Concrete is one of those technologies that was used for centuries—in this case, by the Romans—and then had to be invented again centuries later. But once we rediscovered it, we were hooked. The only thing that humans consume more of, by volume, than water is cement, concrete's key ingredient. We use so much of the stuff that the cement industry is responsible, all on its own, for 5 percent of global carbon dioxide emissions.
The idea is simple enough: you mix together small particles (like sand) and slightly bigger particles (like tiny pebbles) with water and some sort of paste that will set and keep the whole thing together. (It's this last bit that's the tricky part.) The Romans made concrete of lime and volcanic rock, and they had some pretty good ideas about what it should be used for:
But at some point between the rule of the emperor Hadrian and the end of this past millennium, the recipe for Roman concrete was lost. In the second half of the 1700s, in England, engineers were experimenting once again with heating up limestone to create a powder that, when wet, would set and hold together. In 1757, the engineer John Smeaton found that the best limes to use in mortars were the ones with high amounts of clay, and he used this "hydraulic lime," to help build the third iteration of the Eddystone lighthouse, off the south coast of England:
Smeaton's lime set off a craze among builders in England to come up with a better "Roman cement." The key to making a good concrete was to improve on the composition of the paste that kept the whole thing together. In 1824, a paper in Leeds reported on some progress: "We hear that Joseph Aspdin, bricklayer of this town, has obtained a patent for a superior cement representing Portland stone."
This was, at the very least, an ingenious bit of marketing. Stone from Portland, an island on the southern coast of England, had been quarried for centuries and used in buildings like Westminster and the Tower of London. In his patent, Aspdin was vague about what exactly he had accomplished: it said that he had used a "specific quantity" of limestone and clay, but specified neither the quantities or the temperature at which they were fired.
Although he's often credited as the creator of Portland cement, the mix that Aspdin came up with probably isn't the one that's used all over the world today. (It's unlikely that he got the heat high enough for key chemical reactions to take place.) Still, from 1824 on, engineers, chemists and builders worked to create better "Portland cement." By 1844, another inventor had nailed down the modern, high-heat process, in which the components are brought up to 1,400 °C (2,552 °F). Since then, Portland cement has made concrete one of the most popular construction materials in the world.
Because it takes so much energy to produce, chemists and engineers are now working to find an equally strong and durable substitute. To do so, they're looking at everything from re-creating actual Roman cement to subbing in eco-friendly blocks made of earth and recycled concrete. In the meantime, though, engineers have managed to refine the possibilities of the concrete we do have: During the construction of the Burj Khalifa for example, concrete was pumped more than 1,900 feet above the ground. And on the other end of the building spectrum, this concrete tent can be inflated one day and set overnight:
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