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The Secrets of Ancient Roman Concrete

The Secrets of Ancient Roman Concrete

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History contains many references to ancient concrete, including in the writings of the famous Roman scholar Pliny the Elder, who lived in the 1st century A.D. and died in the eruption of Mt. Vesuvius in A.D. 79. Pliny wrote that the best maritime concrete was made from volcanic ash found in regions around the Gulf of Naples, especially from near the modern-day town of Pozzuoli. Its virtues became so well-known that ash with similar mineral characteristics–no matter where it was found in the world–has been dubbed pozzolan.

By analyzing the mineral components of the cement taken from the Pozzuoli Bay breakwater at the laboratory of U.C. Berkeley, as well as facilities in Saudi Arabia and Germany, the international team of researchers was able to discover the “secret” to Roman cement’s durability. They found that the Romans made concrete by mixing lime and volcanic rock to form a mortar. To build underwater structures, this mortar and volcanic tuff were packed into wooden forms. The seawater then triggered a chemical reaction, through which water molecules hydrated the lime and reacted with the ash to cement everything together. The resulting calcium-aluminum-silicate-hydrate (C-A-S-H) bond is exceptionally strong.

By comparison, Portland cement (the most common modern concrete blend) lacks the lime-volcanic ash combination, and doesn’t bind well compared with Roman concrete. Portland cement, in use for almost two centuries, tends to wear particularly quickly in seawater, with a service life of less than 50 years. In addition, the production of Portland cement produces a sizable amount of carbon dioxide, one of the most damaging of the so-called greenhouse gases. According to Paulo Monteiro, a professor of civil and environmental engineering at the University of California, Berkeley, and the lead researcher of the team analyzing the Roman concrete, manufacturing the 19 billion tons of Portland cement we use every year “accounts for 7 percent of the carbon dioxide that industry puts into the air.”

In addition to being more durable than Portland cement, argue, Roman concrete also appears to be more sustainable to produce. To manufacture Portland cement, carbon is emitted by the burning fuel used to heat a mix of limestone and clays to 1,450 degrees Celsius (2,642 degrees Fahrenheit) as well as by the heated limestone (calcium carbonate) itself. To make their concrete, Romans used much less lime, and made it from limestone baked at 900 degrees Celsius (1,652 degrees Fahrenheit) or lower, a process that used up much less fuel.

The researchers’ analysis of Roman concrete sheds light on existing modern concrete blends that have been used as more environmentally friendly partial substitutes for Portland cement, such as volcanic ash or fly ash from coal-burning power plants. Monteiro and his colleagues also suggest that adopting materials and production techniques used by the ancient Romans could produce longer-lasting concrete that generates less carbon dioxide. Monteiro estimates that pozzolan, which can be found in many parts of the world, could potentially replace “40 percent of the world’s demand for Portland cement.” If this is the case, ancient Roman builders may be responsible for making a truly revolutionary impact on modern architecture–one massive concrete structure at a time.

Drilling Into The Secrets Of Roman Concrete

The ruins of the Roman Forum. Credit: THINK Global School/flickr/CC BY-NC-ND 2.0

Over two thousand years ago, the ancient Romans built piers, breakwaters, and other structures out of concrete—and some of those structures still stand today. Now, researchers are trying to understand the chemical and geological processes that work together to give that ancient concrete such durability. Using microscopy, x-ray diffraction, and spectroscopic techniques, they’ve developed a map of the crystalline microstructures within the concrete. According to their research, a slow infusion of seawater into concrete made with a type of volcanic ash found near Rome gradually creates crystals of a material called aluminous tobermorite, which actually strengthens the concrete as it ages.

Marie Jackson, a geology and geophysics research professor and one of the authors of a report on the work , says that understanding Roman concrete could give modern materials scientists ideas for how to strengthen modern structures, and could even lead to new materials, such as concretes that soak up and trap nuclear waste.

Roman concrete

Concrete was usually covered as concrete walls were considered unaesthetic. Roman builders covered building walls with stones or small square tuff blocks that would often form beautiful patterns noting that brick faced concrete buildings were common in Rome especially after the great fire of 64 AD.

Roman concrete formula

Ancient Roman concrete vault in Rome

Concrete was made by mixing with water: 1) an aggregate which included pieces or rock, ceramic tile, pieces of brick from previously demolished constructions, 2) volcanic dust (called pozzolana) and 3) gypsum or lime. Usually the mix was a ratio of 1 part of lime for 3 parts of volcanic ash. Pozzolana contained both silica and alumina and created a chemical reaction which strengthened the cohesiveness of the mortar.

There were many variations of concrete and Rome even saw the Concrete Revolution which represented advances in the composition of concrete and allowed for the construction of impressive monuments such as the Pantheon. For example, Roman builders discovered that adding crushed terracotta to the mortar created a waterproof material which could be then be used with cisterns and other constructions exposed to rain or water.

Romans mastered underwater concrete by the middle of the 1st century AD. The city of Caesarea gives us an impressive example of Roman construction. The production technique was quite incredible: the mix was one-part lime for two-parts volcanic ash, and it was placed in volcanic tuff or small wooden cases. The seawater would then hydrate the lime and trigger a hot chemical reaction which hardened the concrete.

Caesarea harbor before and today - Robert Teringo, National Geographic Society

Was Roman concrete better than modern concrete?

Actually it has been argued that the concrete used by the Romans was of better quality than the concrete in use today. Recent research from US and Italian scientists has shown that the concrete used to make Roman harbors in the Mediterranean was more resistant than modern concrete (known as Portland cement).

The production process was dramatically different. Portland cement is made by heating clays and limestone at high temperatures (various additives are also added) while the Romans used volcanic ash and a much smaller amount of lime heated at lower temperatures than modern methods.

For example, Roman harbors remain intact today after 2,000 years of waves breaking on the harbors' breakwaters whereas Portland concrete begins to erode in less than 50 years of sea battering. The concrete from ancient Rome also had bending properties that Portland concrete does not have due to its lime and volcanic ash, which explains why it does not crack after a few decades.

The Ancient Romans' Concrete Recipe Could Help Us Beat Back Rising Seas

Ancient Roman concrete was more durable than any developed before or since. "It's the most durable building material in human history," Philip Brune, a researcher at DuPont Pioneer who studies the engineering of ancient Roman monuments, told theWashington Post. "And I say that as an engineer not prone to hyperbole."

Indeed, academics study the properties and chemical mixture of the concrete, which was made with volcanic ash found in Italy and is particularly well suited to marine structures. Now some researchers are wondering if the secrets of this ancient building material could help us adapt to a world of rising seas.

As global temperatures rise, sea ice is melting and causing the sea level to rise at a faster rate than during the 1900s. Exactly how much it will rise is dependent on a number of variables, but there is a high likelihood that rising sea levels will force us to reinforce infrastructure around coastal cities. Venice is already sinking.

One of the most direct solutions for a coastal city is to construct a seawall. These structures do not need to hold back the ocean constantly, but rather are built to block the water from the city during high tide and storms that can cause flooding. The Malecón in Havana, Cuba, for example, is a five-mile-long roadway and seawall that guards the infrastructure of the city. Seawalls are used around the world in countries like the United Kingdom and Australia.

It turns out the ancient Romans had the perfect recipe for water-resistant concrete. The material, called opus caementicium by the Romans, is made from a hydraulic cement, meaning it can set underwater or in wet conditions. The Romans mixed this cement with volcanic ash found in regions around modern Naples. The volcanic ash added a mineral called phillipsite to the concrete, and a study published Monday in the American Mineralogist reveals that aluminous tobermorite crystals grow in the Roman concrete when it is exposed to water. These crystals, the scientists believe, could provide the structural reinforcement that makes Roman concrete so durable.

Between 22 and 10 BCE, the Romans constructed an underwater concrete foundation for the harbor of the ancient city of Caesarea in what is now Israel. These marine structures are still intact today, more than two millennia later. Researchers studying ancient Roman concrete suggest the material could be imitated with modern resources to build seawalls around cities at risk of flooding from the ocean.

Marie Jackson, an archeologist with the University of Utah, is attempting to recreate this type of concrete using volcanic rocks found in the ocean around San Francisco. She notes that artificially producing aluminous tobermorite requires a large amount of heat and energy simply to synthesize a small amount. If we hope to add this material to modern concrete, it will likely be more cost efficient to harvest it from the natural source of heat and energy where it forms: volcanoes.

If cities around the world are forced to build seawalls due to rising oceans, a version of Roman concrete could provide an alternative to steel structures. This enduring concrete only hardens and becomes more durable as it is exposed to the saltwater of the sea.

Pliny the Elder, the famed Roman author, historian, and philosopher, once wrote an ode to concrete. "As soon as it comes into contact with the waves of the sea and is submerged becomes a single stone mass, impregnable to the waves." With the waves rising around us, we could have more need for this concrete than ever before.

Nature’s laboratory

The Neapolitan Province in southern Italy is an ideal place to dive into the science of natural hazards and how they have played into daily life and innovation over thousands of years. Densely populated and peppered with dozens of volcanoes, the region ranks as one of the most hazardous on Earth. The ruins of a Roman harbor and an emperor’s villa can be found offshore, sunken like Atlantis as a result of unrest in Earth’s crust. “Not many places on Earth experience this kind of seismicity and volcanism, while being an ancient town and functioning as a modern society,” Vanorio said. “That’s the beauty of the place.”

Underlying the seminar’s excursions and daily lessons in geophysics, the properties of Roman concrete and 3D modeling from drone images was a larger exercise in finding connections between different fields of study. It’s no accident that students chosen to participate in the seminar represented a wide range of majors, including computer science, physics, classics, chemical engineering and political science.

The ancient Italian city of Pozzuoli was shaped by volcanic activity. (Image credit: Kurt Hickman)

“Not many places on Earth experience this kind of seismicity and volcanism, while being an ancient town and functioning as a modern society.”

—Tiziana Vanorio

Assistant Professor of Geophysics

“There are still scientific questions that we don’t know how to answer,” said Vanorio, who discovered natural processes deep in the subsurface of Campi Flegrei that mirror those in Roman concrete, and has used historical texts to shed light on strengths and the characteristics of both volcanic and engineered materials. “The more we leverage knowledge across different disciplines, the more we can address and solve those problems.”

For Amara McCune, BS ’18, who joined a previous seminar in the region led by Vanorio in 2016, the intermingling of geophysics with dives into the region’s culture proved a powerful mix. “The unique combination of learning about Pompeii, volcanic uplift and Rome while being on-site, hearing from local guides and having archaeological and geological experts point out features of a location made for an incredibly rich learning experience,” she said.

Roman Seawater Concrete Holds the Secret to Cutting Carbon Emissions

Drill core of volcanic ash-hydrated lime mortar from the ancient port of Baiae in Pozzuloi Bay. Yellowish inclusions are pumice, dark stony fragments are lava, gray areas consist of other volcanic crystalline materials, and white spots are lime. Inset is a scanning electron microscope image of the special Al-tobermorite crystals that are key to the superior quality of Roman seawater concrete. (Click on image for best resolution.)

The chemical secrets of a concrete Roman breakwater that has spent the last 2,000 years submerged in the Mediterranean Sea have been uncovered by an international team of researchers led by Paulo Monteiro of the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), a professor of civil and environmental engineering at the University of California, Berkeley.

Analysis of samples provided by team member Marie Jackson pinpointed why the best Roman concrete was superior to most modern concrete in durability, why its manufacture was less environmentally damaging – and how these improvements could be adopted in the modern world.

“It’s not that modern concrete isn’t good – it’s so good we use 19 billion tons of it a year,” says Monteiro. “The problem is that manufacturing Portland cement accounts for seven percent of the carbon dioxide that industry puts into the air.”

Portland cement is the source of the “glue” that holds most modern concrete together. But making it releases carbon from burning fuel, needed to heat a mix of limestone and clays to 1,450 degrees Celsius (2,642 degrees Fahrenheit) – and from the heated limestone (calcium carbonate) itself. Monteiro’s team found that the Romans, by contrast, used much less lime and made it from limestone baked at 900 ˚ C (1,652 ˚ F) or lower, requiring far less fuel than Portland cement.

Cutting greenhouse gas emissions is one powerful incentive for finding a better way to provide the concrete the world needs another is the need for stronger, longer-lasting buildings, bridges, and other structures.

“In the middle 20th century, concrete structures were designed to last 50 years, and a lot of them are on borrowed time,” Monteiro says. “Now we design buildings to last 100 to 120 years.” Yet Roman harbor installations have survived 2,000 years of chemical attack and wave action underwater.

How the Romans did it

The Romans made concrete by mixing lime and volcanic rock. For underwater structures, lime and volcanic ash were mixed to form mortar, and this mortar and volcanic tuff were packed into wooden forms. The seawater instantly triggered a hot chemical reaction. The lime was hydrated – incorporating water molecules into its structure – and reacted with the ash to cement the whole mixture together.

Pozzuoli Bay defines the northwestern region of the Bay of Naples. The concrete sample examined at the Advanced Light Source by Berkeley researchers, BAI.06.03, is from the harbor of Baiae, one of many ancient underwater sites in the region. Black lines indicate caldera rims, and red areas are volcanic craters. (Click on image for best resolution.)

Descriptions of volcanic ash have survived from ancient times. First Vitruvius, an engineer for the Emperor Augustus, and later Pliny the Elder recorded that the best maritime concrete was made with ash from volcanic regions of the Gulf of Naples (Pliny died in the eruption of Mt. Vesuvius that buried Pompeii), especially from sites near today’s seaside town of Pozzuoli. Ash with similar mineral characteristics, called pozzolan, is found in many parts of the world.

Using beamlines,, 12.2.2 and 12.3.2 at Berkeley Lab’s Advanced Light Source (ALS), along with other experimental facilities at UC Berkeley, the King Abdullah University of Science and Technology in Saudi Arabia, and the BESSY synchrotron in Germany, Monteiro and his colleagues investigated maritime concrete from Pozzuoli Bay. They found that Roman concrete differs from the modern kind in several essential ways.

One is the kind of glue that binds the concrete’s components together. In concrete made with Portland cement this is a compound of calcium, silicates, and hydrates (C-S-H). Roman concrete produces a significantly different compound, with added aluminum and less silicon. The resulting calcium-aluminum-silicate-hydrate (C-A-S-H) is an exceptionally stable binder.

At ALS beamlines and, x-ray spectroscopy showed that the specific way the aluminum substitutes for silicon in the C-A-S-H may be the key to the cohesion and stability of the seawater concrete.

Another striking contribution of the Monteiro team concerns the hydration products in concrete. In theory, C-S-H in concrete made with Portland cement resembles a combination of naturally occurring layered minerals, called tobermorite and jennite. Unfortunately these ideal crystalline structures are nowhere to be found in conventional modern concrete.

Tobermorite does occur in the mortar of ancient seawater concrete, however. High-pressure x-ray diffraction experiments at ALS beamline 12.2.2 measured its mechanical properties and, for the first time, clarified the role of aluminum in its crystal lattice. Al-tobermorite (Al for aluminum) has a greater stiffness than poorly crystalline C-A-S-H and provides a model for concrete strength and durability in the future.

Finally, microscopic studies at ALS beamline 12.3.2 identified the other minerals in the Roman samples. Integration of the results from the various beamlines revealed the minerals’ potential applications for high-performance concretes, including the encapsulation of hazardous wastes.

Lessons for the future

Environmentally friendly modern concretes already include volcanic ash or fly ash from coal-burning power plants as partial substitutes for Portland cement, with good results. These blended cements also produce C-A-S-H, but their long-term performance could not be determined until the Monteiro team analyzed Roman concrete.

Their analyses showed that the Roman recipe needed less than 10 percent lime by weight, made at two-thirds or less the temperature required by Portland cement. Lime reacting with aluminum-rich pozzolan ash and seawater formed highly stable C‑A-S-H and Al-tobermorite, insuring strength and longevity. Both the materials and the way the Romans used them hold lessons for the future.

“For us, pozzolan is important for its practical applications,” says Monteiro. “It could replace 40 percent of the world’s demand for Portland cement. And there are sources of pozzolan all over the world. Saudi Arabia doesn’t have any fly ash, but it has mountains of pozzolan.”

Stronger, longer-lasting modern concrete, made with less fuel and less release of carbon into the atmosphere, may be the legacy of a deeper understanding of how the Romans made their incomparable concrete.

This work was supported by King Abdullah University of Science and Technology, the Loeb Classical Library Foundation at Harvard University, and DOE’s Office of Science, which also supports the Advanced Light Source. Samples of Roman maritime concrete were provided by Marie Jackson and by the ROMACONS drilling program, sponsored by CTG Italcementi of Bergamo, Italy.

Scientific contacts: Paulo Monteiro, [email protected], 510-643-8251 Marie Jackson, [email protected], 928-853-7967

For more information, read the UC Berkeley press release at

“Material and elastic properties of Al-tobermorite in ancient Roman seawater concrete,” by Marie D. Jackson, Juhyuk Moon, Emanuele Gotti, Rae Taylor, Abdul-Hamid Emwas, Cagla Meral, Peter Guttmann, Pierre Levitz, Hans-Rudolf Wenk, and Paulo J. M. Monteiro, appears in the Journal of the American Ceramic Society.

“Unlocking the secrets of Al-tobermorite in Roman seawater concrete,” by Marie D. Jackson, Sejung Rosie Chae, Sean R. Mulcahy, Cagla Meral, Rae Taylor, Penghui Li, Abdul-Hamid Emwas, Juhyuk Moon, Seyoon Yoon, Gabriele Vola, Hans-Rudolf Wenk, and Paulo J. M. Monteiro, will appear in American Mineralogist.

Modern Technology Unlocks New Doors During Research

In a previous research project led by Jackson, the team managed to collect samples of Roman marine concrete from various ports along the Italian coast. Then the researchers mapped the samples using an electron microscope, before drilling down to an extremely high resolution with X-ray micro-diffraction and Raman spectroscopy. With the help of advanced technology they managed to distinguish all the mineral grains produced in the ancient concrete over centuries. "We can go into the tiny natural laboratories in the concrete, map the minerals that are present, the succession of the crystals that occur, and their crystallographic properties. It's been astounding what we've been able to find," Jackson said as Science Alert reports .

Jackson showed particular interest in the presence of aluminous tobermorite, a hardy silica-based mineral that is very rare and difficult to make in the lab, yet can be easily found in big doses when it comes to ancient Roman concrete. Apparently, aluminous tobermorite and a similar mineral called phillipsite appears to grow in the concrete thanks to the sea water splashing around it, gradually dissolving the volcanic ash within and giving it free room to develop a fortified structure from these interlocking crystals. "The Romans created a rock-like concrete that thrives in open chemical exchange with seawater," Jackson says, pointing out the gigantic difference of ancient and modern concrete, which in contrast to ancient Roman concrete appears to erode as saltwater rusts the steel reinforcements and washes away the compounds that hold the material together.

Ancient Romans made world’s ‘most durable’ concrete. We might use it to stop rising seas.

Two thousand years ago, Roman builders constructed vast sea walls and harbor piers. The concrete they used outlasted the empire — and still holds lessons for modern engineers, scientists say.

A bunch of half-sunken structures off the Italian coast might sound less impressive than a gladiatorial colosseum. But underwater, the marvel is in the material. The harbor concrete, a mixture of volcanic ash and quicklime, has withstood the sea for two millennia and counting. What's more, it is stronger than when it was first mixed.

The Roman stuff is “an extraordinarily rich material in terms of scientific possibility,” said Philip Brune, a research scientist at DuPont Pioneer who has studied the engineering properties of Roman monuments. “It's the most durable building material in human history, and I say that as an engineer not prone to hyperbole.”

By contrast, modern concrete exposed to saltwater corrodes within decades.

The mystery has been why the ancient material endured. “Archaeologists will say they have the recipe,” said Marie Jackson, an expert in ancient Roman concrete at the University of Utah. (Pliny the Elder once wrote an ode to concrete “that as soon as it comes into contact with the waves of the sea and is submerged becomes a single stone mass, impregnable to the waves.") But it's not the complete picture: It's one thing to assemble the ingredients, another to know how to bake the cake.

To that end, Jackson and her colleagues peered into the microscopic structures of concrete samples, extracted from the sea walls and piers as part of a project called the Roman Maritime Concrete Study. “This rocklike concrete is behaving, in many ways, like volcanic deposits in submarine environments,” Jackson said.

Where modern concrete is designed to ignore the environment, Roman concrete embraces it. As the scientists report in a study published Monday in the journal American Mineralogist, Roman concrete is filled with tiny growing crystals. The crystals, like tiny armor plates, may keep the concrete from fracturing.

The scientists subjected the concrete samples to a battery of advanced imaging techniques and spectroscopic tests. The tests revealed a rare chemical reaction, with aluminous tobermorite crystals growing out of another mineral called phillipsite. Brune, who was not involved with the study, called the work a “significant accomplishment.” He likened it to the scientists biting into a cake of mysterious flavor and determining that the baker used organically sourced dark chocolate.

In this instance, the key ingredient proved to be seawater. As seawater percolated within the tiny cracks in the Roman concrete, Jackson said, it reacted with the phillipsite naturally found in the volcanic rock and created the tobermorite crystals.

“Aluminous tobermorite is very difficult to produce,” she said, and requires very high temperatures to synthesize small amounts. Cribbing from the ancient Romans might lead to better production of tobermorite, which is prized for its industrial applications, she noted.

The Romans mined a specific type of volcanic ash from a quarry in Italy. Jackson is attempting to recreate this durable concrete using San Francisco seawater and more abundant volcanic rocks. She has several samples sitting in ovens and jars in her lab, which she will test for evidence of similar chemical reactions.

Why 2,000 Year-Old Roman Concrete Is So Much Better Than What We Produce Today

One of the fascinating mysteries of Ancient Rome is the impressive longevity of some of their concrete harbour structures. Battered by sea waves for 2,000 years, these things are still around while our modern concoctions erode over mere decades.

Now scientists have uncovered the incredible chemistry behind this phenomenon, getting closer to unlocking its long-lost recipe. As it turns out, not only is Roman concrete more durable than what we can make today, but it actually gets stronger over time.

Researchers led by geologist Marie Jackson from the University of Utah have been chipping away at the mysteries of Roman concrete for years, and now they have mapped its crystalline structure, figuring out precisely how this ancient material solidifies over time.

Modern concrete is typically made with portland cement, 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.

This aggregate has to be inert, because any unwanted chemical reaction can cause cracks in the concrete, leading to erosion and crumbling of the structures. This is why concrete doesn't have the longevity of natural rocks.

But that's not how Roman concrete works.

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 as aggregate, which would then continue to react with the material, ultimately making Roman cement far more durable than you'd think it should be.

In a previous research project led by Jackson, the team had already gathered samples of Roman marine concrete from several ports along the Italian coast.

Drilling for Roman concrete samples in Tuscany, 2003. Photo: J. P. Oleson

Now the researchers mapped the samples using an electron microscope, before drilling down to an extremely high resolution with X-ray microdiffraction and Raman spectroscopy. With these advanced techniques they could identify all the mineral grains produced in the ancient concrete over centuries.

"We can go into the tiny natural laboratories in the concrete, map the minerals that are present, the succession of the crystals that occur, and their crystallographic properties," says Jackson.

"It's been astounding what we've been able to find."

Jackson was particularly interested in the presence of aluminous tobermorite, a hardy silica-based mineral that's actually pretty rare and difficult to make in the lab, yet is abundant in the ancient concrete.

As it turns out, aluminous tobermorite and a related mineral called phillipsite actually grows in the concrete thanks to the sea water sloshing around it, slowly dissolving the volcanic ash within and giving it space to develop a reinforced structure from these interlocking crystals.

"The Romans created a rock-like concrete that thrives in open chemical exchange with seawater," says Jackson.

That's pretty crazy, and is exactly the opposite of what happens in modern concrete, which erodes as saltwater rusts the steel reinforcements and washes away the compounds that hold the material together.

Making concrete the way Romans once did would be a boon to the modern building industry, especially when it comes to coastal structures, like piers that are constantly battered by the waves, or fanciful tidal lagoons to harness energy from waves.

But unfortunately the recipes have been lost to the tooth of time, so our only shot at recreating the ancient material is to reverse-engineer it based on what we know about its chemical properties.

And it's not like we can replace all the world's cement with the historical stuff, because not everywhere can we access the right volcanic ingredients.

"Romans were fortunate in the type of rock they had to work with," says Jackson. "We don't have those rocks in a lot of the world, so there would have to be substitutions made."

But if Jackson and her colleagues can crack the recipe, modern marine engineers could tap into the potential of a material that doesn't need steel reinforcements, can last for centuries, and makes fewer carbon emissions to boot.

Scientists have long puzzled over the elusive recipe for ancient Roman concrete, which has withstood the test of time better than any of the concrete that’s been poured in the 20th century. Now, Time reports that Maria Jackson from the University of Utah claims to have unravelled the mystery, and furthermore believes that the ancient Roman process could influence modern-day construction.

Jackson’s findings, published in American Mineralogist, claim the unbreakable strength of ancient Roman concrete is due to a rare chemical reaction that takes place when the mineral aluminium tobermorite is exposed to sea water. The reaction strengthens the mortar and prevents cracks from forming or widening.

The longer the concrete is submerged in sea water, the stronger it becomes, as a mineral mixture of silica oxides and lime grows between the volcanic rock aggregate, which in turns hardens all the components into a single, unyielding piece. Jackson explained how this is different from our current concrete to Time:

This may explain the ancient observation of the Roman scientist Pliny the Elder, who wrote in 79 AD that the concrete, “as soon as it comes into contact with the waves of the sea and is submerged, becomes a single stone mass, impregnable to the waves and every day stronger.”

The Pantheon in Rome, still in use over 2,000 years after it was built, is a testament to the strength of ancient Roman concrete. Once a Roman temple, it has been in continuous use throughout history, and since the 7th century has been used as a church dedicated to “St. Mary and the Martyrs.”

Jackson notes that the Roman process was actually much more eco-friendly than our modern method, which is known to produce carbon dioxide. She believes that the old ways of concrete production could teach us a lot, but she notes that the ancient Romans had a greater access to volcanic ash, a primary ingredient, than most countries do today.

Jackson said she is currently experimenting with several substances that could act as a substitute for volcanic ash in the concoction, which would also require lime, sea water, and aluminium tobermorite. She has also proposed that the construction of a planned tidal lagoon in the United Kingdom utilize the ancient Roman concrete in place of steel.

She said that the ancient concrete would be ideal for the tidal lagoon, as the concrete would strengthen with the tide, rather than deteriorating over time. However, she did note that it would take about 120 years to know if the recipe will stand the test of time as well as that of the Romans.

Either way, she believes the ancient concrete would last at least twice as long as our modern concrete.

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New studies of ancient concrete could teach us to do as the Romans did

ROMACONS drilling at a marine structure in Portus Cosanus, Tuscany, 2003. Drilling is by permission of the Soprintendenza Archeologia per la Toscana. Credit: J.P. Oleson

Around A.D. 79, Roman author Pliny the Elder wrote in his Naturalis Historia that concrete structures in harbors, exposed to the constant assault of the saltwater waves, become "a single stone mass, impregnable to the waves and every day stronger."

He wasn't exaggerating. While modern marine concrete structures crumble within decades, 2,000-year-old Roman piers and breakwaters endure to this day, and are stronger now than when they were first constructed. University of Utah geologist Marie Jackson studies the minerals and microscale structures of Roman concrete as she would a volcanic rock. She and her colleagues have found that seawater filtering through the concrete leads to the growth of interlocking minerals that lend the concrete added cohesion. The results are published today in American Mineralogist.

Roman concrete vs. Portland cement

Romans made concrete by mixing volcanic ash with lime and seawater to make a mortar, and then incorporating into that mortar chunks of volcanic rock, the "aggregate" in the concrete. The combination of ash, water, and quicklime produces what is called a pozzolanic reaction, named after the city of Pozzuoli in the Bay of Naples. The Romans may have gotten the idea for this mixture from naturally cemented volcanic ash deposits called tuff that are common in the area, as Pliny described.

The conglomerate-like concrete was used in many architectural structures, including the Pantheon and Trajan's Markets in Rome. Massive marine structures protected harbors from the open sea and served as extensive anchorages for ships and warehouses.

Modern Portland cement concrete also uses rock aggregate, but with an important difference: the sand and gravel particles are intended to be inert. Any reaction with the cement paste could form gels that expand and crack the concrete.

"This alkali-silica reaction occurs throughout the world and it's one of the main causes of destruction of Portland cement concrete structures," Jackson says.

A video abstract of the remarkable properties of Roman concrete. Credit: University of Utah

Rediscovering Roman concrete

Jackson's interest in Roman concrete began with a sabbatical year in Rome. She first studied tuffs and then investigated volcanic ash deposits, soon becoming fascinated with their roles in producing the remarkable durability of Roman concrete.

Along with colleagues, Jackson began studying the factors that made architectural concrete in Rome so resilient. One factor, she says, is that the mineral intergrowths between the aggregate and the mortar prevent cracks from lengthening, while the surfaces of nonreactive aggregates in Portland cement only help cracks propagate farther.

In another study of drill cores of Roman harbor concrete collected by the ROMACONS project in 2002-2009, Jackson and colleagues found an exceptionally rare mineral, aluminous tobermorite (Al-tobermorite) in the marine mortar. The mineral crystals formed in lime particles through pozzolanic reaction at somewhat elevated temperatures. The presence of Al-tobermorite surprised Jackson. "It's very difficult to make," she says of the mineral. Synthesizing it in the laboratory requires high temperatures and results in only small quantities.

For the new study, Jackson and other researchers returned to the ROMACONS drill cores, examining them with a variety of methods, including microdiffraction and microfluorescence analyses at the Advanced Light Source beamline 12.3.2 at Lawrence Berkeley National Laboratory. They found that Al-tobermorite and a related zeolite mineral, phillipsite, formed in pumice particles and pores in the cementing matrix. From previous work, the team knew that the pozzolanic curing process of Roman concrete was short-lived. Something else must have caused the minerals to grow at low temperature long after the concrete had hardened. "No one has produced tobermorite at 20 degrees Celsius," she says. "Oh—except the Romans!"

"As geologists, we know that rocks change," Jackson says. "Change is a constant for earth materials. So how does change influence the durability of Roman structures?"

The team concluded that when seawater percolated through the concrete in breakwaters and in piers, it dissolved components of the volcanic ash and allowed new minerals to grow from the highly alkaline leached fluids, particularly Al-tobermorite and phillipsite. This Al-tobermorite has silica-rich compositions, similar to crystals that form in volcanic rocks. The crystals have platy shapes that reinforce the cementing matrix. The interlocking plates increase the concrete's resistance to brittle fracture.

This microscopic image shows the lumpy calcium-aluminum-silicate-hydrate (C-A-S-H) binder material that forms when volcanic ash, lime, and seawater mix. Platy crystals of Al-tobermorite have grown amongst the C-A-S-H cementing matrix. Credit: Marie Jackson.

Jackson says that this corrosion-like process would normally be a bad thing for modern materials. "We're looking at a system that's contrary to everything one would not want in cement-based concrete," she says. "We're looking at a system that thrives in open chemical exchange with seawater."

Modern Roman concrete

Given the durability advantages of Roman concrete, why isn't it used more often, particularly since manufacturing of Portland cement produces substantial carbon dioxide emissions?

"The recipe was completely lost," Jackson says. She has extensively studied ancient Roman texts, but hasn't yet uncovered the precise methods for mixing the marine mortar, to fully recreate the concrete.

"Romans were fortunate in the type of rock they had to work with," she says. "They observed that volcanic ash grew cements to produce the tuff. We don't have those rocks in a lot of the world, so there would have to be substitutions made."

She is now working with geological engineer Tom Adams to develop a replacement recipe, however, using materials from the western U.S. The seawater in her experiments comes from the Berkeley, California, marina, collected by Jackson herself.

Roman concrete takes time to develop strength from seawater, and features less compressive strength than typical Portland cement. For those reasons, it's unlikely that Roman concrete could become widespread, but could be useful in particular contexts.

Jackson recently weighed in on a proposed tidal lagoon to be built in Swansea, United Kingdom, to harness tidal power. The lagoon, she says, would need to operate for 120 years to recoup the costs incurred to build it. "You can imagine that, with the way we build now, it would be a mass of corroding steel by that time." A Roman concrete prototype, on the other hand, could remain intact for centuries.

Jackson says that while researchers have answered many questions about the mortar of the concrete, the long-term chemical reactions in the aggregate materials remain unexplored. She intends to continue the work of Pliny and other Roman scholars who worked assiduously to discover the secrets of their concrete. "The Romans were concerned with this," Jackson says. "If we're going to build in the sea, we should be concerned with it too."

Watch the video: Ancient discoveries: Ancient Roman concrete recipe found; Earliest human remains - Compilation (August 2022).