The world’s most common building material produces 5 percent of global CO2 emissions. But what if we needed less of it?
A few days before Christmas in 1824, Joseph Aspdin appeared before the King of England clutching a piece of paper. Beneath the title “Artificial Stone” was a one-paragraph recipe, peppered with technical terms like “argillacious” and “calcined.” Neither King George IV nor Aspdin—a bricklayer applying for a patent—could have known that this little paragraph was the foundation of the modern city. It contained the formula for Portland cement.
Aspdin's innovation, named after high-quality limestone from the Isle of Portland in England, is one of the great triumphs of modern engineering. It was simpler and hardened more quickly than Roman cement, and helped us build bigger and better bridges, skyscrapers, and factories. Unfortunately, it's also one of the greatest polluters of our time. That’s because when cement is mixed with sand and water, the result is the most-consumed material in the world—concrete.
To grasp the importance of cement, try imagining a world without concrete. It's almost as difficult as imagining a person without any bones. “We'd still be constructing our major buildings with brick and mortar,” says Robert Courland, author of the book Concrete Planet. Homes, hospitals, and schools would take far longer to build and repair. In the developing world, shelter would be scarcer and less reliable. “The whole world would look like the 19th century,” Courland says.
Cement production hasn't changed much since the days of Joseph Aspdin. Limestone and clay are still ground into powder, mixed with water, baked in a furnace, and ground into powder again. As Asdpin described in 1824, “this powder is to be mixed with a sufficient quantity of water,” resulting in a versatile paste. The main thing that’s changed in the last two centuries is the sheer quantity of cement we use. Every year, humans lay down three tons of concrete for every person on Earth. (At that rate, it would take just 100 years to lay a 1-inch layer across the entire surface of the moon.)
Here's the catch: “It's one of the dirtiest industrial processes out there,” says Courland. There are two reasons for this. First, when limestone is processed into cement, it emits the greenhouse gas CO2. Second, cement-producing furnaces typically burn fossil fuels, often coal, to achieve the blistering temperature of 2500 degrees Fahrenheit. On a global scale, the resulting emissions boggle the mind. Concrete produces 5 percent of global CO2 emissions—as much as the entire country of Russia.
Of course, we rarely notice concrete for the same reason it has such a vast impact: It's everywhere. What's more, end users can't easily see its environmental impacts. Polluters like cars and power plants have exhaust pipes that billow smoke. Sidewalks and foundations, by contrast, can seem deceptively clean and inert.
Engineers are just starting to address this challenge. They analyze materials like concrete with the term “embodied energy,” which measures the amount of energy that goes into the final product. Embodied energy calculations are sort of like nutrition facts for building materials—they help tally the total burden of man-made objects, the way candy bar wrappers tell us how many calories we're consuming.
Recent research suggests that when it comes to concrete, we can do better. Several universities and corporations have set up research partnerships to make concrete more sustainable. In 2014, engineers at the Massachusetts Institute of Technology reported that by tweaking the ratio of starting ingredients, they had created cement samples with twice the strength of standard Portland cement. Franz-Josef Ulm, director of MIT's Concrete Sustainability Hub, says that stronger materials allow engineers to make do with less total cement—potentially achieving the same construction goals while emitting 60 percent less CO2.
Other inefficiencies are subtler and more surprising. In a side room of Ulm's MIT lab, a small mechanical wheel rolls over a long strip of plastic. As the wheel moves along, it makes a slight indentation in the plastic. According to Ulm, this is exactly what happens when a car drives down a paved road. However stiff concrete and asphalt might seem, both materials flex and compress.
“As the wheel moves, it deforms the pavement because of its weight,” Ulm says. “Which means we are always driving slightly uphill.” This effect increases fuel consumption by 1 to 3 percent, meaning that for every dollar we spend on gas, a few pennies are wasted because of the squishiness of our roads. This might not seem like much, Ulm says—“but if you take this at the scale of a state or a nation, 1 to 3 percent is enormous.” Pavement is quite literally where the rubber meets the road, and to fully account for the burden of concrete, we have to add up lots of little inefficiencies like this one.
Even ubiquitous steel-reinforced concrete is riddled with problems. Steel was supposed to increase the strength and lifespan of structures, but according to Courland, the design backfired. In just a few decades, steel reinforcement rusts and causes concrete to crack—which, ironically, yields a material with only a fraction of the longevity of millennia-old concrete. “If the Romans had used steel reinforcement in making the Pantheon, then it wouldn't have survived the empire that built it. It would have long since fallen apart,” says Courland.
Alternatives already exist, but there's little incentive to implement them. For example, Courland says that concrete reinforced with fiberglass is stronger and longer-lasting than steel-reinforced concrete. But in the cost-cutting field of construction, such innovations are a tough sell. “When you go outside and look around, most of the buildings you see were done by the lowest bidder,” says Courland. Worse, when a steel-reinforced concrete bridge erodes, construction companies profit from rebuilding it. The result can be something like a race to the bottom, in which cities are built of needlessly low-quality materials, over and over again.
Ulm's research may be a sign that this is starting to change. His lab is partly funded by concrete companies, which already use his team's high-strength concrete in specialty applications like oil wells. Still, when it comes to modern building practices, innovations are embraced remarkably slowly. When John Aspdin came up with “Artificial Stone” in 1824, it revolutionized construction within decades. Ulm says that today, the road “between invention and innovation” is so filled with obstacles that engineers call it the “valley of death.” Even dramatic improvements in our materials face a long uphill journey to adoption.
We need concrete. “It's a marvelous material,” Courland admits. Its versatility and simplicity have adapted it for everything from urban sidewalks to rural schoolrooms to modern sculpture—which is exactly why concrete has become a symbol for practicality and action. We think of “concrete solutions” as the ones that lead to real change.
Maybe it's time to help our favorite material live up to its name. To do that, we'll have to acknowledge its shortcomings. What better way to reduce our footprint than by improving the ground we walk on?