Human World

Brent Constantz builds cement like corals do

Biomineralization expert Brent Constantz of Stanford University was inspired to make a new type of cement for buildings by the way corals build reefs. The process of making this cement actually removes carbon dioxide – a greenhouse gas, thought to cause global warming – from the air. The company Constantz founded, called Calera, has a demonstration plant on California’s Monterrey Bay. The installation takes waste CO2 gas from a local power plant and dissolves it into seawater to form carbonate, which mixes with calcium in the seawater and creates a solid. It’s how corals form their skeletons, and how Constantz creates cement. This interview is part of a special EarthSky series, Biomimicry: Nature of Innovation, produced in partnership with Fast Company and sponsored by Dow. Constantz spoke with EarthSky’s Jorge Salazar.

I understand that your method of making cement, modeled on the way corals build reefs, is an example of what’s called “biomimicry.” Would you explain what biomimicry is?

Biomimicry is really the study of evolution. And it’s the study of the function of biological structures. Historically, paleontologists just studied the structural morphology of fossils, because paleontologists only had the shapes of fossils to look at. When we’re studying biomimicry, we’re studying how evolutionary structures are adapted to their environment, how they function. And they’re the result of evolution.

So, for example, we look at an organism like the corals that build reefs. Building reefs, the corals have developed an incredible ability to calcify. They’re the most prolific mineralizers on the planet. They form great structures like the Great Barrier Reef. In doing so, they’re able to make more mineral than any other organism we’ve ever seen. They’ve adapted specialized structures.

In biomimicking what corals do, we’re really trying to mimic, in some cases, how they can mineralize so rapidly, so prolifically, to make the largest biological structures on the planet, like the Great Barrier Reef.

Coral life. Image Credit: Toby Hudson

What’s the simplest way you could explain your process of taking CO2 and making concrete from it?

There’s a natural interaction between CO2, which is a gas, and water. They come into equilibrium together and the CO2 is dissolved in water. The colder the water is, the more CO2 is dissolved into it. This forms another molecule, CO3, which we call carbonate. It’s the carbonate in carbonated water. The higher the concentration of CO2, the more carbonate you form. When we interact water with something with very high concentrations of CO2, like the flue gas of a power plant, we get much, much more CO2 dissolved in water to form carbonate.

That’s what Calera does. Across the street here at Moss Landing, there’s a 110 foot high absorber – it’s just a vertical carwash, which is spraying sea water through this big, vertical column. At the base of the column comes the flue gas from this power plant. It comes up from the base of the column, and it goes up and goes over the top. On its way out, with the sea water spraying through it, that same reaction occurs. The CO2 goes to CO3 as it dissolves in the water.

Sea water has calcium. When the calcium sees the carbonate, you form calcium carbonate, the solid. That’s what limestone is. That’s how corals form their shells. So that’s the basic process. The solids that form – it looks like milk – fall to the bottom and are separated. They’re dried out using the waste heat from the hot flue gas. There’s a way to trap the heat of the hot flue gas – it’s called a heat exchanger – so there’s no burning of fossil fuel to dry it out. That produces a powder in a spray dryer, which is akin to a machine making powdered milk. And that is the cement. The cement can be used to make aggregate, synthetic rock like synthetic limestone, or it can be kept dry as a cement and used in a concrete formulation.

What’s new about this process?

Calcium carbonate precipitation, which is what I just described, is really one of the most common chemical processes today. It’s been around for over a hundred years. Calcium carbonate is used as a filler in plastics and food products. It’s very ubiquitous. What’s different about what we’re doing to make concrete and cement is that when we talk about solids that are crystalline minerals, there are different forms of these minerals. For example, carbon in diamonds has the same chemical composition. They’re just carbon. So graphite and diamond are the same. But they look very different. That’s because they have different crystallographic structures. And that’s what we’re doing over here, is we’re forming different crystallographic structures – in this case of calcium carbonate – which have very different properties. Some of them have properties that make them very good for cement, so that when you add water to them, they’ll recrystallize apart into something like synthetic limestone.

Road through old forest. Image Credit: Chris Willis

What in nature inspired you think about how concrete is made?

If you look at man’s history, the main thing we’ve left behind is the built environment. If we look at civilizations 5,000 years ago, we see today, the pyramids, for example. When we look at the last few centuries in Europe, we see these massive buildings, bridges, dams, and roadways.

When you go forward a hundred years from now, you’ll see that, looking back, there’s been this transition from using stone and ancient mortars that are derived from limestone, to concrete. Concrete is, in fact, the most-used building material today. The main thing that our generation is going to leave behind for new generations is massive amounts of concrete.

So concrete represents this incredible reservoir to store something. Instead of mining limestone and what’s called calcite to make Portland cement, and mining limestone to make the aggregate to mix with the Portland cement to make concrete, our process provides this reservoir to form a massive structure like the Great Barrier Reef, which is the largest biological structure on the planet, not like a man-made structure. The inspiration was as much as anything just in the sheer volume of material transport we’re talking about.

In fact, from a mass point of view, the amount of concrete being made today is the largest mass transport in the history of the planet. If you look at all the aggregate that’s being moved and all the cement that’s being moved for concrete, asphalt, and road base, and we look at the formation of a structure like the Barrier Reef, it represents billions of tons of CO2 that’s been taken from the atmosphere through the ocean. Through biomineralization, it’s been incorporated into these mineral structures that sequester the carbon dioxide forever.

So, in a broader sense, from a large-scale mass balance, moving these massive amounts of CO2, which are outpacing all our efforts today to mitigate CO2 with wind, solar, tidal, low-emission cars, new types of transmission and everything, and putting the CO2 into the built environment and storing it there as a profitable activity, really is what we see in the natural world.

How do you see the situation today of the way things are made in the “built envirnonment”?

There’s been a fair amount of money put behind a first-generation approach, jumping directly to the industrial method, to use traditional chemical engineering approaches to achieve the end, rather than mimicking the processes that are used in nature.

My hope would be to see that we embrace the more biomimetic pathway to these processes, which are more sophisticated and more complicated and follow what nature actually does. I believe very sincerely that beneficial use of carbon, reutilizing this carbon in a productive, economically sustainable way is truly one of the only solutions that we have.

Because, energy efficiency is where we’ll get many gains. We’re still going to see this tremendous rise in carbon dioxide in the atmosphere because of all the new point-sources of carbon dioxide that are developing around the world with new coal-fired power plants and new cement plants. Even if we try and push renewables as hard as we possibly can, we’re still mainly going to see our electrical power coming from the production of coal around the world, and CO2 levels are going to continue to rise. We absolutely have to come up with a program where we can capture all that CO2 and we can do something with it.

We have to create a model where developing countries and developed countries can work on the same technologies and actually make a profit pulling this CO2 out of coal plant emissions and use it for products that are already in their economy, like concrete, road base, filler for asphalt and other things that can be done with these materials. I don’t believe that there’s another reservoir available where we can put that much carbon dioxide. Yet we have this beautiful market for concrete that is just perfect for introducing this technology today, and solving the concrete industry’s carbon problem at the same time, bringing new, prosperous economies to the countries that choose to follow this process.

What change do you want to see in how we create the built environment?

I think we do need to really go back to basics when we think of the built environment. When we look at structures that were constructed before we had steel, for example, we know that we found out about these principles differently. The pyramids weren’t only constructed the way they were because they liked the shape. It’s because they were using no steel. In order to build structures out of stone without steel, you need to think about the whole structure differently.

Another way we need to rethink the built environment is, for example, roads. Most concrete is used in roads today. And here in the U.S., we only build our roads when they’re built of concrete a few feet thick at the most. And typical roads in Europe are several feet thick. And they last much longer. And reasons for that are related to this whole thinking of the economics of the road building. But imagine if that road is now placed to sequester carbon dioxide. The thicker the road, the longer it lasts. The more carbon dioxide we’re sequestering.

So today, architects think, how can I minimize the amount of concrete I’m using in my material? Because we’re interested in minimizing the carbon footprint as much as possible. Instead, we can see the built environment as a place to sequester carbon dioxide.

April 27, 2012
Human World

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