Where does concrete come from, and where does it go when a building comes down?

In numbers

• ≈ 8%cement's share of global carbon dioxide emissions (2018)
• ≈ 1,450 °Ctemperature of a cement kiln
• ≈ 30%of the production CO₂ that concrete reabsorbs over time

In five stops

1. 1

   Where were its atoms born?

   The calcium, silicon, oxygen and iron in concrete formed inside stars and stellar explosions. These atoms became part of Earth's rocks and over millions of years moved from rock to sea, from sea to living things, and from living things back into rock.

2. 2

   How did nature prepare it?

   Limestone (calcium carbonate, CaCO₃) mostly forms in the sea: corals, shellfish and tiny plankton take calcium and carbon from the water to build shells and skeletons; when they die these pile up on the sea floor and turn to rock. Sand and gravel are fragments of rock worn away by erosion.

3. 3

   What did people do?

   The Romans made strong concrete with lime and volcanic ash; the dome of the Pantheon in Rome, finished in AD 128, is still the largest unreinforced concrete dome in the world. Modern Portland cement was patented by Joseph Aspdin in 1824. In a cement plant, limestone and clay are fired at about 1,450 °C; the limestone breaks down and its carbon escapes into the air as carbon dioxide.

4. 4

   What happens when we use it?

   When cement is mixed with water it doesn't just dry, it reacts: it forms new crystals with the water, and these crystals bind the sand and gravel together like stone. That's why concrete can harden even under water. It is the most widely used building material in the world today.

5. 5

   Where does it go next?

   Concrete can last for centuries. Over that time it slowly reabsorbs carbon dioxide from the air, making up for about 30% of the carbon dioxide released when its cement was made. Concrete from demolished buildings can be crushed and reused as road fill or as aggregate for new concrete.