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Concrete Doesn’t Have to Be an Ecological Nightmare

Clockwise from upper left: the Pantheon in Rome; Boston City Hall; Saarinen’s TWA terminal at JFK airport; Zaha Hadid’s Sheikh Zayed Bridge in Dubai. Photo: Getty Images

Imagine your refrigerator is made of concrete, a two-ton slab buckling the kitchen floor — that’s your individual share of the world’s concrete production this year alone. Next year, you’ll be statistically responsible for another, slightly larger refrigerator-size block and another the year after that. Humanity has an insatiable thirst for that gray ooze of ground-up rock and water. But the same substance that grows the world’s cities is also heating the air and doing its part to bring on climate catastrophe. We are building our way to oblivion.

In theory, that combination of understanding and dread should result in a revolution. “It’s a golden moment for experimental materials in architecture because there’s an increasing recognition that things have to change and our systems no longer seem so inevitable,” says architect David Benjamin, a co-founder of the experimental firm the Living. Yet a host of nimble innovators and aspiring disruptors are smacking against the slow-moving juggernaut that is the worldwide construction industry. When it comes to designing a viaduct, a seawall, or an apartment tower, no responsible engineer (or building-code enforcer) will choose a promising invention over the old familiar products. When you’re ordering 100,000 tons of anything, you don’t take chances.

Concrete is ancient, ubiquitous, and still miraculous. A couple of thousand years ago, some resourceful Roman near the coastal village of Pozzuoli noticed that when ash from Vesuvius got wet, it turned into a thick paste. Mixed with lime, that volcanic powder was powerful enough to bind sand and crumbled rock, and that flowing goo hardened to near indestructibility, even underwater. Today, Pozzuoli, a lovely spot for a seaside lunch just outside Naples, lends its name to pozzolan, a crucial ingredient in cement.

Pourable, shapable, abundant, durable, versatile, reliable, fireproof, bombproof, earthquake-resistant, and cheap — what more could you ask of a building material? Add some steel reinforcement bars and concrete’s what the modern world is made of. It provides the backbone of skyscrapers and the brawn for dams but also the sensuous curves of Eero Saarinen’s TWA Terminal and Zaha Hadid’s Sheikh Zayed Bridge in Abu Dhabi. In megacities all over the world, the poor start building their homes by stacking concrete blocks, and rich penthouse-dwellers have their concrete floors burnished to a high gloss. Even architecture that looks fragile and improvised usually sits on a concrete slab. In 2001, the Burkina Faso–born German architect Francis Kéré returned to his native village and tweaked its traditional techniques of making mud bricks so that the new school he designed wouldn’t crumble in the rain; he had effectively shown the villagers how to mix concrete.

There’s a catch, of course, a fatal flaw that was there in the earliest concoctions. The Romans knew the binding properties of quicklime, but centuries later, its role in turning soil into structure had to be discovered all over again. In the early 19th century, Joseph Aspdin, a visionary bricklayer from Leeds, stirred together clay and crushed limestone in a pot that he heated on his kitchen stove. The mixture burned, the fumes stank, and the result was a fine white powder that, when diluted with water, petrified into what he called artificial stone. In a stroke of marketing genius, he called the stuff Portland cement, after the pure white Portland stone found in monuments and mansions all over Britain (including Buckingham Palace). The process took decades to refine and longer to become a global phenomenon, but the basic chemical facts remain. Limestone is laced with carbon, and heating it releases greenhouse gas — lots of it. Today, cooking one ton of cement yields nearly a ton of CO2, and concrete plants pump out nearly 8 percent of the world’s carbon emissions. Since an ever-growing population can’t wean itself from its primary building material, warding off climate change is going to require a cleaner way to make it.

The good news is that the technologies to do that may already exist, at least in the sense that cement existed in Aspdin’s kitchen. The challenge is to make the leap from tabletop potion to the scale of Hoover Dam.

Concrete may not be sexy, but it’s got its share of jaunty entrepreneurs, each with a different strategy. Brimstone Energy — a start-up headquartered in Silicon Valley, backed by venture capital, and founded by a couple of Caltech grads — is betting on a different kind of rock. The company envisions replacing limestone with a substance that is both carbon-free and freely available: calcium silicate. “The cheapest way to run a cement plant is to have it sit on top of the quarry. We need to sit on top of one of the most common rocks in the world,” says co-founder Cody Finke. He has a simple answer to the question of where his next-generation plants can be located: wherever there are rocks.

A fluent and persuasive salesman, Finke makes the transition to zero-carbon construction seem almost frictionless and inevitable, partly because the current supply chain is drying up. Until recently, the industry was finding economies in recycling fly ash and slag — waste products from coal plants, which are now growing scarce, and steel plants, which are concentrated in China. The rising cost of those raw materials, and the complexities of transporting them long distances, makes a new technology that much more appealing. Brimstone’s approach offers another elegant advantage, too. A concrete plant consumes massive doses of energy, and until the day when solar or wind power can nourish such a monster, it will need to feed on fossil fuels. But one byproduct of the Brimstone process, magnesium hydroxide (the chemical that settles your stomach when you pop some antacids), naturally absorbs about the same quantities of CO2 as the plant’s operations churn out. Voilà, carbon neutrality.

More poetically, the North Carolina–based company Biomason recruits an army of living creatures to produce a material famous for entombing nature. Some strains of bacteria secrete calcium carbonate; bribed with a nutritious diet of wastewater and agricultural castoffs, these microorganisms can be prodded into growing biological cement. Introducing a population of those industrious critters into a batch of traditional concrete can make columns and foundations self-healing, especially underwater. Billions of unpaid microscopic laborers spit out crack filler and maintain the structure’s integrity.

Biomason has promoted bacteria from maintenance staff to the creative team, fabricating this new cement from scratch. “This is the way to decarbonize the industry,” says founder Ginger Dosier. “We’re in the age of biology.” She’s not alone in that belief. The federal government’s Defense Advanced Research Projects Agency (DARPA) funded the company’s research into biocement for marine projects, and Biomason recently licensed its technology to a major Danish manufacturer, IBF, which plans to grow its own carbon-free cement in Europe. It’s a small company with a lot of muscle at its shoulders.

In a generation or two, tiny brainless organisms could be responsible for grinding out much of the built environment. As long ago as 2014, David Benjamin’s firm the Living installed a cluster of biodegradable towers in the courtyard of MoMA PS1. The bricks were made from mycelium — the mushroom threads that grow on rotting tree trunks — mixed with ground-up corn stalks. Since then, other versions of that process have yielded some fungal fashions, but it could eventually evolve into mass-produced bricks or substitutes for the concrete blocks used to fill in walls. “We’re probably just a couple of steps and a couple of years away from mycelium materials being used in architecture,” Benjamin says.

Optimism stokes an equal and opposite reaction: skepticism. But even an expert with a panoramic view of concrete science like MIT professor Franz-Josef Ulm can sound breathless and hard-nosed at the same time. Retracing two decades of history, Ulm notes that the strategies for decarbonization have changed. A couple of decades ago, it was tinkering with the ingredients of cement, then it was coming up with ways to bury carbon in concrete. Scientists have developed additives that made the finished product much denser and tougher, which should have cut the need but instead wound up increasing demand. Towers that might once have been designed with steel structures are now built with concrete columns and beams. Now he sees a different set of possibilities. “The big bang in the last three to five years is that you add another function,” Ulm says. Concrete is an effective insulator, but it can be coaxed into conducting electricity with the addition of a material called nanocarbon black. That means a pair of slabs with a positive charge on one side and a negative charge on the other and a divider between them can double as a massive capacitor, warehousing energy and releasing it as needed. A building’s structure acts as its own solar-powered battery. “This is not science fiction I’m talking about,” Ulm insists. “It’s available now.”

Even starry-eyed would-be Joseph Aspdins recognize that the path from almost-there to a new business-as-usual is strewn with stumbling blocks of regulation, building codes, habit, economics, and politics.

Still, they remain implacably upbeat, and they have data to support their sunniness. In the meantime, traditional concrete manufacturers have made progress in reducing carbon output, and various trade associations have committed to hitting carbon neutrality in the next 30 years. “The industry is finally open to innovation,” Ulm says. “Things are going pretty well.”

Concrete Doesn’t Have to Be an Ecological Nightmare