Concrete And Global Warming: A Mixed Bag Concrete may be responsible for as much as five percent of the global carbon footprint, taking into account the energy required to make it and the CO2 emitted by the kilning process. But a new study by engineer Liv Haselbach finds that concrete may also be an important CO2 sink.
NPR logo

Concrete And Global Warming: A Mixed Bag

  • Download
  • <iframe src="" width="100%" height="290" frameborder="0" scrolling="no" title="NPR embedded audio player">
  • Transcript
Concrete And Global Warming: A Mixed Bag

Concrete And Global Warming: A Mixed Bag

  • Download
  • <iframe src="" width="100%" height="290" frameborder="0" scrolling="no" title="NPR embedded audio player">
  • Transcript


Up next, if you look at all the sources of greenhouse gases out there, from cars to power plants to factories, probably one that does not immediately come to mind is concrete - concrete, yeah, you know, the stuff that they make cement out of.

It's always been assumed that the cement used to make concrete could contribute as much as five percent of the global carbon footprint once the energy to make it and the carbon dioxide emitted as it is kilned are taken into account. We've always heard about - it takes - a lot of CO2 is emitted when they make concrete and the cement inside of it.

But new research published in the Journal of Environmental Engineering indicates that concrete may actually act as a carbon sink, as well. That means the carbon gets absorbed into the concrete as it's sitting out there, out in the open once it's made, and the material can actually absorb carbon dioxide and lock it away inside the porous structure that we know that concrete has. And while it's not enough to cancel out the environmental cost of concrete production, that absorption does shift the balance somewhat, and joining me now to talk about this is one of the authors of the report.

Liv Haselbach is an associate professor in the department of civil and environmental engineering at Washington State University in Pullman, Washington. Welcome to the program.

Dr. LIV HASELBACH (Washington State University, Pullman): Yeah, hello.

CONAN: Thank you very much, Dr. Haselbach. Tell us why you changed your mind on this. What happened with the thinking?

Dr. HASELBACH: Well, actually they've known for a long time that concrete absorbs carbon dioxide. It's a process they usually refer to as carbonation because when they make cement, they use limestone, and CO2 is released. But later on as you make concrete and add water and add the rocks and have the concrete, the CO2 in the air wants to go back in, and Mother Nature wants to form back the stable product, which is limestone again.

So they've known for a long time that this is happening, but it doesn't usually get well into interior structure of, let's say, a concrete building or things like that.

However, they've always looked at it just in case it changes the chemistry to affect the structure.

FLATOW: Right.

Dr. HASELBACH: And now that we're looking at the carbon footprint of all our things, I decided to look at it again in the context of how much carbon is actually in there. So the new thing is not that it's going in but the fact that there's more in there than we had expected.

FLATOW: And how much more?

Dr. HASELBACH: Well, what I'm looking at is something different than limestone. Limestone is a solid, you know, calcium carbonate solid. It's the stable form, but as it comes in, you know, it doesn't instantly become rock. Rock takes a little while to form, and so it's in there in some other forms and more of hydrated, or that's the water-based forms and other not-as-stable forms. And that's what I was looking at. And it isn't that I found it - I guess they've known it for a long time.

You know, 50 years ago, they mentioned it. It just wasn't important. We went back, and we investigated. In fact, it's about five percent of what the CO2 was released in the process.

FLATOW: And that's the ingredient, the calcium carbonate, you're saying, that's put in there.

Dr. HASELBACH: Yeah, it becomes calcium carbonate. It becomes rock again. So the concrete wants to become rock again, and as it's doing it, there's a little bit more carbon dioxide in there than we had thought, and that's good news.

FLATOW: Does it also just sit there out in the open and suck up CO2?

Dr. HASELBACH: Yeah, that's what we're hoping. This is preliminary research that we've been working on, and there's other researchers looking into it also. We're really hoping that we can design concrete's life cycle so that by the time it's reused again for, let's say, fill somewhere or a road base, that we can actually make it almost carbon neutral.

FLATOW: 1-800-989-8255 is our number. We're talking with Liv Haselbach of Washington State University in Pullman about cement. You know, I got lectures many years ago: There's a difference between cement and concrete.

(Soundbite of laughter)

Dr. HASELBACH: Yeah, there is a difference. Cement is the gluey part, and concrete is when you add the rock together.

FLATOW: There you have it. We're going to come back after we take a little short break and talk more with Dr. Haselbach. Our number, 1-800-989-8255. Also you can Twitter us. You can send a tweet to @scifri, @-S-C-I-F-R-I. Also in Second Life, we're taking your question. The avatar is there. Get your free T-shirt. 1-800-989-8255. Stay with us. We'll be right back.

(Soundbite of music)

FLATOW: I'm Ira Flatow. This is SCIENCE FRIDAY from NPR News.

(Soundbite of music)

FLATOW: You're listening to SCIENCE FRIDAY from NPR News. I'm Ira Flatow, talking with Dr. Liv Haselbach, who is an associate professor in the department of civil and environmental engineering at Washington State University in Pullman. We're talking about concrete, cement, concrete and carbon dioxide.

Let me run an idea just past you that I heard over the years, and maybe you can tell me if it's true or not…


FLATOW: …about concrete. Is it true that concrete never stops drying?

Dr. HASELBACH: It is the most amazing material ever. I'm an environmental engineer, not a concrete engineer, and I came in for the environmental aspects of it, but yeah it's true. Inside the Hoover Dam there, that stuff is still changing. And that's why you can get this carbon dioxide back into it because as I said, the stable form that it wants to eventually change back to is solid rock, just like it started for - as.

FLATOW: Is that right?


FLATOW: So even the Roman aqueducts that are like crude concrete, they're still drying out, too?

Dr. HASELBACH: Well, they're probably still changing, but if they've been there for so long, they may have formed back into a stable form. I can't answer that, since I haven't studied them, but I'd imagine so.

You know what? It's like you know when we're using the bio-based fuels, how we're trying to use Mother Nature to make our energy for us and solar power. We're trying to use Mother Nature to make our energy for us. Well, this whole process is the same thing. Mother Nature wants to put the CO2 back into this product. So we bar the rock, we make our structure, and if we work it right, we can put it back into the cycle to make it renewable.

FLATOW: So here's a question from Second Life. So should we be considering switching back to concrete highways instead of asphalt?

Dr. HASELBACH: That's a really good question. Pavement is what I do a lot of studies on, and honestly I'm studying more specialty pavements that they use for storm-water management, which have a lot of surface areas.

What you need is a lot of surface area so that the CO2 comes back in a relatively short amount of time, which is a couple of years instead of many, many decades kind of time frame.

The consideration for the highways has other considerations, too. Concrete's a great pavement material, asphalt's a great pavement material. Some are used for different reasons, and it has to do with whether they're flexible or not.

But yeah, it is something they're going to ask. They're going to look at the carbon footprint of the different pavement materials, and this is important information to make the correct environmental decision, I would say.

FLATOW: Let's go to Harvey in Oakland. Hi, Harvey.

HARVEY (Caller): Yes. Now does the carbon…

FLATOW: Whoop, lost him.

Unidentified Man: Oh no.

FLATOW: Someone was asking, I guess it was Harvey maybe, wanted to know if you used carbonated water to make the concrete with the cement, would that help?

Dr. HASELBACH: You know, it probably does. And when you make concrete, and it's rolling around in the truck, probably some CO2 is going in it then, too. There are actually companies that are now manufacturing certain concretes by power plants to use the emissions from them. It's really cool.

FLATOW: Seltzer in your concrete. There's a concept of making seltzer. Okay, it didn't go very far, not a good concept. What about ways - can you engineer concrete now to then - you compared it to biology. Could we engineer concrete to absorb more CO2?

Dr. HASELBACH: That is what I'm hoping will happen, and there are several other researchers and companies that are interested in it. And that's why I started this research is because I would like to engineer it so that we have the lowest carbon footprint possible.

FLATOW: And so what kind of things could you do?

Dr. HASELBACH: Well, some of it has to do with some products that we're looking into that have a lot of surface area. It's like when you build a building, and you implode it, then you have all this concrete rubble. Well, think of all the surface area there. Perhaps when it's crushed it could be left out in certain conditions - it has to have some water to it, too - that you could reabsorb it fast enough before you use it as a recycled aggregate.

So in other words in the life cycle is where most people are putting it in, but there are other - there was some architects and engineers I was speaking to yesterday was wondering well, if you have different layers of concrete, some more porous than others in a structure, will you bring it back in faster? Very good questions, and very, very demanding engineering challenges to try and design it and implement it that way.

FLATOW: 1-800-989-8255. Let's get some FAQs in from Carl(ph) in Durham. Hi, Carl.

CARL (Caller): Hey there. I had a couple questions regarding some things that I read about concrete.


CARL: One is: How much carbon is emitted per ton of concrete produced?

Dr. HASELBACH: I don't have the exact number, but basically I thought it was about a ton for every ton. Now that's not concrete. That I believe is - no, I think it's about a ton per ton, and the reason why it's emitted is for two reasons.

One of it has to do with the fact that you're using energy when you make cement and the transportation and everything else. And energy, you know, we can obviously change the carbon footprint with that, but again the rest of it is the chemical reaction. The main ingredient's limestone. Limestone has carbon dioxide in it. It's released when you make cement. That's the main ingredient for cement, but we're hoping to bring it back in.

FLATOW: You're hoping to bring it back in. Roger, Roger's in Erwinville(ph).

Unidentified Woman: (Unintelligible).

FLATOW: You have to turn off your radio.

Unidentified Woman: (Unintelligible) when you make cement.

FLATOW: Whoops, sorry about that. So where do you go from here in your research? What's your next step?

Dr. HASELBACH: Well, one of them is one of the first things that gentleman I think from Oakland asked: Should I put carbonated water in it when I'm making the concrete? So one of the first things is should we put a little bit more CO2 in it as we're mixing up the concrete? That's one of the first things that I hope to be looking at next year. How much is it initially when you do it?

And then the other things I really like to study right now are the recycled uses of it. If we have concrete that's already in building that we haven't done something special to, can we, when we demolish the buildings, can we reuse it and get the carbon dioxide in before it's reused?

FLATOW: And so concrete is not normally recycled, you're saying?

Dr. HASELBACH: Concrete used to not be recycled as much, and except that recently, with everything becoming more sustainable, it has now gone from very little to nearly all is looked at as being recycled.

FLATOW: You've got to get all that rebar and things out of it.

Dr. HASELBACH: Yeah, but you know, everyone wants to recycle the metal, too.

FLATOW: Oh that's right, the metal.

Dr. HASELBACH: So the opportunities now that people are looking for more green construction is making the opportunities for carbon dioxide sequestration in concrete much more feasible.

FLATOW: Very fascinating. Thank you very much, Dr. Haselbach, for being with us.

Dr. HASELBACH: Oh, you're welcome.

FLATOW: Liv Haselbach, associate professor in the department of civil and environmental at Washington State University in Pullman.

Copyright © 2009 NPR. All rights reserved. Visit our website terms of use and permissions pages at for further information.

NPR transcripts are created on a rush deadline by Verb8tm, Inc., an NPR contractor, and produced using a proprietary transcription process developed with NPR. This text may not be in its final form and may be updated or revised in the future. Accuracy and availability may vary. The authoritative record of NPR’s programming is the audio record.