- This is the most important construction material in the world.

It's super cheap.

It can take up to 14,000 pounds per square inch of pressure.

(hydraulic press hisses) And it makes buildings like this and this and this possible.

(graphics clicking) Because concrete is so awesome, we use 10 billion tons of it every year, which means it's responsible for 8% of global carbon emissions.

That's a lot.

Optimistically, that means that even a small change in how concrete is made could have a huge impact on emissions.

And the changes that are on the horizon right now are not just small, they could virtually eliminate our single largest chunk of carbon emissions.

(text dings) (drill buzzes) So how is concrete made?

Well, the first step is to take calcium carbonate, alumina, silica, and iron oxide, throw it into a kiln and heat the whole thing up to 1,450 Celsius.

And when you do that, the first thing that happens is that the calcium carbonate decomposes in the heat to form calcium oxide and carbon dioxide.

And your CO2 problem, it is mostly right here for two reasons.

First, the kiln is usually heated up using fossil fuels.

But the second and more important reason is that for every molecule of calcium oxide you're making, you are also making a molecule of CO2, and this is usually released to the atmosphere.

The next step is this behemoth of a reaction right here.

And if you are wondering, by the way, what the tildes mean, so was I, (chuckles) had to go look this up.

It turns out it means the same thing as it does in math.

It means that these formulas are approximate.

Then you take all of this stuff and you add some calcium sulfate.

And at that point what you have is called cement.

And then you take the cement and you add some sand, pebbles and water.

And the next thing that happens is that two of the chemicals from over here, which are called alite and belite, reacts with the water to form calcium silicate hydrate, which is essentially, the glue that holds everything together.

It is this reaction that turns this wet and goopy stuff (concrete burbles) into hard concrete.

(concrete thunks) (knuckles tap) Let me take you back to 1973, to this very short nature paper, that at first seems like it has absolutely nothing to do with concrete.

(paper rustles) But let's look at figure one.

These are bacteria growing on a Petri dish.

And these white circles, at first, might look like sugar crystals, but they're not.

When the chemists, who wrote this paper, analyzed this material, they discovered that it was calcite, a mineral that is one of the two main forms of calcium carbonate, and one of the two main components of limestone.

Now, limestone is mostly made of calcite and dolomite, which I could not find on the internet.

Calcite is calcium carbonate, and dolomite is calcium magnesium carbonate.

Anyway, back to the bacteria.

In 1973, the researchers already knew that bacteria living in the ocean could make calcite, but this was the first paper showing that terrestrial bacteria could do the same thing.

In fact, the researchers showed that over 210 species of bacteria were capable of producing calcite.

Which suggests that many more, maybe even most bacteria can do it as long as the environment is right.

And this is where the story takes kind of a surprising turn.

(jaunty music) This is the John Ericsson Memorial, in downtown DC.

And the special thing about it is that it's low enough for me to show you the actual stone.

See how the stone isn't a perfectly sharp angle and it's worn away and there's cracks and crevices?

This is weathering, essentially, stone going bad.

Here in DC there aren't that many really old buildings, but in Paris, (fingers snap) Paris, (fingers snap) but in Paris, there are stone buildings that have been subjected to the elements for almost 1,000 years, which brings us to this patent, which I can only sort of read because it's in French.

(reads in French) And what this patent is describing, in rather poetic French, is a way to help preserve really old stone, whether it be statues, monuments, buildings, whatever.

Essentially, the process involves taking bacteria, sticking them in a solution that is primed for calcite production, and then painting that on old stone.

Once on the stone, the bacteria get to work producing calcite all around them and that includes filling in the cracks.

And in 2017, we got the clearest picture yet of what that looks like on the bacterial scale.

This is a bacterially produced chunk of calcite.

Now, it may look like any other chunk of calcite, but let's zoom in.

See these things?

They look like impressions made in something soft, like peanut butter.

But remember, we are looking at what is essentially, stone.

This is a mineral, it's calcite.

So what you're seeing here is where the bacteria were when they built calcite all around themselves, entombing themselves in stone.

(drill buzzes) The next key innovation was the idea that you could use bacteria to make concrete that's actually capable of healing itself.

The basic idea is that you put a bunch of dormant bacteria into concrete, and then later, if a crack forms and lets water in, those bacteria wake up and start producing calcite, which seals up the crack before it can get any bigger and really screw up the structure.

(drill buzzes) But if bacteria can heal existing concrete, then that raises the question, could they build new concrete from scratch?

And the super short answer to that question is yes.

This sidewalk, for example, was grown by Sporosarcina pasteurii bacteria, employed by a startup called Biomason.

Now, Sporosarcina pasteurii, which I had to learn to pronounce, builds calcite using the urea hydrolysis pathway.

Now, this right here, this is urea.

It is the main non-water component of pee.

And the bacteria use an enzyme called urease to split urea up into two pieces, carbonic acid and ammonia.

And the next thing that happens is a series of reactions that happen whenever carbonic acid is dissolved in water, which it would be inside of bacteria.

These reactions give you the carbonate ion, CO3 2-, which ordinarily, wouldn't do all that much.

But suppose there are calcium ions present because we put them there.

The next thing that happens is that the calcium and the carbonate react with each other to form calcium carbonate.

And this precipitates out of solution as calcite crystals.

Now, for all this to work you need to give the bacteria urea, calcium and food, and that costs money and energy.

But there is an alternative method, from a startup called Prometheus Materials.

Prometheus's solution is to use a completely different set of bacteria called cyanobacteria.

You might know them as blue-green algae, and they use photosynthesis to make calcite.

They make calcite from light, which is wild.

Now, I'm not gonna go through all the gory details here because they're actually not as well understood as the urea hydrolysis pathway is.

But very broadly speaking, the bacteria use an enzyme called carbonic anhydrase, to convert carbon dioxide and water into carbonic acid.

That carbonic acid loses a proton, forming bicarbonate, which reacts with calcium, that we provide, to form solid calcite.

Solid calcite, rock basically, from light.

So one process can be done in the dark but you have to feed the bacteria.

The other process doesn't require food but does require energy in the form of light.

Either way, you gotta pay the piper.

What if we could replace the billions of tons of regular concrete we use every year with bio-concrete?

With carbon neutral bio-concrete, with carbon negative bio-concrete?

That's a pretty amazing dream.

But we are very far from that reality today because whether you are feeding the bacteria or growing them under sunlight or growing them under artificial light, you are doing something that at least, for now, makes bio-concrete less convenient to produce or more expensive to produce, or both, than regular concrete.

And we all know the free market doesn't exactly go out of its way just because something is good for the environment.

Plus bio-concrete isn't nearly as strong as regular concrete yet.

So to truly compete, bio-concrete will need to be just as good or better than regular concrete when it comes to price, convenience, and strength.

Here's to hoping.