It is estimated that 5 percent of global emissions of carbon dioxide comes from the manufacture of cement. Some clever manufacturers are finding ways to not only reduce the carbon footprint of the industry but possibly reverse it! That is to say, it seems possible to capture CO₂ and store it in concrete, i.e. sequester it, thus reducing the amount of atmospheric CO₂, a green house gas which contributes to global warming.

A story from the NYTimes.com, Concrete Is Remixed with Environment in Mind, compares the process of making concrete from cement to following a recipe that can be tweaked. The story describes the kinds of modifications being tested and their effects in terms of CO₂ emissions. To understand how these variations work, one should understand that the making of concrete from mixing water and cement is not simply a dehydration process. Rather, the water in the cement is a reactant, which is consumed during chemical reactions. These reactions chemically alter the mixture such that strength and durability are achieved.

One variation enables the cement to flow better and reduces the amount of water needed; another speeds up the reactions so the concrete is ready for use faster. In Minneapolis, the concrete in the new I-35W bridge contains titanium dioxide, which in sunlight chemically reacts to break down organic air pollutants and “bleach” them, giving the concrete a bright, white appearance. The story includes a photo of the bridge.

But what does CO₂ have to do with all this? According to the Times article, “About a ton of CO₂ is emitted for every ton of cement produced. The basic manufacturing process involves burning limestone and other minerals at about 2,700 degrees Fahrenheit to create an intermediate product called clinker.” Some entrepreneurs see an opportunity to fill a market niche: make a concrete that can sequester, or store, CO₂. Here are two approaches described in the article:

Novacem, a British startup, is developing a cement that does not use carbonates and can make concrete that absorbs carbon dioxide. . . . At a site adjacent to a gas-fired electricity generation plant in Moss Landing, Calif., the Calera Corporation is developing a process to bubble power plant flue gases [emissions from burning coal, for example] through seawater or other brackish water, using the CO₂ in the gases to precipitate carbonate minerals for use as cement or aggregates in concrete. The process mimics, to some extent, what corals and other calcifying marine organisms do.

In the article’s multimedia offerings, there is a link to a six-minute podcast, an interview with the article’s author, Henry Fountain. In the interview, Fountain recaps some of the facts and processes described in the article, providing good reinforcement for the reader or an alternative to reading for those with reading disabilities.

How to Turn This News Event into an Inquiry-Based, Standards-Related Science Lesson

An investigation into the concrete industry connects to the Physical Science and Science and Technology content standards of the National Science Education Standards. It also provides an excellent avenue for connecting to real-world issues.

Ask students: What would your life/world be like in the absence of concrete? Allow them plenty of time with this one. Then ask what is concrete; where does it come from; how long have people been using it; how is it made? What’s the difference between cement and concrete? Where in the world would you suspect the greatest amount of concrete manufacturing is occurring? Did they guess China? The Three Gorges Dam is just the tip of the iceberg there.

Can students imagine that the making of concrete emits tons of carbon dioxide into the atmosphere? Can they imagine that the making of concrete could be a solution to both reducing greenhouse gases and consuming waste materials from other manufacturing processes?

For some lessons focused on these kinds of questions, see the lesson plans on the Portland Cement Association web site: Lesson 1: The Uses of Concrete; Lesson 2: What are the Parts of Concrete?; Lesson 3:A Further Look at the Content of Concrete; Lesson 4: A Collection of Aggregates; Lesson 5: So, You Think Concrete Dries Out?

Here’s a related article published April 22, 2009, after the original date of this blog post, Restrictions sought for Cement Plants, also from the NYtimes.com.

The following are some related resources from the National Science Digital Library Middle School Portal: The Civil Engineer: A Day in the Life; Carbon Dioxide and Climate Change: and What Is the Carbon Cycle?