Jack Cutmore-Scott

It holds the other three ingredients, gravel, sand and water, together.

Unfortunately, it's impossible to make cement without generating carbon dioxide.

The essential ingredient in cement is calcium oxide, CaO.

We get that calcium oxide from limestone, which is mostly made of calcium carbonate, CaCO3.

We extract CaO from CaCO3 by heating limestone.

What's left is CO2, carbon dioxide.

So for every ton of cement we produce, we release one ton of carbon dioxide.

As tricky as this problem is, it means concrete could help us change the world a third time by eliminating greenhouse gas emissions and stabilizing our climate.

Right now, there's no 100% clean concrete, but there are some great ideas to help us get there.

Cement manufacturing also produces greenhouse gas emissions by burning fossil fuels to heat the limestone.

Heating the limestone with clean electricity or alternative fuels instead would eliminate those emissions.

For the carbon dioxide from the limestone itself, our best bet is carbon capture, specifically capturing the carbon right where it's produced, before it enters the atmosphere.

Devices that do this already exist, but they aren't widely used because there's no economic incentive.

Transporting and then storing the captured carbon can be expensive.

To solve these problems, one company has found a way to store captured CO2 permanently in the concrete itself.

Other innovators are tinkering with the fundamental chemistry of concrete.

Some are investigating ways to reduce emissions by decreasing the cement in concrete.

Still others have been working to uncover and replicate the secrets of Roman concrete.

They found that Pliny's remark is literally true.

The Romans used volcanic ash in their cement.