Age-Old Recipe for Concrete Is Water, Cement, Sand, and Rocks

A race has begun among cement companies to find a more sustainable recipe for the old standby

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This podcast is part of the Sustainable Design radio program, a collaboration between IEEE Spectrum and the National Science Foundation.

Susan Hassler: The world’s most widely used building material is also one of its oldest: concrete. The recipe of water, cement, sand, and rocks has barely changed since Roman times. But now there’s a race on among cement companies to find a more sustainable recipe for the old standby. Phil Ross reports.

Phil Ross: Engineer Maria Juenger, at the University of Texas in Austin, has built her career around “green” concrete. But she’ll be the first to tell you: Regular old concrete already has a lot going for it.

Maria Juenger: It’s made out of local materials, it produces less CO2, uses less energy, produces less air pollution in general, and less water pollution than most other competitor building materials. 

Phil Ross: But making the cement in concrete releases CO2. And since so much cementis made, its impact is significant. Cement alone creates 5 percent of man-made CO2 emissions. There are ways of using less cement in concrete. But to make the cement itself “greener”?Well, that means tinkering with the time-honored recipe.

Phil Ross: At the Texas Lehigh Cement plant outside Austin, Texas, the whole of cement manufacturing is on display. Quality control manager Jim Jarl points to the towering succession of machines.

Jim Jarl: Raw material goes in one end, product comes out the other. We use basically a limestone and a soft marle, which has calcium and silica, and black dirt.

Phil Ross: Those raw materials are heated to extreme temperatures. In the process, the limestone releases CO2. It’s an unavoidable part of the chemistry, says Jarl.

Jim Jarl: It’s a point of contention for environmentalists that we’re releasing carbon dioxide, but you can’t make concrete without it.

Phil Ross: Unless…

Phil Ross: Unless a new cement was developed, says Maria Juenger.

Maria Juenger: So some of the work we’ve been working on is, how can we either not use limestone—so eliminate the calcium carbonate to begin with—or to create a cement that uses less of it, and the only way that you can create a cement that uses less of it is by changing the chemistry of the cement itself.

Phil Ross: One of the promising new cements she’s studying is called calcium sulfoaluminate cement—we’ll call it CSA for short. CSA cement has been used in China for over 30 years but has only recently caught the eye of international cement companies.

[mixer sounds]

Phil Ross: The University of Texas engineers mix CSA cements up in the lab in a big KitchenAid mixer. The recipe uses the same basic ingredients as the cement we saw at the plant, plus aluminum. Adding aluminum means the cement requires less limestone and lower temperatures. Taken together, that could reduce CO2 emissions by up to a third. And the CSA cement does well in testing. Concrete made with it is as durable as conventional concrete and as impermeable to many chemicals.

Maria Juenger: Your goal is to reach the same physical properties on any sort of observable mechanical testing level but to get at a completely a different route by having a completely different chemical reaction—different reaction products but have the same observable result.

Phil Ross: Her lab is methodically testing different “recipes” for CSA cements, to find the optimal blend. However, the aluminum ingredient in CSA cements presents its own sustainability challenge. Extracting aluminum from the mineral bauxite is costly and energy intensive. Luckily, there’s a recycled alternative.

Katy Gustashaw: I’ll take ’em out and you can feel ’em. ’Cause they are different, it’s kind of interesting.

Phil Ross: Katy Gustashaw is a Ph.D. student in Maria Juenger’s lab. She’s studying concretes made from fly ash, a waste product of burning coal. She pours tiny mounds of the ash onto filter paper.

Katy Gustashaw: So there’s two different types of fly ash, class F and class C. But all of these are different fly ashes, and you can see the various colors, first of all....

Phil Ross: International cement companies are racing to develop a CSA cement. Those cements could make use of fly ash, though for now the recipes are under wraps. It’s probably only a matter of time until U.S. regulations expand to allow CSA cements in concrete. But ultimately, there won’t be just one “green concrete.” New kinds of concrete will use ever more recycled material, in ever more inventive ways, Gustashaw predicts.

Katy Gustashaw: Because they’re being produced—end of the day, they’re being produced. So being able to find a way to utilize those materials is something I think is really interesting and rewarding.

Phil Ross: In Austin, Texas, I’m Phil Ross.

This podcast is part of the Sustainable Design radio program, a collaboration between IEEE Spectrum and the National Science Foundation.

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