In 2017 a team led by Marie Jackson (University of Utah) published in American Mineralogist an analysis of cores drilled from the Roman harbour piers of Caesarea Maritima and Portus: structures that had been submerged in seawater for nearly two thousand years, the most aggressive condition possible for a cementitious binder. The finding isn't that Roman concrete simply "held up" — it's that it grew stronger over time, not weaker. The key is a mineral that almost never forms in modern concrete: aluminous tobermorite.
It was not "poured": it was laid in layers
A first difference, often overlooked, concerns how the material was placed. Modern concrete is a fluid mix poured into formwork and vibrated. Roman opus caementicium was instead built in layers: a bed of lime and pozzolana mortar was spread, fist-sized pieces of stone or brick, the caementa, were placed on it by hand, and the whole was tamped with rammers to compact it before moving on to the next layer. In walls, this core was held between two faces of brick or small stone blocks acting as permanent formwork. The mix contained very little water, and tamping removed most of the voids: two conditions that are still among the most important factors in the durability of any concrete today.
The choice of aggregates was deliberate too. In the Pantheon's dome the builders used increasingly light materials as they rose: pieces of travertine and tuff in the lower parts, then crushed brick, and towards the top pumice and volcanic scoria, so as to reduce weight precisely where the dome becomes thinner and more delicate. Vitruvius, in the second book of his treatise, describes with admiration the "powder" of Pozzuoli, capable of hardening even under water, and this property explains why the Romans shipped it to build distant harbours, such as that of Caesarea Maritima on the coast of present-day Israel.
For works at sea the Romans developed a specific technique. At Caesarea, the harbour built by Herod the Great at the end of the 1st century BC, large timber forms were lowered onto the seabed and filled directly under water with lime and pozzolana mortar and rubble: the mix set while submerged, without the need to drain the area. In the early 2000s a group of archaeologists and engineers reconstructed a block of this kind in the harbour of Brindisi, using materials and methods as close as possible to the ancient ones, precisely to study how the concrete behaves in its first years under water.
The chemistry: pozzolana versus clinker
Modern Portland concrete relies on the hydration of clinker — calcium silicates fired at 1,450°C — which produces calcium silicate hydrate (C-S-H), a phase that's relatively stable but chemically "closed": once it forms, it no longer evolves significantly. Roman opus caementicium follows a different path. The binder isn't clinker but lime mixed with pozzolana, a volcanic ash rich in reactive silica and alumina: for marine works the ash from the Bay of Pozzuoli, in the Phlegraean Fields, which gave it its name; for buildings in Rome, the ashes of the Alban Hills volcanoes, a few kilometres from the city. The pozzolanic reaction between lime and pozzolana is slower than Portland setting — it takes weeks, not hours — but it produces crystalline phases that keep transforming for centuries.
When seawater seeps into microcracks in Roman concrete, it doesn't degrade it: it triggers the growth of aluminous tobermorite and phillipsite crystals directly inside the fissures, reinforcing the matrix instead of corroding it. It's a form of passive chemical self-repair that requires no human intervention. In modern Portland concrete, seawater does the exact opposite: it carries chlorides that attack the steel reinforcement, the one weak point in an otherwise durable system.
In 2023 an MIT research team led by Admir Masic added a second mechanism. Analysing samples of Roman concrete, the researchers observed small white lumps of unmixed lime, long interpreted as a sign of careless workmanship. Their hypothesis is that the Romans mixed quicklime, at least in part, directly with the pozzolana and water, a technique called hot mixing, because the reaction of quicklime with water gives off heat. The resulting lumps would work as calcium reservoirs: when a crack reaches them and water seeps in, the calcium dissolves and recrystallises as calcium carbonate, sealing the crack. In laboratory tests, samples prepared this way closed cracks within a few weeks.
These results should be read with caution. The samples studied come from a limited number of buildings, conditions of two thousand years ago cannot be fully reproduced, and the Roman structures we see today are the ones that survived: many others collapsed or were demolished and do not appear in the statistics. But the overall picture is clear: the durability of Roman concrete does not depend on a lost secret, but on a combination of pozzolanic chemistry, an unreinforced material and massive structures working almost entirely in compression.
Why we no longer build this way
If Roman cement is so durable, why did it disappear for over a thousand years, and why don't we use it today? The answers are less romantic than the chemistry. First: high-quality natural pozzolanas are geographically localised resources, not available everywhere in the world at the same quality — Portland is standardisable and can be produced wherever there's limestone. Second: Roman opus caementicium has no tensile strength and can't be reinforced — it works for domes and piers in pure compression, not for the slabs, cantilevers or slender structures that reinforced concrete makes possible. Third: the slow setting times (weeks) are incompatible with industrial construction schedules. The Pantheon took years to be cast, layer upon layer, with aggregates chosen for decreasing density from the base to the top of the dome — a process about as far as you can get from casting a modern floor slab in a single day.
The point isn't "going backwards". It's that research into low-impact cements (see the in-depth piece on geopolymers and low-carbon cements) explicitly looks to Roman pozzolanic chemistry as a model: activated aluminosilicate binders that form durable phases without going through the high-temperature firing of clinker. The difference is that today the reactivity of pozzolana or its industrial substitutes (fly ash, blast-furnace slag) can be engineered, instead of depending on one specific volcanic deposit.
Part of the Roman lesson has in fact already entered everyday practice, even if few people know it. The European standard for common cements, EN 197-1, includes among others pozzolanic cements, designated CEM IV, in which part of the clinker is replaced by natural pozzolanas or fly ash, and blast-furnace cements, CEM III, in which the replacement is ground slag, a by-product of steelmaking. In Italy pozzolanic cements have long been widely used, precisely because volcanic pozzolanas are available. These cements harden more slowly than pure Portland, but they produce concretes more resistant to sulphate and seawater attack, and they have a lower carbon footprint because they contain less clinker.
For marine works, blast-furnace and pozzolanic cements are often the recommended choice for exactly these reasons. The price to pay is greater care on site: slow curing requires keeping the concrete moist for longer and allowing suitable striking times, especially in cold weather. It is, in the end, the same patience Roman concrete demanded, translated into the rules of an industrial building site.
The real comparison: like-for-like structures
An honest comparison means not comparing apples with oranges. The Pantheon doesn't have to withstand traffic load cycles, has no reinforcement to protect from corrosion, and its mass (dome thickness up to 6 metres at the base) is oversized by any modern economic standard. The reinforced-concrete structures that show signs of decay today after 50-60 years — viaducts, multi-storey car parks, coastal buildings — almost always fail for the same reason: insufficient concrete cover, too high a water-cement ratio, an exposure class underestimated at the design stage. It isn't that "modern cement is inherently less durable" — it's that it's built to much tighter safety and cost margins, and preventive maintenance is often put off. Reinforced concrete designed to the EN 1992-1-1 exposure classes with correct cover (see the in-depth piece on concrete pathologies and diagnostics) has a design service life of 100 years even in a marine environment — about a twentieth of the Pantheon's present age, but still twice the fifty-year design life of an ordinary building.
The rules for achieving durable reinforced concrete in a marine environment are now well codified. The EN 206 standard defines specific exposure classes, from XS1, for structures near the coast exposed to salt-laden air, to XS3, for tidal and splash zones, and for each it sets a maximum water/cement ratio, a minimum cement content and a minimum strength class. Eurocode 2 sets the cover, the thickness of concrete protecting the reinforcement, which in these classes normally exceeds four centimetres. The collapse of the Morandi bridge in Genoa in 2018 brought back to the centre of public debate a fact well known to engineers: corrosion of reinforcement, hidden inside the concrete, can progress for decades without obvious signs, and a structure's lifespan depends as much on inspection and maintenance as on design.
Where to see Roman concrete
The Pantheon remains the most impressive example: a dome about 43 metres across in unreinforced concrete, still intact and in use after almost nineteen centuries. But to understand how the material is made it is worth looking at ruins, where the concrete is exposed in section. At Trajan's Markets in Rome you can see the concrete cores enclosed between brick facings and the cross vaults; at Ostia Antica, in partly collapsed walls, the layers of mortar and caementa are clearly visible. In the Bay of Pozzuoli, the Underwater Archaeological Park of Baia makes it possible to observe, from a glass-bottomed boat or by diving, the remains of Roman villas and harbour structures now under water as a result of bradyseism: it is the best place to see the material described in this article in the environment for which it was designed.