The secret of the Colosseum: Roman concrete could save our future

From the ruins of Rome's Colosseum to the aqueducts that remain standing after more than 2,000 years, Roman concrete has been an enigma for engineers and scientists for millennia. In contrast, the estimated lifespan of many of our modern structures is barely 50 years, a time span that, compared to Roman longevity, seems astonishingly short, almost ridiculous. What did Roman engineers know that we, with all our technology, seem to have forgotten? Today, with their secret now revealed, scientists are wondering whether, in the 21st century, applying their technique could help us save the planet.

Both then and now, buildings and large structures are constructed from concrete, a material fundamental to our civilization. And while technology has advanced exponentially in this field, long-term durability has certainly regressed. This suggests that 'innovation' in modern concrete has focused more on aspects such as production speed than on longevity. This is not, therefore, a mere lack of knowledge, but rather a choice of priorities that reflects two different philosophies, Roman and ours, when it comes to construction.

Now, just as we know the secrets of its formula, a study recently published in the journal ' iScience ', by Cell Press, has focused on a very different aspect, and asks whether 'returning' to Roman concrete could improve the sustainability of modern concrete production, which is one of the world's biggest polluters.

Today, concrete production has a more than considerable environmental impact, contributing to air pollution and accounting for approximately 8% of global anthropogenic CO2 emissions and 3% of total global energy demand. Given these figures, the search for more sustainable alternatives has become a priority in the race to "decarbonize" the construction industry. Not to mention that building demolition, for example, releases large amounts of concrete dust into the atmosphere, a dangerous pollutant. And this is precisely where Roman concrete comes into play.

Both Roman and modern concrete have one fundamental component in common: limestone. When heated to extremely high temperatures, limestone decomposes, releasing CO2 and producing calcium oxide. The latter, when combined with other key minerals and water, forms a paste that acts as a binder. Modern concrete is made by mixing cement with various types of sand and gravel. It is reinforced with steel beams (reinforced concrete), something the Romans did not do .

Roman concrete, however, known as opus caementicium, was an extraordinary material that required no reinforcement and whose composition and manufacturing method radically distinguished it from its modern counterpart. The Romans used a mixture of quicklime, water, and, crucially, volcanic ash, which they called pozzolan. This ash, abundant in places like Pozzuoli, near Naples, was not simply an inert filler, but an active ingredient that, when mixed with lime, created a matrix that not only set but also gained strength over time, especially in humid or marine environments.

For a long time, researchers assumed that the key to Roman durability lay solely in pozzolan. However, a 2023 study by MIT and Harvard revealed a crucial detail: the 'hot mix' process. Instead of 'slaking' the quicklime with water before mixing, as is normally done, the Romans added it directly with the ashes and aggregates. This generated an exothermic reaction, meaning it released heat, creating tiny fragments of quicklime, 'lime clasts,' previously thought to be mere defects or impurities in the mix. But that wasn't the case; it was a well-thought-out strategy, a true stroke of genius, and, ultimately, the secret to the durability of their buildings.

When a crack appeared in Roman concrete and rainwater managed to seep in, these tiny lime fragments reacted chemically with it, generating calcium carbonate crystals that filled and sealed the crack from within. In other words, the concrete "repaired itself," as if it had its own "immune system" that allowed it to respond to damage, similar to how living organisms repair their tissues.

Now that the secret is known, this study asks whether, precisely because of its great durability, Roman concrete could also be a more sustainable alternative for today's construction.

Led by engineer Daniela Martínez of the Universidad del Norte in Colombia, the authors of the new study compared the environmental footprint of both types of concrete. They used models to estimate the volume of raw materials required (such as limestone and water) and the amount of CO2 and air pollutants produced. Given the variability in Roman recipes, they compared multiple ancient formulas with different proportions of limestone and pozzolan. They also analyzed the sustainability of ancient and modern production techniques, and the use of different energy sources (fossil fuels, biomass, or renewable energy).

The results were surprising. Contrary to expectations, it turned out that Roman concrete production generated CO2 emissions similar to, or even higher than, those of modern concrete. "Contrary to our initial expectations," explains Mafrtínez, "the adoption of Roman formulations with current technology may not produce substantial reductions in emissions or energy demand." This suggests that simply replicating the ancient recipe is not a panacea for the emissions problem.

However, the research did find one advantage for Roman concrete when it comes to air quality. Using the ancient formula resulted in fewer emissions of air pollutants such as nitrogen oxide and sulfur oxide, substances harmful to human health. These reductions, which ranged from 11% to 98%, were maintained regardless of whether Roman concrete production was fueled by fossil fuels, biomass, or renewable energy, with the latter generating the greatest reductions.

But the real advantage of Roman concrete lies, once again, in its exceptional durability. And this is where the scales tip in its favor as a more sustainable long-term option, especially for high-use applications like roads and highways, which typically require regular maintenance and replacement. "When we consider the lifespan of concrete," says Martínez, "that's when we begin to see the benefits."

Sabbie Miller, an engineer at the University of California, Davis and co-author of the study, underscores this point: “In cases where extending the use of concrete can reduce the need for new materials, more durable concrete has the potential to reduce environmental impact.” Imagine, for example, a modern concrete bridge that needs to be repaired or replaced every 50 years, compared to a Roman structure that remains functional for 1,000 years. The lower frequency of construction, demolition, and transportation of materials associated with greater durability translates into significant energy and emissions savings.

However, making this comparison is not straightforward. Modern concrete has only been produced for the last 200 years, and unlike modern reinforced concrete, ancient Roman structures did not use steel bars to increase their strength. Paulo Monteiro of the University of California, Berkeley, also a co-author of the study, cautions: "Corrosion of steel reinforcement is the main cause of concrete deterioration, so comparisons must be made with great caution." The presence of metal reinforcement in modern concrete introduces a complex variable that did not exist in Roman constructions. Corrosion of steel, resulting from exposure to water and oxygen, can cause internal expansions that crack the concrete, compromising its structural integrity.

The proposal to "return" to Roman concrete has already generated various reactions in the scientific community. Manuel F. Herrador, PhD in civil engineering and professor of Concrete Structures at the University of A Coruña, while appreciating the quality of the study, qualifies expectations. "The formulation of Roman concrete is well known because it has been recorded," explains Herrador. "Indeed, we know that it is a more durable concrete than those commonly used today, but also that it is less resistant, takes longer to set, depends on components (such as volcanic ash) that cannot be easily obtained anywhere, and in some of its most striking uses (I am referring to mixtures with seawater) is incompatible with the steel reinforcements that are essential in our reinforced and prestressed concrete structures."

The lessons learned from Roman concrete, according to this expert, are already being incorporated into modern engineering. Current regulations already contemplate the use of ash additions, which are, in fact, commonly employed in structures with special durability requirements.

Furthermore, Herrador points out that there are more promising lines of research for the decarbonization of cement, such as so-called "green cements." These new materials explore the use of other industrial byproducts, such as bottom ash from blast furnaces or waste from the wood industry, offering innovative avenues for reducing the carbon footprint. A notable example is blast furnace slag (GGBS) cement, which uses a byproduct of steelmaking, significantly reducing the need for Portland cement clinker, the most emissions-intensive component. Another is fly ash cement, which incorporates a residue from coal combustion in power plants, diverting these materials from landfills and leveraging their pozzolanic properties.

The study, however, makes it clear that sustainability in construction does not necessarily mean an exact replication of ancient techniques, but rather a deep understanding of their principles. The Romans built for eternity, whereas today speed prevails, resulting in a kind of "planned obsolescence" in many of our buildings. "There are many lessons we can learn from the Romans," concludes Daniela Martínez. "If we can incorporate their strategies into our modern innovative ideas, we can create a more sustainable environment."

In the future, the researchers plan to conduct more detailed analyses to compare the performance and lifespan of Roman and modern concrete in different scenarios. The goal is not simply to copy, but to learn and adapt. The intrinsic durability of Roman concrete teaches us that longevity is a fundamental pillar of sustainability. If we can design materials that, while initially requiring similar energy to produce, last twice or three times as long, the long-term resource savings and emissions reductions will be immense.