Concrete has been prized as a building material for over two millennia. This simple blend of powdered cement and other mineral ingredients hardens quickly when mixed with water. It's easily molded, and when it sets, it's strong, yet flexible. So it's no wonder concrete is the most widely used man-made building material in the world. Unfortunately, making concrete is not an environmentally friendly process. But some are trying to make concrete "greener," without sacrificing its low cost and legendary durability.
Concrete is very strong and, if it's made correctly, can be very durable. In fact, the Romans built structures like the Pantheon 2,000 years ago, and the structure still stands proud. Concrete's so-called mechanical properties also explain its universal popularity as a building material.
"Also, it is very cheap compared to with other building materials," says Christian Meyer, head of Columbia University's Department of Civil Engineering, adding, "And you can get it almost everywhere… because the ingredients that you need [to make concrete] you find [almost] everywhere."
Every year, nearly a cubic meter of concrete is produced for every man, woman and child on the planet. Meyer says satisfying that demand requires a lot of land.
"If you want to produce 10 billion cubic meters of concrete, you need 10 billion meters of material. That's the conservation of mass."
And where do you get that?
"You have to quarry the stone. You have to quarry the aggregate and get the sand from sand pits. That's a huge amount of material!"
When staying power isn't a good thing
In fact, concrete's very durability is also one of its biggest drawbacks.
"It's not a material that comes down easily when it's put up," says architect and Columbia University professor Michael Bell, who chairs an ongoing conference about architecture and materials, including concrete. "So whatever we are building in Beijing, Shanghai, or in India right now, those buildings are going to stay. Or, if they aren't going to stay, getting them down is going to produce a massive amount of waste."
Until recently, most of the world's used concrete went into landfills, at enormous environmental cost. But today, some manufacturers, especially in Europe, are crushing used concrete and recycling it for use as an ingredient in new concrete. The United States lags behind in these efforts, but many American state highway departments are interested in the process.
A major source of carbon dioxide
The greatest environmental threat associated with concrete is that the processing of one of its key ingredients, cement, releases large amounts of carbon dioxide, or CO2, a greenhouse gas associated with global warming. About a metric ton of carbon dioxide is created for each metric ton of cement produced, and the process now accounts for as much as 7 percent of the world's industrial carbon dioxide emissions.
Why so much carbon dioxide? First, it is created as a natural chemical byproduct when the ingredients in traditional cement - called Portland cement - are combined. Second, cement is made by heating a chemical mixture - mostly limestone, clay, iron ore and other minerals - in a kiln to high temperatures usually exceeding 1,500 degrees centigrade. The energy for that heat usually comes from burning oil or coal, both of which release carbon dioxide during combustion.
Finding greener alternatives
Fortunately, there are ways to reduce the carbon footprint of the cement we make and use.
"The most common one is that we simply use less cement and replace it with some other material that has cementitious [cement-like] properties that happen to be byproducts of other industrial processes," says civil engineer Christian Meyer.
The most common substance to fit that bill that is "fly ash," which is a byproduct of coal combustion. U.S. federal law now requires that power plants either capture this fly ash so it doesn't pollute the atmosphere or pay a hefty fine for the fly ash that is released. But it was found that this fly ash has excellent cement-like properties.
"In fact," says Meyer, "it can react together with the cement and the water and produce better concrete if you use fly ash than if you don't use fly ash. It's also cheaper than cement. So it's a 'win- win' situation here."
It is projected that power plants in the future may be built alongside cement factories, so that the waste from one industry may be easily used as the raw material for another. There is a similar "green" use for the slag created as a byproduct of steel manufacture.
New types of cement are being developed or are already in use. One is a white cement that is easily pigmented to create colorful visual alternatives to the drab gray of traditional cement. That reduces visual pollution. Another uses titanium-oxide particles that break down many of the organic pollutants already in the air and help clean the atmosphere. That was the cement of choice for architects and engineers who built a beautiful new concrete highway bridge in Minneapolis to replace the one that had collapsed in 2007, killing 13 people.
In an era when nations around the world are industrializing, and cities are growing at an unprecedented pace, making the world's favorite building material a little greener is an achievement as solid as… concrete.