Concrete = cement + sand + gravel. Cement is the most important man-made material on Earth. Offset your carbon footprint on Wren: https://wren.co/veritasium . For the first 100 people who sign up, I will personally pay for the first month of your subscription!

▀▀▀
A huge thank you to Nevada Ready Mix for being willing to bury me in concrete, especially Elu Chavez and Mike Sherwood. https://www.nevadareadymix.com

And to Brandon Birchak of Six Foot Productions for providing the big fish bowl, safety equipment, planning and filming: https://www.sixfootcreations.com

▀▀▀
References:  
Instant stone (just add water), Roots of Progress, https://rootsofprogress.org/instant-stone-just-add-water
https://rootsofprogress.org/cement-redux

Cement Chemistry and Sustainable Cementitious Materials
https://www.youtube.com/@cementchemistryandsustaina9629

Ahmad, S., Lawan, A., & Al-Osta, M. (2020). Effect of sugar dosage on setting time, microstructure and strength of Type I and Type V Portland cements. Case Studies in Construction Materials, 13, e00364. – https://ve42.co/Ahmad2020

Seymour, L. M., Maragh, J., Sabatini, P., Di Tommaso, M., Weaver, J. C., & Masic, A. (2023). Hot mixing: Mechanistic insights into the durability of ancient Roman concrete. Science advances, 9(1), eadd1602. --  https://ve42.co/Seymour2023

▀▀▀
Special thanks to our Patreon supporters:
Emil Abu Milad, Tj Steyn, meg noah, Bernard McGee, KeyWestr, Amadeo Bee, TTST, Balkrishna Heroor, John H. Austin, Jr., Eric Sexton, john kiehl, Anton Ragin, Benedikt Heinen, Diffbot, Gnare, Dave Kircher, Burt Humburg, Blake Byers, Evgeny Skvortsov, Meekay, Bill Linder, Paul Peijzel, Josh Hibschman, Mac Malkawi, Juan Benet, Ubiquity Ventures, Richard Sundvall, Lee Redden, Stephen Wilcox, Marinus Kuivenhoven, Michael Krugman, Cy 'kkm' K'Nelson, Sam Lutfi.

▀▀▀
Written by Derek Muller
Edited by Trenton Oliver
Filmed by Raquel Nuno, Austin Bradley and Bryson
Animated by Ivy Tello & Mike Radjabov
Additional video/photos supplied by Getty Images & Pond5
Music from Epidemic Sound & Jonny Hyman: the Bill Wurtz inspired ‘Skyscrapers are made of sea shells’
Produced by Derek Muller, Petr Lebedev, & Emily Zhang

And while that's happening, I'm going to explain everything you need to know about this substance.

to clear up is the difference between cement and concrete, because people often mix these up, okay?

Now concrete is cement plus aggregate, so plus gravel and sand, and this is filling up really rather quickly.

Cement is the most important man-made substance on the planet.

We use more of it than any other substance apart from water.

Every year, 500 kilograms of cement are created for every man, woman, and child on earth.

And that amount of cement can make 2 cubic meters of concrete, which is about 2 of these big fish bowls.

I don't think people realize just how important concrete is.

Every year we make a certain weight of goods out of copper, more out of aluminum, and then you have glass, asphalt, lime, iron is a big 1 for all the steel, and then there's ceramic and wood.

But by far, the solid product we make the most of is cementitious material, essentially cement.

We use as much of it as we do all other materials combined.

It's strong and durable and inexpensive, and it is so easy to produce that people have been making a version of it for thousands of years.

To make primitive cement the key ingredient is limestone, which is basically calcium carbonate.

If you heat it up to around a thousand degrees Celsius that drives off CO2 out of the rock, leaving calcium oxide, which is also known as quicklime.

Now if you grind up the calcium oxide and mix it with water, you get an exothermic reaction that creates calcium hydroxide, which you can pour into a mold, and over a time it will absorb CO2 from the atmosphere turning back into calcium carbonate as the water evaporates.

Now that's a pretty good primitive cement, it's the first way that people ever made cement, but there are several drawbacks to it.

I mean for 1 thing you can't make the molds very big because otherwise CO2 can't penetrate all of the material and harden it.

Plus it's not gonna work underwater where there is no CO2 to harden your mixture.

The Romans discovered the solution to these problems.

They added volcanic ash called pozzolana to the crushed limestone before heating it up.

And they discovered this cement was much stronger and much more durable.

They used it to create the largest unreinforced concrete dome in the world, the Pantheon, which has stood for 2, 000 years.

And they built concrete piers into the sea, which hardened underwater, some of which are still standing today.

Like it's already like really weighing down my feet and making me slightly concerned about being able to get out.

What I can tell you is that concrete is about 3 times as dense as water.

I just want to make sure that you're not going to have some suction.

I'm going to try to lift a leg just to see how bad it's going to be.

What I like is having my feet on the bottom, and I'm not sure I can get them back down there after I lift it

I'm going to try to lift this leg pretty slowly.

So at least up to this point I am able to get out.

My fear is that the concrete is so dense that when it gets up around my chest it may apply a lot of pressure making it hard to breathe.

It's like being in a trench and having to cave in on you.

You cave in your lungs so you can breathe.

Just once they contract and the weight gets against your lungs.

But we do have oxygen on hand just in case.

Earlier, I practiced how I could get out of the bowl if I'm in trouble.

So how did Roman concrete harden underwater and in thick slabs that CO2 couldn't possibly penetrate?

There is a claim that it was superior to modern concrete.

For centuries, the Roman recipe was lost.

It was only discovered in a book in a Swiss monastery in the 1400s.

And since then, architects, scientists, and engineers have been experimenting with different cement recipes to try to achieve the best result.

Now it turns out that that incredible strength, durability, and ability to set underwater of the Roman cement, that came from the pozzolana, that volcanic ash that was added.

And nearly 2, 000 years later, people discovered that adding clay or shale to the limestone before it was crushed and heated produced the same effect.

And the reason is that all of these materials contain silica and silica totally changes the chemistry of the cement.

It means that it doesn't need to dry in order to harden.

In fact, the water becomes an integral part of that hardened concrete, so that it actually achieves maximum strength when it sets underwater.

So this is the compressive cylinder curing room.

We're required to maintain the concrete samples in 100% humidity, so we've chose to submerge them in water in a lime bath.

Every time concrete suppliers pour concrete on a job site, They cast sample cylinders of the material so they can later test it to ensure it has the strength required.

So we actually intercept a little bit of the concrete going into whatever structure they're pouring, a slab, a wall, we'll intercept it into a wheelbarrow, take the wheelbarrow over to our testing station and start casting these compressive cylinders.

Every day we're breaking these cylinders, checking it for strength.

You can see these have been broken today.

So samples are tested at 7, 14, and 28 days when the concrete is said to have reached full strength.

So what we do is we list the date that there will be placed in the compression machine on top of every cylinder.

In reality, it will continue strengthening after that.

Samples are placed in a hydraulic press and then the pressure is increased on them until they fail.

We want to maintain around 30 psi per second.

You don't want to shock it by applying too much load too fast.

This middle number is showing us our strength in pounds per square inch.

And then the top number, the big number, is showing us the force of pounds we're applying.

What are we going to get up to, like 7, 000 PSI?

It's amazingly just like, you can't see anything happening.

Like the strongest concrete in the world.

Well, the strongest concrete in the world is going

Today, virtually all concrete is made with a cement formulation discovered in the 1840s.

But the name is really just a marketing term.

You know, they claimed that the gray color of the cement would resemble these very desirable rocks which were quarried near the town of Portland, England.

But the Portland cement was made by crushing up limestone and then mixing it with a certain percentage of shale or clay to provide the silicates.

And that was all ground up into a fine powder and put in a kiln and heated to very high temperatures.

And what comes out are these nodules that are really hard.

Now it's suspected that cement chemists in the past may have produced clinker by accident when they overcooked their lime mixtures.

But since this clinker is so hard to grind down they just considered it waste.

But if you do grind it down the cement that it produces is far superior to basically any other chemistry we've discovered which is why it is so commonly used today.

Now there are lots of compounds inside Portland cement.

The most common 1 is tricalcium silicate.

Like I am now floating in concrete, which is pretty ridiculous because most of my body is out of the material, but because it's 3 times as dense as water, you can float just up to your waist.

I did not expect to be able to like float in concrete.

My feet are here and my shoulders are up and I'm just being pushed up out of it.

Once you have the cement powder, the other things you need to make concrete are the aggregates, sand and gravel.

Then they're ground up to be particular sizes.

There are very strict requirements about the sizes and shapes of the aggregate, which is going to get poured into the concrete, because of course that will affect the strength of the concrete that results.

Yeah, this is just a well-graded concrete sand.

The most well-rounded material helps the contractor as far as their finishability goes.

You know, when they're running that trowel over that slab, they want a rounded particle, not a jagged, crushed particle.

Do you really want like a spherical sand?

A rounded, yeah, a spherical sand, like a river stone, like a river sand.

Designers on the strip want to pour concrete elevated.

They want to lighten up the load of the deck.

We'll incorporate some lightweight aggregate in place of this normal weight.

Typical concrete will weigh 150 pounds per cubic foot, normal weight.

This gets us about 110 pounds per cubic foot.

The aggregates are hauled to the concrete plant in big trucks.

And then they travel on big conveyor belts up into storage piles.

These get loaded into big hoppers and then weighed out and poured into the trucks.

Control operators can open this gate and drop material into this hopper, down into the mixer truck below.

This is where the batch operator's controlling the plant.

Right here in front of our batch operator, William is the actual recipe that he's gonna dispense into the concrete mixer.

Each line represents a different component in the concrete.

We have an intermediate 3 8ths rock below it.

Over here in this column, he's targeting, for instance, 3 quarter rock.

He wants to hit 13, 000 pounds of material for that load.

The computer is gonna try to hit 13, 042 pounds of 3 quarter rock.

Now if he goes out of tolerance and he overshoots a target it's gonna change to red, a red color.

Up here on this screen we're actually showing the holding hopper, this material he's weighing is directly above the mixer truck that just pulled under the plant.

All right, I'm going to take a look inside the truck here.

That's what it looks like down inside a cement truck.

So I was interested to see what would be the difference in strength between pure cement, cement with sand, and cement with sand and gravel, like the typical concrete mix that they would make.

So I had them make up these special cylinders and test them in the hydraulic press.

You might think that since pure cement has the most glue it would be the strongest.

So if we lessen or lower the amount of cement per unit volume, we're gonna have less strength.

But when we tested the pure cement cylinder, it fractured a lot as the load was applied.

My prediction was based on the strength it was 2 weeks ago, which we have here.

We got 14 days of 6, 600 so the reason would be it would probably break another 1, 000 psi above that, but it definitely gained a lot.

Prediction was based on t weeks ago, which we we ha days of 6600.

So reason w break another 1000 psi ab gained a lot.

I was surprised to see that all the cylinders broke under about the same pressure.

Think to me, really interesting that you could have all of that cement, right?

But you don't get appreciably stronger than like this or that.

Cement is the most expensive part of concrete.

So if you can get away with reducing it down to 30% in the mix and still get the same strength characteristics, well then you should definitely do that.

The other thing that was interesting was pure cement seemed to flake and chip more as it was loaded, so it seemed like adding the aggregate actually helped the sample cohere and stay together even under all that load.

So was Roman concrete superior to modern concrete?

I mean, there were some surprising advantages that Roman concrete had.

For example, it was actually less well mixed than modern concrete.

So there were little blocks of undissolved calcium oxide or quicklime that remained inside Roman concrete.

And then what happened is when the concrete cracked and water got in there, it would dissolve that calcium oxide, forming calcium hydroxide, and then you would get the new growth of calcium carbonate.

So Roman concrete was actually self-healing.

But I would say, by and large, you know, when we look back at the Roman structures, we only see the ones that have survived to this day, so there is a survivor bias.

We could make very strong concretes that could last an incredibly long time, but we choose not to because it's just cheaper and we don't expect our buildings to last that long.

Before the concrete goes out on site, they have to make sure it's the right consistency for the customer, not too dry and not too runny.

This can be adjusted with water, but you don't really want to do that because that can also affect the strength.

So here they use modern chemicals like super plasticizers.

Up here on the top left you have our dispensing units for all our chemical admixtures.

And that's what they're adding now, a super plasticizer.

It makes the concrete easier to work and spread around without really changing the water content very much.

To check that the consistency is right, they perform something called a slump test or spread test.

We're gonna fill the cone to the top, nice and level.

And we're gonna pull that metal cone off measure the distance it travels on the board.

The concrete they're pouring on me should have a spread of 27 inches.

So why is it so important that you've got the right consistency of concrete?

You want it wet enough to flow and fill up whatever container you're trying to fill.

So the question you're probably wondering is, how long can I stay in here before the concrete hardens?

The usual answer is about 4 hours without agitation.

And that's why when you see concrete trucks driving down the road, that drum has to be turning to keep the concrete agitated and prevent it from setting up.

But what happens if the truck breaks down or there's a traffic jam or something breaks?

Then at times, concrete does harden inside the drum of these trucks and that is a terrible outcome.

See all Veritasium transcripts on Youtube

Getting Buried In Concrete To Explain How It Works