Amongst the myriad secrets lost to the ancient world was the formula for Roman concrete: uncannily more robust than today’s mixtures, there are ancient Roman seawalls and harbor piers, built two millennia ago, that are still standing today, whereas modern concrete would require maintenance every few decades under the same aquatic conditions. But now, a new study may have uncovered how the ancients made their long-lasting concrete, suggesting that the secret of this forgotten formula may not truly be set in stone.

Members of The Roman Maritime Concrete Study has been studying the chemical composition of samples of concrete from ancient aquatic structures, such as the now-submerged breakwaters at Caesarea built by King Herod between 22 and 10 BCE. Unlike modern mixtures, Roman concrete not only can withstand the corrosive effects of seawater, but it also appears to be hardened by exposure to it.

The problem in trying to back-engineer a substance such as concrete is that cement hardens and sets due to a chemical reaction that turns the mixture from a lime-based goop into a rock-like structure — meaning that the chemical composition of what’s being studied is not the same as what was originally poured by the ancient construction teams.

Analyzing the microscopic structure of the samples unveiled the presence of aluminous tobermorite crystals that were growing out of another mineral, called phillipsite. The crystals, with their microscopic structures lending additional strength to the overall material, are the products of a chemical reaction between the phillipsite and the seawater that the concrete was cast into — meaning that the briny environment that would normally be eroding the structure was, quite literally, strengthening it instead.

Replicating this process on a large scale in the same manner that the ancients did would be problematic, however: the phillipsite-rich volcanic ash the Romans used in their mixture came from a specific quarry in Italy, a site that would be unable to provide enough material for the world’s construction needs. However, Roman concrete expert Marie Jackson, with the University of Utah, is currently researching alternate ways to produce aluminous tobermorite crystals within concrete mixtures.

While this discovery might appear to only be useful for the improvement of the construction of modern marine structures, there may be a dire need for such robust materials in the near future, as rising sea levels threaten coastal populations, necessitating the construction of expansive seawalls. Modern concrete requires the addition of steel reinforcement to lend tensile strength, but the structures built out of ancient Rome’s mixture didn’t use steel, and is still standing today — a robustness that will be required if future seawalls that would need to last for generations to be effective. 

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