings across the empire, has endured for over two thousand years. In a study publishing July 25 in the Cell Press journal iScience, researchers investigated whether switching back to Roman concrete could improve the sustainability of modern-day concrete production. They found that reproducing the ancient recipe would require comparable energy and water and emit similar amounts of CO2. However, the authors suggest that the heightened durability of Roman concrete might make it a more sustainable option because it could reduce the need for replacement and maintenance.

“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.

Making more sustainable concrete remains an important challenge in the race to decarbonize the construction industry. Modern concrete production contributes to air pollution and is responsible for approximately 8% of global anthropogenic CO2 emissions and 3% of the total global energy demand. Since previous studies have suggested that Roman concrete might be more sustainable than modern concrete, the researchers decided to put this hypothesis to the test.

“We were interested in how we can draw lessons from their methods to inform some of the climate-mitigation challenges that we currently face in our built environment,” says Martinez.

The key raw ingredient in both Ancient Roman and modern concrete is limestone. When heated to extremely high temperatures, limestone decomposes to produce CO2 and calcium oxide, which can be combined with other key minerals and water to form a paste that binds the concrete (or mortar) together. Whereas the Romans incorporated locally available rocks, volcanic debris called “pozzolan,” and recycled rubble from demolition projects into their concrete, modern concrete is made by mixing cement with various types of sand and gravel.

To compare the sustainability of producing Roman and modern concrete, the researchers used models to estimate the volume of raw materials required (e.g., limestone and water) for each concrete type and the amount of CO2 and air pollutants produced. Since Roman concrete was not made uniformly, they compared multiple ancient recipes that used different proportions of limestone and pozzolan. For the Roman recipes, they also compared the sustainability of ancient and modern production techniques and the use of different forms of energy (e.g., fossil fuels, wood or other biomass, or renewable energy).

To their surprise, the researchers showed that, per volume of concrete, producing Roman concrete results in similar—and, in some cases, more—CO2 compared to modern concrete formulations.

“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 researchers estimated that Roman concrete production would result in lower emissions of air pollutants such as nitrogen oxide and sulfur oxide, which are harmful to human health. These reductions, which ranged from 11%–98%, were present whether Roman concrete production was fueled by fossil fuels, biomass, or renewable energy, but renewable energy resulted in the biggest reductions.

In addition to being potentially less harmful to people, Roman concrete is also thought to be more durable, which could make it a more sustainable option over time, especially for high usage applications like roads and highways, which typically require regular maintenance and replacement. “When we take concrete’s service life into consideration, that’s when we start seeing benefits,” says Martinez.

“In cases where prolonging the use of concrete can reduce the need to manufacture new materials, more durable concrete has the potential to reduce environmental impact,” says author and engineer Sabbie Miller of the University of California, Davis, USA.

However, it’s very difficult to make this comparison, because modern concrete has only been produced for the past 200 years, and, 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.

In the future, the researchers plan to develop more in-depth analyses to compare the performance and lifespan of Roman and modern concrete in different scenarios.

“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.”