Concrete, one of the dirtiest substances ever concocted, could help save the planet | Aleksei Zotov/UCG/Universal Images Group via Getty ImagesNo, Rome was not built in a day, but it was built at breathtaking speed, its temples, amphitheaters, and aqueducts rising with the help of a magical blend of volcanic ash and lime. Today, that miraculous mortar has a less than enchanting name: concrete.“Romans lived through a true architectural revolution, and at the foundation of this revolution, literally, was a concrete discovery,” said Admir Masic, a materials science professor at MIT who made waves a few years ago for unearthing the fast-hardening secret sauce for why Roman concrete is so durable. “They were able to take a pile of dirt and mix it together, add water, and turn it into solid rock of any arbitrary size. It literally changed everything.” And yet, for a millennia after the fall of Rome, the world largely forgot about concrete, until, in the 18th century, an English mason found that mixing limestone and clay could form a paste that hardened underwater. Another few decades later, concrete as we now know it was reborn, and once again, it changed everything. In the centuries since, concrete has come to reshape our way of life, with disastrous results for the climate. What early concrete evangelists didn’t realize was that the modern way of producing the substance released giant amounts of carbon dioxide into the atmosphere. As it has become increasingly central to modernity, it has helped fuel environmental catastrophe.Today, however, a new generation of concretophiles like Masic have pored over the poured mixture in search of another miracle, a greener, more durable kind of concrete. And they’ve found multiple answers, from concrete that can heal its own cracks — as the Pantheon has for millennia — to electroconductive concrete capable of storing energy to a blend that allows concrete to soak up more CO2 than it emits. Together, they suggest that in a not-so-distant future, concrete — yes, concrete — one of the dirtiest substances ever concocted, could help save the planet, transforming our built environment while actually intervening in the climate for the better. How concrete built (and polluted) the modern worldIt’s no wonder that luminaries of the neo-concrete age, Thomas Edison among them, marveled at the stuff just as their Roman predecessors had. They saw it as a harbinger of utopia, of a world in which anything and everything — a home, an interstate highway, a water pipe, a subway system, or a grand piano — could be molded and poured for a fraction of ordinary costs. “As to concrete dogs to stand warningly in the front yard and concrete cats to purr stonily under a concrete kitchen range,” the New York Times teased Edison in 1911 when he described his plans for a concrete home finished with concrete furnishings, “he made no announcement.”The beguiled inventor’s utopian vision of a world remade by concrete would come true in more ways than he realized. Yes, it changed the way we build forever. Muddy roads and rotting wooden sidewalks gave way to smooth gray pavement, and newly cast sewer systems saved millions of lives in the deathly dirty cities of the early industrial era. Concrete, when poured over rods of steel, proved strong enough to prop up colossal skyscrapers that Julius Caesar could only have dreamed of. But there was a trade-off: Every time Edison fired up his trademark 150-foot kilns, scorching the limestone inside into blistering pebbles of cement clinker, he released hundreds of tons of carbon dioxide into the atmosphere. Yankee Stadium was built not by Babe Ruth’s swing alone, but with around 30,000 cubic yards of Edison concrete, whose creation released at least 5,400 tons of CO2, the equivalent of driving a gas-powered car 12.5 million miles, or to the moon and back more than two dozen times. But this is nothing compared with the roughly 30 billion tons of concrete the world now uses each year, enough to have turned the gluey gray goo into the second-most used material on Earth. (The first is water. Glug glug.) The Anthropocene has been built atop a foundation of concrete, its weight eclipsing the biomass of all living things, and its annual carbon tab responsible for up to 8 percent of global emissions, the equivalent of the entire country of India. Critically, unlike the astonishingly enduring remnants of Rome, our modern concrete decays quickly, so it must be repoured time and time again, which only makes it more environmentally catastrophic. But it doesn’t have to be this way, not anymore. “The future of concrete is to think differently, to go away from claiming that cement is bad,” Masic told me. “Our goal is to turn this around and make concrete part of the solution instead of being a problem.”This is why we can have nice concretePer ton, concrete actually emits far less CO2 by volume than other materials like aluminum, steel, plastic, or glass. The problem is that we use so much of it, even a slightly negative footprint can quickly become enormous. But “the upside is,” said Sabbie Miller, a professor at UC Davis who focuses on sustainable concrete, “if you’re able to come up with some small change, then those small changes will also scale” in ways “that could have a huge effect on modern society and its burdens on the environment.” One of those solutions takes a cue from the Romans, who made concrete differently than we do today. Their once-mysterious method ensured that hulking structures like the Colosseum or the Aqua Virgo, an aqueduct built during Augustus’ reign and still in use today, could survive for millennia while New York City’s Brooklyn-Queens Expressway or the Washington Bridge crumble after just a few decades of use. It’s not just that we drive cars over our concrete freeways while the Colosseum has been mostly free of even the bulkiest chariot wheels for millennia. The answer, according to Masic’s research, lies in the Romans’ ingenious recipe for cement, the glue that binds concrete together. When the ancients brewed a batch of cement, they left big white chunks of dormant lime in the final mixture. Later, if a crack formed, any water would react with the idle lime, filling the fissure with new stone. Our modern concrete has no such lime clasts, meaning that if water seeps into a crack, it degrades the structure from within, rusting the rebar that made our big towers possible, leading to a grim condition known as “concrete cancer.” Masic has recently turned his research into a startup called DMAT that reserves unreacted lime in concrete, giving it self-healing properties he says could extend a structure’s lifespan by about 50 percent, which in and of itself would reduce its carbon footprint by 40 percent. If the construction industry normalized such concrete or other mixtures that emit, say, half as much CO2 as usual, it would keep hundreds of millions of tons of it out of the atmosphere each year. But why stop there? Some startups have gotten much better in recent years at producing concrete capable of quite literally breathing in more CO2 than it emits in a process that takes advantage of concrete’s natural porousness. It could, in theory, turn our cities into concrete rainforests by turning the material into a storage reservoir. As Miller put it, “what if we can use our concrete to undo some of our environmental damages that we’ve already caused?” In the Bahamas, the startup Partanna has already built what it’s called the world’s first carbon net-negative home, an unassuming 1,250-square-foot abode that the company claims will sequester 182.6 tons of CO2 in its lifetime, the equivalent of hundreds of trees. Meanwhile, Masic’s lab has been piloting ways to turn concrete into a giant battery, capable of storing solar power during the day and releasing it at night, a green innovation that could cancel out its own carbon footprint within a few years of use. Masic believes that electroconductivity could be key to catalyzing a new era of concrete — more so even than his neo-Roman ventures. “It might hold the secret,” he said, “for turning a bad guy into something amazing.” But as promising as these innovations are, it will take time for them to ripple out from model homes and into the concrete that actually surrounds us. American concrete startups have lost billions of dollars in federal funding and expected procurement contracts under the Trump administration. Some states, most notably California, are helping to pick up the slack, but investment still lags behind potential. There’s also the challenge of convincing builders running on paper thin margins that their old ways of doing things might fit better in a new mould. But concrete’s staggering scale means that we can’t afford to keep pouring it without considering its ugly trade-offs. More importantly, perhaps, we can’t afford to miss out on the opportunity that such a mammoth, transformative material presents to change everything yet again.