Why Roman concrete was more sustainable than modern materials

By Stephen Beech

Ancient Roman concrete is more sustainable than modern building materials, according to new research.

The Romans used concrete to build aqueducts, bridges and buildings across their vast empire, which have endured for over 2,000 years.

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 similar energy and water and emit similar amounts of carbon dioxide (CO2).

But the research team suggests that the "heightened durability" of Roman concrete might make it a more sustainable option because it could reduce the need for replacement and maintenance.

Study author and engineer Doctor Daniela Martinez, of Universidad del Norte, Colombia, said: “Studying Roman concrete can teach us how to use materials in a way that can maximise the longevity of our structures, because sustainability goes hand-in-hand with durability.”

Modern concrete production contributes to air pollution and is responsible for around 8% of global anthropogenic CO2 emissions and 3% of the total global energy demand.

As previous studies had suggested that Roman concrete might be more sustainable than modern versions, the research team put the hypothesis to the test.

Dr. Martinez said: “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.”

She explained that 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.

While 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 research team used models to estimate the volume of raw materials required 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, including fossil fuels, wood or other biomass, or renewable energy.

The findings, published in the journal iScience, showed that, per volume of concrete, producing Roman concrete results in similar - and, in some cases, more - CO2 compared to modern concrete formulations.

Dr. Martinez said: “Contrary to our initial expectations, adopting Roman formulations with current technology may not yield substantial reductions in emissions or energy demand.

“Using biomass and other alternative fuels to fire kilns may prove more effective in decarbonising modern cement production than implementing Roman concrete formulations.”

But the researchers estimated that Roman concrete production would result in lower emissions of air pollutants such as nitrogen oxides and sulfur oxides, which are harmful to human health.

The reductions, which ranged from 11% to 98%, were present whether Roman concrete production was fuelled 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 believed to be more durable, which could make it a more sustainable option over time, especially for high usage applications such as roads and motorways, which require regular maintenance and replacement.

Dr. Martinez said: “When we take concrete’s service life into consideration, that’s when we start seeing benefits."

Co-author Professor Sabbie Miller of the University of California, Davis, said: “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.”

But the researchers said the comparison is difficult to make because modern concrete has only been produced for the past 200 years, and, unlike modern reinforced concrete, ancient Roman structures did not use steel bars to increase strength.

Co-author Professor Paulo Monteiro, of the University of California, Berkeley, said: “Corrosion of steel reinforcement is the main cause of concrete deterioration, so comparisons should be made with great care.”

The research team plans to develop more in-depth ways to compare the performance and lifespan of Roman and modern concrete in different scenarios.

Dr. Martinez added, “There are a lot of lessons that we can draw from the Romans.

“If we can incorporate their strategies with our modern innovative ideas, we can create a more sustainable built environment.”