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This article is part of Fortune‘s Blueprint for a climate breakthrough package, guest edited by Bill Gates.

In Emperor Trajan’s Rome, every week was “Infrastructure Week.” During his prosperous reign, from AD 98–117, Trajan commissioned massive public works projects: new bridges and roads, aqueducts, public baths, a vast maritime port on the Ostia coast just west of the city of Rome. (Trajan’s predecessor, Augustus, was no slouch either.)

If you’ve ever visited Italy, you’ve no doubt gawked at these long-standing feats of engineering.

One, in the center of Rome, has fascinated historians, engineers, and geologists for years: Trajan’s Market. Sitting across the street from the Roman Forum, the ancient marketplace is a grand, sturdy structure befitting any capital city with the ambitions of a global trading power. You enter through a vaulted concrete corridor built 1,900 years ago. Even if you’re no fan of concrete architecture, the edifice is still worth appreciating for its groundbreaking design and durability. It’s survived earthquakes, smog, marauding hordes, and throngs of tourists. And there’s not a bit of metal reinforcement holding it up.

Climate Package 2021-Ancient Roman Concrete-Trajan's Market — Rome’s Trajan Market was built with Roman concrete, which can last centuries or even millennia.

Even more impressive are the concrete ports of that epoch, says Marie D. Jackson, a professor in the geology and geophysics department at the University of Utah. They’re still remarkably intact. Compare that with many of the crumbling waterfront projects of the 20th century.

“If you’ve ever walked along the Embarcadero of the San Francisco waterfront, those are concrete [seawalls] made in the 1950s, and you just see huge blocks that are falling off, and there’s steel reinforcement bleeding out into the concrete,” she observes, adding to the chorus of impact studies that have made a similar lament over the past decade.

Climate Package 2021-Ancient Roman Concrete-Embarcadero — The Embarcadero in San Francisco, constructed in the 1950s, is already decaying.

The structures of the ancient Roman empire drew Jackson to Italy in the 1990s to study why they have withstood the test of time while our modern concrete architecture begins to chip away after a few decades. (“Cement” and “concrete” are terms often used interchangeably, but on a building site there’s a difference: Concrete is the final product made from cement.)

In 2013, Jackson was part of an international team of researchers that reverse engineered the original formula for super-stable Roman concrete. (Spoiler: It involves a mix of lime and local volcanic rock.) The discovery rocked the geeky world of cementitious materials science, and now it’s exciting a new generation of builders who’ve been looking for a greener substitute for the now-ubiquitous version, Portland cement.

What came after the Romans

Developed in 19th-century England, Portland cement—so named by inventor Joseph Aspdin for its resemblance to the limestone found on the Isle of Portland that juts into the English Channel—has become the go-to building block of the world’s cities and road networks. Good old Portland cement is cheap and versatile, but its life span is measured in mere decades; upkeep is a constant headache. And making the stuff is brutal on the environment. Portland cement is produced in kilns that run at extremely high temperatures. And the cooking process creates a chemical reaction called calcination that emits heavy greenhouse gas emissions. All told, roughly 8% of global carbon emissions come from the production of Portland cement.

Roman concrete, on the other hand, has a track record of lasting centuries or even millennia—notwithstanding the occasional barbarian invasion. It’s lighter and, depending on the application, requires little to no steel reinforcement. The production of Roman concrete, meanwhile, utilizes far less energy, generating a carbon footprint that’s a fraction of that of Portland cement.

Thanks to a small team of scientists like Jackson, ancient Roman concrete is on the map again.

Following the publication of Jackson’s work in various scientific journals, the U.S. Department of Energy took notice. In 2019, the agency awarded Jackson and her research team a $1.4 million ARPA-E grant to develop a modern-day version of the same material that built the Roman empire—and bring it to the marketplace.

“We have to innovate to find a replacement for that material that is both low-cost and environmentally friendly,” she tells Fortune. “And that’s what we’re doing.”

Jackson’s team partnered with the DOE R&D lab, the Savannah River National Laboratory, and with a consortium of construction industry players, including GlassWRX, a South Carolina–based materials science startup.

“Take Rome to the masses”

There’s an obvious problem with Roman concrete. It’s a formulation rich in the local volcanic stone, a variety of rock called tuff, found on the Italian peninsula and few other places on the planet. Ancient Romans made concrete by mixing lime and the indigenous volcanic tuff, packing the blend into wooden forms. They also relied on Mother Nature to help fortify the final product. For example, the engineers of the time understood that seawater, coming into contact with the raw cement blend, would instantly trigger “a hot chemical reaction. The lime was hydrated—incorporating water molecules into its structure—and reacted with the ash to cement the whole mixture together,” producing super-sturdy building blocks, Jackson’s team explained in its 2013 paper.

For today’s builders, there’s simply not enough of the raw materials of Roman concrete to satisfy the world’s building needs, particularly in a rapidly urbanizing planet. (As Bill Gates wrote on his blog in 2019, “the world will add 2 trillion square feet of buildings by 2060—the equivalent of putting up another New York City every month for the next 40 years.”)

Jackson’s development team knew that if they couldn’t mine the material out of the Italian countryside, they’d have to develop a substitute closer to home that’s just as good. “We started thinking, ‘Hey, let’s take Rome to the masses, rather than the masses to Rome,’” says Thomas Adams, the lead engineer, and Jackson’s partner on the Roman concrete project. “By the time this project is over, we expect to have at least one, and hopefully more active demonstration projects, where we have gone through and reinvented Roman concrete outside of Rome.”

That’s where GlassWRX comes in. Working with Jackson, the firm has developed a building material with many of the same strength and durability attributes of ancient Roman concrete. It’s made in part from recycled glass.

The technical term for this GlassWRX material is “engineered cellular magmatic,” or ECM. “With ECMs we can make better, longer-lasting, more eco-friendly concretes. We can create materials that are better at cleaning our polluted air and treating our water,” the company wrote in a recent presentation explaining the technology.

The first generation of ECM isn’t quite versatile enough to displace quick-dry Portland cement in most kinds of construction projects. “But once the world’s engineers get their hands on this, the hacker-factor thing comes into play. They’ll figure out ways to install this really, really quickly,” Adams says, adding that he can see early-use cases in bridge abutments, retaining walls, and seawalls.

The city of Beaufort, S.C., is looking at the technology for exactly that. Local officials are in discussions with GlassWRX to develop a waterfront construction project with this ECM. Jackson says it would be the first construction project since the ancient Roman era involving such a durable building material—one that’s expected to last for centuries and requires no cement or steel rebar supports. “We’ve seen nothing like this in modern times,” she says of the material.

The GlassWRX’s scientific team says the ECM will last, conservatively, for 200 years. Portland cement, particularly used in marine settings, lasts 25 to 30 years, experts say, before it starts to degrade.

That shelf life is what so intrigues the U.S. Department of Energy. With so much focus on rebuilding America’s crumbling infrastructure, and to do so in a low-carbon way, the race is on to find a greener, more durable substitute for Portland cement.

“Portland cement is sort of the building block of the industrial civilized world,” observes Phillip Galland, the CEO and cofounder of GlassWRX. “Now it’s time to move into smart cities, and smart societies that focus on sustainability, and resiliency to climate change. And that is where the new materials can come in and have a societal impact.”

Trajan, that empire builder, just might have competition.