Green Concrete

Stone circles, pyramids, great walls, even sandcastles, humans have always loved to build things. And though they have many building materials to choose from, none is more popular than concrete.

Thanks to its unique strengths, it’s the most widely used solid material in the world ^[1,2,3]. In fact, concrete - together with the other man-made materials - now weighs more than all the plants and animals on earth combined ^[4,5,6].

Unfortunately, concrete production releases harmful greenhouse gases, and accounts for 8% of global carbon emissions ^[7,8]. If concrete was a country, it would be the third biggest polluter on the planet ^[9,10].

So, if humans want to build a cleaner world, they’re going to need solutions; and green concrete is the key.

Primitive forms of concrete were used to make floors over 10,000 years ago in the Middle East ^[11,12,13], but the first to truly master it were the Ancient Romans who used it to build the Pantheon, which remains the world’s largest unreinforced concrete dome to this day ^[14,15,16].

Unfortunately, the secrets of concrete were lost to the sands of time ^[11-16]. It took until 1824 before British bricklayer Joseph Aspdin rediscovered how to make cement, the key ingredient in concrete, while mucking about in his kitchen ^[12,17,18] and he paved the way for a concrete that could rival that of Ancient Rome.

Today, humans produce 30 billion metric tons of the stuff a year ^[19]. That's enough to pave over Austria with six inches of concrete, every year! ^[20,21,22]. And with demand growing, so are emissions. But how exactly does concrete generate CO₂ anyway?

Well, concrete is essentially an artificial rock. Like a sandcastle, it’s made from sand, plus gravel and water. But add cement, the magic ingredient that binds it all together ^[23-27] and the mixture becomes solid as a rock.

Unfortunately, cement also creates nearly all the emissions ^[26-29]. To understand why, we need to look at how it’s made.

The recipe is remarkably simple, but the process is quite complicated:

First, take a generous portion of limestone, which is usually made of the shells and fossils of sea creatures that lived millions of years ago. It contains the calcium needed for cement as well as a store of trapped carbon ^[30-33].

To access the calcium, you crush it up and then add a dash of clay, and a sprinkle of sand. This mixture is then preheated and fed into a rotary kiln to mix them together at a sizzling 1450°C until the limestone burns. This forms a calcium-rich material called clinker ^{[23-27,34-37]}.

But burning fossilized sea creatures releases a lot of CO₂, and because fossil fuels are burned to generate the heat, even more CO₂ is produced ^[38-42].

Still following? We are here!

Next, mix the clinker with gypsum and other additives, then grind them together in a grinding mill to produce cement powder ^[34,35].

All that’s left to do is mix the cement with water, sand, gravel, and other aggregates ^[23-27] and Voila! You’ve made concrete!

Along with a whole bunch of pollution. And as you can see, most of the emissions come from producing cement, with the biggest chunk produced by burning all that limestone! ^[38-43]

In fact, every 1000 kilograms of cement in the mix produces 900 kilograms of CO₂ ^[43,44,45].

So how can humans reduce the emissions of their favorite fake rock? Well, there are several solutions available ^[43-50].

The easiest solution in the short term is simply using concrete more efficiently ^[43-48,50]. At the moment, most structures are built with a lot more concrete than they actually need, so many architects are now optimizing designs to use less concrete, lowering demand and reducing emissions.

Efficiency is also improved by crushing up old concrete and reusing it in new concrete mixtures ^[51,52,53], and exciting new technologies like electric recycling are also being tested ^[54]. This uses renewable electricity in an electric arc furnace to recycle both concrete and steel together, reducing the emissions of steelmaking and creating cement in the process. Better yet, this cement is theoretically zero carbon ^[54].

While this is a huge improvement, the human appetite for concrete is so large that recycled concrete is just never going to meet demand.

The next strategy is to replace the fossil fuels needed for production. Grinding mills can be powered by clean renewable electricity from wind and solar ^[55,56]. But it’s difficult to use electricity alone to reach the toasty temperatures of a kiln, so alternative fuels like green hydrogen ^[57-61] or sustainable biomass ^[61-64] can be used. By eliminating the need for fossil fuels like coal, these alternative fuels can make a big dent in CO₂ emissions ^[50].

But unfortunately there are a few problems…

Biomass is mainly derived from wood and obtained by cutting down trees, making it hard to produce sustainably ^[65,66,67]. And while green hydrogen has promise, producing enough requires a massive expansion of the renewable electricity needed to make it ^[68,69,70].

So, for the time being, these solutions are a long way from making a serious impact. And, even with these fuels powering the kiln, you still have to burn limestone which is responsible for over half the emissions of cement production ^[38-43].

Clearly humanity still needs other methods.

Carbon capture is often hailed as the answer ^[71-78]. Used to catch the CO₂ as it is released and store it underground, it can reduce emissions, but it can’t catch them all ^[50,79,80]. Worse, it's difficult and extremely pricey to capture, transport, and store all that carbon and it’s not always that effective! ^{[43,77,78,81]}

But what if some of that CO₂ could be used in concrete production itself?

Enter Carbon Curing:

This popular method involves pumping some of the captured CO₂ back into the concrete during mixing where it becomes chemically embedded, potentially strengthening the concrete and lowering its costs ^[82-91]. It’s a neat solution but it can only deal with a fraction of the emissions from cement production ^[92].

So, to truly achieve green concrete, humans are changing the recipe, and one of the best ways they are doing this is by adding ingredients called SCMs to replace some of the polluting cement ^{[45-49,93-97]}.

These are made with fine powdery materials like fly ash from coal plants or slag from the steel industry. Since these are both waste products, this helps clean up the environment too, and because SCMs reduce the amount of cement that’s needed, they can lower emissions by between 15 and 40% ^[48,97]. Better yet, they also improve concrete’s strength and durability ^[98,99,100].

Unfortunately, on their own they can never replace cement entirely, but there is a solution… an alkaline solution!

If you take substances like the industrial waste products used for SCMs and mix them with alkaline solutions, it’s possible to create a binding agent that can replace cement entirely ^[101-104]. And because you don’t need to dig, crush, grind or burn the ingredients for conventional cement, the resulting concrete can reduce emissions by up to 90% ^[49,104-107].

It’s called alkali-activated concrete, and although it has yet to be deployed at scale ^{[108,109,110]}, it’s already being used around the world ^[111-123].

Humans love building things more than ever, building an area the size of Manhattan every 3 days, but can they do this without heating the planet? ^[124]

Well, yes!

By investing in research and development, deploying the full range of available solutions, and by expanding renewable electricity generation to support them, humanity can finally perfect the recipe for green concrete and build a zero-carbon future!