The incipit of an item at Catalyst (it's a transcript of an interview):

Narration: What’s the fast growing source of greenhouse gas?

You might be surprised but it’s cement.

Cement’s already the 3rd largest man-made source of carbon dioxide - more than two billion tonnes of it a year. That’s after fossil fuels and defrorestation.

And because of all the construction going on around the world, cement’s carbon footprint is growing rapidly.

We desperately need greener concrete.

Professor Jannie van Deventer’s team at the University of Melbourne has found one. And surprisingly it’s chemically similar to a cement used by the ancient Romans.

Professor Janine van Deventer: The good properties of geopolymeric concrete are also present in the old Roman structures, so old roman concrete be made along similar lines in terms of the chemistry.

Narration: Over time the Roman knowledge was lost. The cement we know and use was invented in the 19th Century. The problem with it is its fundamental chemistry.

Narration: 60% of the carbon emissions from cement manufacture come from the chemical reaction required to make it.

Calcium carbonate is heated until breaks down into calcium oxide – which is needed in the cement – and the by-product Carbon dioxide.

An environmentally-friendly cement will need a completely different chemical reaction.

... here's the rest ...