Concrete on Rare Earth
April 17, 2025 at 7:19 AM by Dr. Drang
A recent episode of the BBC Radio 4 science and environment show, Rare Earth, discussed concrete. I approached the show with some trepidation. Concrete has become an environmental whipping boy because of the great quantities of carbon dioxide released during the manufacture of cement. But the show was fair. It didn’t shy away from concrete’s problems, but it also discussed how valuable concrete is to society. It’s well worth a listen.
Like most civil/structural engineers, I have a fondness for concrete. It’s a great building material, and it’s hard to imagine the modern world without it. In the show, they talk about it mostly in the context of buildings, but roads, bridges, harbors, tunnels, dams, and wastewater systems all depend on concrete.
But its carbon footprint is undeniably huge, primarily because of cement. Cement is the powder that reacts with water to form the hard material that binds together sand and gravel into concrete. It’s made by burning limestone, clay, and a few other materials in giant kilns, and that’s where the environmental trouble lies.
Here’s a fun image from the first edition (1956) of Troxell and Davis’s Composition and Properties of Concrete, showing the various parts of cement production in pictorial form. The kiln is the long tube near the center. It’s set on an incline and rotates slowly, with the raw materials turning into cement clinker (nodules of cement that get ground into powder) through a series of reactions as they make their way down the slope.
Several of these reactions, called calcination, produce carbon dioxide. The simplest is
where heat drives off carbon dioxide from limestone (calcium carbonate) to produce quicklime (calcium oxide), which is used in later reactions to produce cement itself. It’s this chemistry that makes cement production such a profligate generator of . Even if fossil fuels were eliminated from the heating, grinding, and mixing processes, there’d still be carbon dioxide coming out of cement plants.
The show discusses ways of reducing the produced by cement, mainly by reducing the amount of cement needed in concrete. Materials like fly ash and granulated blast furnace slag are used to replace some of the cement in concrete. And because these are the waste products of other industries—coal-fired power plants and steel mills, respectively—they have an additional environmental benefit.
Like many Radio 4 shows, Rare Earth is available as a podcast (that’s a link to its RSS feed). After listening to this episode, I subscribed.