In the heart of Rome stands one of the Eternal City’s most famous and well-preserved ancient monuments: the Pantheon. Constructed during the reign of Emperor Hadrian in the Second Century C.E, the building has been in near-continuous use for two millennia, first as a temple dedicated to the Olympian Gods, and then as a Catholic Basilica. Home to exquisitely preserved Roman architecture and the tombs of such luminaries as the artist Raphael and King Vittorio Emanuele II, the Pantheon has inspired the design of buildings around the world and remains one of Rome’s most popular tourist attractions, visited by some 6 million people each year. But perhaps its most celebrated feature is its 43-metre coffered dome, crowned by a 9 metre circular oculus or opening that floods the rotunda below with light. Incredibly, this impressive structure is built not, as one might expect, of marble or brick, but rather of concrete. Even more impressively, it contains not one piece of rebar or other structural reinforcement, making it the largest unreinforced concrete dome in the world. Indeed, the Romans were masters of concrete construction, building all sorts of massive structures – from harbours and aqueducts to the famous Coliseum – from this versatile material. This fact may shock and confuse those of us living in the modern world, where one bad winter is often enough to turn concrete roads and bridges into something resembling the surface of the moon. What allowed Roman concrete to last millennia, while modern concrete crumbles after just a few years?

Roman concrete is known to have reached widespread use around 150 B.C., though most archaeologists believe it was first developed nearly a century earlier. Like its modern counterpart, Roman concrete has two basic components: cement, a powder that sets hard when mixed with water; and aggregate, a mixture of gravel and other small, hard particles that adds strength to the mixture. Modern cement, known as Ordinary Portland Cement or OPC, is made by heating limestone – as well as various other ingredients including clay, iron ore, sand, gypsum, and coal ash – in a kiln to produce a lumpy material called clinker. The clinker is then ground down to produce cement powder. The composition of Roman concrete was broadly similar, consisting largely of calcium oxide or quicklime, made by heating limestone in a kiln. This was then mixed with pulvis or pozzolana, a fine, glassy volcanic ash found mainly around the Bay of Naples. Pozzolana was considered the “secret sauce” of Roman concrete – so much so that hundreds of thousands of tons were shipped across the Roman Empire as far as Alexandria, Egypt, for use in construction projects. There were, however, some significant differences between Roman and modern concrete. For example, while modern concrete uses aggregate mainly composed of sand and pea-sized gravel, Roman aggregate or caementa consisted of larger, fist-sized chunks of volcanic stone called tephra and pieces of brick and other recycled building materials.

Like modern concrete, Roman concrete hardened not by drying but via the hydration and crystallization of the Calcium-Aluminium-Silicate compounds – also known as strätlingite – formed from the reaction of quicklime and pozzolana, and could thus set in the absence of oxygen – even underwater. This allowed the Romans to build concrete harbour piers, breakwaters, and other underwater structures without the need to build dry cofferdams. The earliest example of such hydraulic cement being used is in the construction of the harbour at Baeie near Naples in the late 2nd Century B.C.E, while its largest-scale application was the harbour of Caesarea Maritima in modern-day Israel, whose concrete piers are still intact to this day despite being exposed to the Mediterranean Sea for two millennia. This remarkable longevity baffled architects and materials scientists for decades, for while the long-term survival of surface structures like aqueducts is impressive enough, immersion in seawater is ordinarily lethal to concrete, causing it to crumble within decades. Yet not only have the piers at Caesarea Maritima survived, they have actually gotten stronger over time. Even more baffling, in terms of sheer compressive strength Roman concrete is actually ten times weaker than modern OPC-based concrete, yet somehow many times more resilient.

So what is going on here? To uncover the secret to Roman concrete’s remarkable resilience, in 2013 a team of researchers led by Paulo Monteiro, a professor of civil and environmental engineering from UC Berkeley, analyzed samples of concrete from the piers at Caesarea Maritima. To their surprise, they discovered crystals of a rare calcium silicate hydrate mineral called tobermorite. These crystals, they deduced, were formed via the reaction of the mineral phillipsite in the pozzolana ash with seawater that had percolated into the concrete. Not only is tobermorite harder than any of the concrete’s original constituents, but its formation traps molecules like chlorides and sulphates that ordinarily damage concrete – meaning, amazingly, that immersion in seawater actually makes Roman concrete grow stronger and more durable with time. As well as potentially improving the performance of modern concrete, this discovery may have wider applications in industry as aluminous tobermorite is normally requires high temperatures and large amounts of energy to synthesize.

Another key to Roman concrete’s durability lies in one of its most historically baffling features: the presence of brittle white inclusions called lime clasts throughout its structure. For decades, archaeologists attributed these inclusions to poor-quality raw materials or sloppy mixing practices. However, some experts like Admir Masic, professor of civil and environmental engineering at MIT, were skeptical of this explanation:

“The idea that the presence of these lime clasts was simply attributed to low quality control always bothered me. If the Romans put so much effort into making an outstanding construction material, following all of the detailed recipes that had been optimized over the course of many centuries, why would they put so little effort into ensuring the production of a well-mixed final product? There has to be more to this story.”

Indeed, in his Ten Books on Architecture, written in 25 B.C.E, the great Roman architect Vitruvius laid out extremely precise recipes for concrete, while Emperor Augustus, who reigned from 27 B.C.E to 14 C.E, spearheaded major construction and restoration projects in Rome that resulted in the widespread systematization and standardization of concrete manufacture.

In 2022, a team led by Masic conducted spectroscopic analyses of concrete samples from the mausoleum of Roman noblewoman Caecilia Matella, built between 30 and 10 B.C.E. These analyses revealed that the lime clasts are composed of various types of calcium carbonate and oxide, which are much more brittle than the surrounding cement matrix. Whenever the concrete shifts – either due to the ground settling or the earthquakes that commonly plague the region – this brittleness causes cracks to form preferentially within the clasts. However, rain or groundwater percolating through the concrete reacts with the clasts, promoting the growth of calcium carbonate crystals that eventually fill and reinforce the cracks. This reaction effectively makes the concrete self-healing, filling small cracks before they can grow into larger, more dangerous fractures. To test this theory, Masic cast and cracked two samples of modern and Roman concrete and ran a stream of water through them. As predicted, within two weeks the cracks in the Roman concrete had sealed themselves, preventing the water from flowing through.

Masic’s analysis also revealed that the lime clasts could only have formed at high temperatures, indicating that the Romans mixed their concrete very differently than modern builders. Today, quicklime is mixed with water, producing calcium hydroxide or slaked lime – before being added to the concrete mixture. The Romans, however, added quicklime directly to the mixture, a technique known as hot mixing. Not only does hot mixing produce the high temperatures needed to form the self-healing lime clasts, but it also reduces overall curing time, allowing for faster construction.

But clever chemistry isn’t the whole story, and the final key to the remarkable longevity of Roman concrete lies in the specific manner it was cast. According to Vitruvius, unlike modern concrete, which is mixed “wet” and poured, Roman concrete was mixed “dry” with as little water as possible, with the resulting putty-like mixture being troweled into place. Large pieces of aggregate like volcanic tephra and brick or building stone fragments were then laid atop the concrete before another layer of concrete was troweled on top. The whole layup was then tamped down using special tools. By tightly compacting the mixture and reducing its water content, Roman builders minimized the formation of voids within the structure – the main source of weakness in concrete – and promoted the formation of tough strätlingite crystals. Interestingly, the advantages of this construction method were confirmed by a much more recent construction project: the building of the Upper Stillwater Dam in easter Utah in 1987. The concrete used in the dam’s construction was composed of 40% OPC and 60% coal ash, closely replicating the lime-pozzolana chemistry of Roman concrete. This mixture was mixed with minimal water to form a “no-slump” concrete, which was spread in layers and compacted into place using giant vibrating rollers. Later analysis of the dam structure has revealed many of the same self-healing properties exhibited by Roman concrete.

These various chemical, mixing, and layup tricks combined to create what Philip Brune, a research scientist at DuPont Chemical, calls:

“…an extraordinarily rich material in terms of scientific possibility…the most durable building material in human history, and I say that as an engineer not prone to hyperbole.”

Indeed, scientists and engineers around the world are actively studying the secrets of this extraordinary ancient building material in order to improve the efficiency and durability of modern concrete – an urgent task given our current climate crisis. While concrete is an extraordinarily versatile material, with 19 billion tons being used every year worldwide, it has an equally outsized impact on the environment, being responsible for up to 8% of global CO2 emissions. Most of these emissions come from the production of clinker, which requires temperatures of 1,450 degrees Celsius and consumes large amounts of fossil fuels. The use of motor vehicles to transport, pour, and repair concrete also contributes to this carbon footprint – a problem made all the worse by the low durability of modern concrete. Roman concrete, by contrast, is made from ingredients fired at much lower temperatures – thus requiring less fuel – and lasts much longer, greatly reducing the frequency with which it must be replaced. And while Roman concrete’s “secret ingredient”, pozzolana ash, is relatively rare, studies have shown it can be replaced with readily-available coal ash with little reduction in performance. Indeed, Roman-style concrete made with fly ash could cost up to 60% less to produce than regular OPC-based concrete, providing a powerful incentive for builders to switch over.

Thus, with any luck, this wonder material from two millennia ago may very well help ensure that our current civilization will last another two millennia. But then again, what have the Romans ever done for us?