I am about to get buried in concrete. While that’s happening, I’m going to explain everything you need to know about this substance. So the first thing that I want to clear up is the difference between cement and concrete, because people often mix these up. Cement is like the glue; it’s the matrix of stuff. Concrete is cement plus aggregate, like gravel and sand. 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 two cubic meters of concrete. This video is sponsored by Wren.

People don’t realize just how important concrete is. Every year, we make a certain weight of goods out of copper, aluminum, glass, asphalt, lime, iron, ceramic and wood. However, 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. Concrete is liquid rock; you can pour it into any shape you like. It’s strong, durable and inexpensive.

The Romans discovered a solution to the drawbacks of primitive cement. They added volcanic ash called pozzolana to the crushed limestone before heating it up, which made the cement much stronger and more durable. They used it to create the largest unreinforced concrete dome in the world, the Pantheon, which has stood for 2,000 years. They also built concrete piers into the sea, which hardened underwater; some of which are still standing today.

I didn’t expect the concrete to feel so heavy, and it’s already weighing down my feet. I’m worried that when it gets up around my chest, it may apply a lot of pressure, making it hard to breathe. Earlier, I practiced how I could get out of the bowl if I’m in trouble, and we have oxygen on hand, just in case. Do you feel that’s okay?

That’s no problem.

Okay, great. 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. So, was it?

For centuries, the Roman recipe for concrete 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 the incredible strength, durability, and ability to set underwater of the Roman cement, came from the pozzolana, a 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. This strength increases over time, 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.

All right, ready?

Yeah, let’s do it.

I’ll start loading.

Sure.

We want to maintain around 30 psi per second. And it’s important to load at that rate.

Yeah, you don’t want to shock it. 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 gonna get up to, like 7,000 psi?

10,000.

So this is gonna be really strong.

Very strong.

Okay.

Yeah. It’s always fun when we get something over 10,000 in a free show. Bang!

It’ll shake this room.

And it has reinforced concrete walls.

It’s amazingly just like you can’t see anything happening even under all that pressure.

It’s still holding.

I know, right?

Oh, here it goes. It’s starting to dry.

Really?

Here it goes. 11,000.

Mm-hmm.

Over 11,000 now. (Cement cylinder pops) Oh yeah!

It still got a life. Having it, it didn’t even…

Fun. (Derek laughs)

It’s my favorite part.

(Cement cylinder blows) Yeah, that’s cool!

Let’s see.

Yeah, I like that. Like the strongest concrete in the world.

Well, the strongest concrete in the world is gonna be like a lab-only thing.

In a competition, it was above 100,000 psi. Today, virtually all concrete is made with a cement formulation discovered in the 1840s. It’s known as Portland cement, 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. So I made three different samples and tested them.The pure cement sample was the weakest at about 2,400 pounds per square inch.The cement and sand sample was a little bit stronger at 2,800.But the cement, sand, and gravel sample was the strongest at 4,200 pounds per square inch.

Ground up into a fine powder, heated in a kiln and then cooled, clinker is produced. It is thought that cement chemists in the past may have produced clinker by accident when they overcooked their lime mixtures, but since the clinker is so hard to grind down, it was considered waste. However, if the clinker is ground down, it produces a cement that is far superior to other chemistries, which is why it is so commonly used today.

The most common compound found in Portland cement is tricalcium silicate. Surprisingly, it is three times denser than water, which allows one to float in concrete up to their waist.

In order to make concrete, other materials are needed such as aggregates, sand, and gravel, which are blasted out of a quarry and ground to particular sizes. The most well-rounded material helps the contractor with finishability.

The cement powder, aggregates, sand, and gravel are loaded into a concrete mixer truck, where a batch operator controls the plant. The recipe of the concrete is dispensed into the mixer truck, and the computer will try to hit a certain weight target.

Finally, the difference in strength between pure cement, cement with sand, and cement with sand and gravel was tested. The pure cement sample was the weakest at about 2,400 pounds per square inch, the cement and sand sample was a little bit stronger at 2,800, and the cement, sand, and gravel sample was the strongest at 4,200 pounds per square inch. If it’s too dry, it’s gonna have weak spots in it.It’s gonna be harder to work with.And then if it’s too wet, it’s gonna be too runny, and it’s gonna be hard to get it up and in the forms.

I was surprised to see that all the cylinders broke under about the same pressure. This indicated that cement is not necessarily the most important factor in creating strong concrete. Reducing the amount of cement per unit volume can still provide the same strength characteristics.

To ensure the correct consistency of concrete, modern chemicals like superplasticizers are used. This makes the concrete easier to work with without changing the water content too much. The slump test is used to check that the consistency is right. If the concrete is too dry, it will have weak spots, and if it is too wet, it will be too runny and hard to get into the forms. You want it wet enough to flow and fill up whatever container you’re trying to fill, in this case, the sphere with you in it. So the question you’re probably wondering is, how long can I stay in here before the concrete hardens? The usual answer is about four 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. But there is one thing you can do to slow the concrete down from setting and that is to add some regular pop like Coke. The sugars inside this coke actually prevent the setting up process from happening and that can buy you a few hours. So apparently these truck drivers, they drive around with a few two-liter bottles of pop inside their cab and so they can dump it in their load if they have to to prevent it from setting up. So hopefully that means I’ll get out okay too.

But how does concrete actually harden? Well, you have the dry mixture of gravel, sand, and cement powder and then you add water. The water starts dissolving the cement powder grains, so ions enter solution, and some of those ions are calcium hydroxide. So that’s what makes concrete a very basic solution. What do you see? - We’re approaching 12, 11.8. So the pH of concrete can get up to 12 or 13. The pH of this is 11.8 and that’s incredibly basic, which means if it’s on your skin, if it’s on your body, it can be dissolving your skin and cells. Being buried in concrete is sort of like jumping in a bath of bleach. And that’s why I’m actually wearing a dry suit and some latex gloves. Yeah, it’s not something you want to try at home. So do not try swimming in concrete.

So now the ions are dissolved in the solution. And remember, the most common compound in cement is tricalcium silicate. As it reacts with the water, crystals start to form of calcium silicate hydrates, plus other hydrate minerals. And all these crystals grow and become interlocking, causing the concrete to harden. Note that the water is essential to the formation of these crystals. So water is not evaporating, it’s not drying out. It’s actually becoming part of the solid concrete material. And that’s why this chemistry is called cement hydration. It’s also why freshly poured concrete should be kept in as damp an environment as possible. Las Vegas is so dry they frequently set up misters to spray around new concrete to ensure the humidity was high enough.

One of the things I realized while making this video is that limestone, the core component of cement and concrete, well it comes from ancient sea life. Limestone is formed from the skeletons and shells of ancient sea organisms that, you know, died millions of years ago, and then all of that got compressed. And now we use that to make huge skyscrapers and, you know, overpasses, basically every huge piece of infrastructure, it’s made with concrete. And so now when you look at a beautiful city skyline, what you’re really seeing is ancient marine life. Skyscrapers are made of seashells. Derek exclaims: “One, two, three. That’s great! That feels so good. I, like, don’t wanna get out. This is so nice. Has anyone had a time check? This stuff is not gonna set up on me, is it?”

As we’ve seen, concrete is one of the most important materials in the world. It’s made possible most of the large-scale infrastructure we rely on, but it also creates a lot of CO2, an estimated 8% of the global total - more than the entire aviation sector.

But together, we can do something about that. I would personally like to offset one month of your carbon emissions. And I will do that through this video sponsor, Wren. Wren is a website where you can calculate your carbon footprint, see which aspects of your lifestyle make the biggest contribution, and learn how to reduce your impact. And then if you like, you can offset your carbon footprint by funding a diverse mix of carbon reduction projects like tree planting, mineral weathering, and rainforest protection.

For the first hundred people to sign up, I will personally offset your first month of emissions. Just click on the link in the description.

Now, I don’t think we’re going to solve climate change using individual action alone. If you can change your light bulbs or install solar, that is great. But what we really need is change on a systemic scale. And that’s what I like about Wren’s approach. They not only plant trees and protect rainforests, they also support policy groups lobbying for change, like the Clean Air Task Force, which advocates for new technologies and policies to get to a zero-emissions economy.

The way I see it, there are moneyed interests lobbying to keep things the way they are. So we need to band together through organizations like Wren to lobby for change. And if you agree with me, then I invite you to click the link in the description and join me in offsetting our carbon emissions and investing in large-scale systemic change.

So I want to thank Wren for sponsoring Veritasium, and I want to thank you for watching.