The arrow of time flies one way, and with it comes decay.
We build our structures to last as long as possible, but even the toughest materials eventually crack, weaken, and crumble.
Ancient Roman concrete did things a little differently.
Scientists have long known that the concrete built during the Roman Empire seems to grow stronger over time.
Previous research suggested this extraordinary durability was largely due to a reaction between volcanic ash, called pozzolan, and quicklime, which produced exceptionally resilient minerals within the concrete.
Now, scientists have discovered that there's another part of the story: the slow yet steady reactions of carbon dioxide from the air.
"While the pozzolanic reaction is of fundamental importance," says engineer Paulo Monteiro of UC Berkeley, "our findings suggest that carbonation over a long period of time also enhances the durability of concrete and can help it seal cracks as it ages."
The team's discovery, detailed in Science Advances, gives us a new appreciation of even the more mundane Roman structures that were nevertheless imbued with engineering prowess.
One of the wondrous things about Roman engineering is how many structures remain in excellent condition where so many contemporaneous buildings have fallen to rubble.
The Pantheon in Rome is the most famous example – a 2,000-year-old temple capped with an enormous dome of unreinforced concrete, the largest structure of its kind in the world.
But to discover the secrets of Roman concrete, Monteiro, his co-lead Xiaohong Zhu of Beijing University of Technology, and their colleagues turned to an unlikely, much less glamorous source.
In the 2nd century CE, the emperor Hadrian had a villa at Tivoli in Italy, much of which is still – you guessed it – standing.
From there, the researchers extracted a small piece of concrete from a communal toilet that once supported imperial bottoms.
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Using a suite of high-resolution imaging techniques, the researchers examined their sample down to the nanoscale.
As expected, they found evidence of the pozzolanic reaction, in which volcanic ash and lime react to form exceptionally durable minerals within the concrete.
But they found something else, as well.
Over centuries, carbon dioxide from the atmosphere had reacted with leftover lime in the concrete to produce calcite – the same mineral that can be found in limestone.
This was no mere by-product of the aging process, the researchers found.
The calcite appears to have made the concrete stronger. It crystallized within tiny pores and cracks, making the concrete denser and gradually sealing weaknesses that would otherwise have spread over time.
Earlier studies had identified calcite in Roman concrete, but had not studied it in three dimensions or mapped its architecture.
The work, the researchers say, suggests that calcite may have played an overlooked role in Roman concrete's incredible longevity – not replacing the known contribution from the pozzolanic reaction, but working alongside it.
Scientists had already been working on reproducing Roman concrete.
Carbonation happens naturally in lime-based concrete whether you know it's there or not, but understanding the role it plays could give researchers another tool as they try to engineer concrete that lasts longer while producing less carbon.
"Understanding how calcium carbonate crystallization dynamics bind concrete together and contribute to its long-term durability could provide new insights into the long-term mineralogical evolution and natural carbonation of lime-based binders," Monteiro says.
Ancient Roman structures from the grand Pantheon to Hadrian's humble toilet give us mind-blowing examples of concrete that has remained structurally sound for millennia.
That doesn't mean we can simply build as the Romans did.
Modern buildings place far greater demands on their materials, and reinforced concrete faces a challenge Roman engineers never had to contend with: the corrosion of the steel rebar embedded inside it.
What the new findings can do is help researchers as they try to design longer-lasting, more sustainable concrete for the future.
Related: Roman-Era Wreck Reveals How Ships Were Built to Last 2,000 Years Ago
"This study shows how exploring ancient engineering techniques can lead to important revelations," Monteiro says.
"We hope that by unlocking Roman secrets for enhancing concrete durability, we can someday attain sustainable modern infrastructure development."
The research has been published in Science Advances.
This article was fact-checked by Rachel Garner and edited by Clare Watson. While we pride ourselves on our process, we are only human. If you spot a mistake, please let us know.