A Step Closer to Finding the Recipe for Ancient Rome’s Rock-Solid Super-Concrete

A Step Closer to Finding the Recipe for Ancient Rome’s Rock-Solid Super-Concrete

New studies of ancient concrete could teach us more about the amazing techniques of ancient Roman engineering and the secrets behind the incredible longevity of many of their concrete harbor structures.

The Secrets of Ancient Roman Concrete’s Incredible Longevity

When famed Roman author Pliny the Elder described 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 clearly didn’t lie or exaggerate. Battered by sea waves for more than two thousand years, these structures are still around today while our modern buildings erode within only a few decades.

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

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

Science Alert recently reported that scientists have discovered the amazing chemistry behind this phenomenon, getting closer to revealing its long-lost recipe. From what they have noticed so far, it seems like not only was Roman concrete more durable than what we can produce today, but amazingly it gets even stronger with the passage of years.

The team of researchers led by geologist Marie Jackson from the University of Utah has been examining closely the mysteries of Roman concrete for years, and finally now they have begun mapping its crystalline structure and figuring out in detail how this ancient material becomes even more durable over time.

Modern Concrete vs Ancient Roman Concrete

Modern concrete is usually produced by portland cement, a mixture of silica sand, limestone, clay, chalk and other ingredients blended and melted together at extremely high temperatures. However, ancient Romans didn’t create their own concrete this way. They produced it from volcanic ash, lime and seawater, taking advantage of a chemical reaction they had possibly observed in naturally cemented volcanic ash deposits called tuff rocks. Blended 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 the modern mix could ever be.

A magnified piece of Roman concrete consisting of lime, volcanic sand, and rock

A magnified piece of Roman concrete consisting of lime, volcanic sand, and rock (Wikimedia Commons)

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.

Caesarea Roman Concrete bath ruins

Caesarea Roman Concrete bath ruins (CC BY SA 3.0)

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.

Could We Replicate Roman Concrete?

So, the question Jackson and her team are called to answer is if we can produce concrete today the way Romans did and how this could benefit the modern building industry.  Theoretically, we could but the sad truth is that the recipes have been lost throughout the centuries. Realistically, our only chance to recreate the marvelous ancient material is to reverse-engineer it based on what we know about its chemical properties. However, that wouldn’t be easy at all as Jackson informs us, "Romans were fortunate in the type of rock they had to work with. We don't have those rocks in a lot of the world, so there would have to be substitutions made," she says as Science Alert reports, leaving a small window of hope open for the future.

An advanced concrete recipe allowed the Romans to construct magnificent structures that no builder would dare to attempt today. Source: BigStockPhoto

An advanced concrete recipe allowed the Romans to construct magnificent structures that no builder would dare to attempt today. Source: BigStockPhoto

It would seem to us that this creation occurred by chance but the question is open as to whether the Romans worked out their long-lasting concrete recipe by design. Either way, it is a rock-solid reminder that there is still some ancient wisdom that is so far not surpassed by the modern world.

Top image: Interior of the Roman Pantheon dome, still the largest (43.4m/142.4 ft dia.) unreinforced solid concrete dome. (CC BY NC SA 2.0)

By Theodoros Karasavvas

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