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. The first thing to clear up is the difference between cement and concrete: cement is like the glue, the matrix of stuff, while concrete is cement plus aggregate (gravel and sand). Cement is the most important man-made substance on the planet, with 500 kilograms created for every person on earth every year. This amount of cement can make two cubic meters of concrete, which is about two big fishbowls. It’s easy to see why concrete is so important: it’s liquid rock, strong and durable, and can be poured into any shape. People have been making a version of it for thousands of years. To make primitive cement, the key ingredient is limestone, which is heated up to 1000 degrees Celsius to drive off CO2 and create calcium oxide. Grinding up the calcium oxide and mixing it with water creates an exothermic reaction that creates calcium hydroxide, which can be poured into a mold. The Romans added volcanic ash called pozzolana to the crushed limestone before heating it up, creating a much stronger and more durable cement. I practiced how I could get out of the bowl if I’m in trouble, but 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. Was it?
For centuries, the Roman recipe for creating concrete was lost until it was discovered in a book in a Swiss monastery in the 1400s. Since then, architects, scientists, and engineers have been experimenting with different cement recipes to try to achieve the best result. It turns out that the incredible strength, durability, and ability to set underwater of the Roman cement came from the pozzolana, a type of volcanic ash that was added.
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. This is because all of these materials contain silica, which totally changes the chemistry of the cement. This means that it doesn’t need to dry in order to harden. In fact, the water becomes an integral part of the hardened concrete, allowing it to achieve maximum strength when it sets underwater.
Today, virtually all concrete is made with a cement formulation discovered in the 1840s known as Portland cement. This name is really just a marketing term as it was claimed that the gray color of the cement would resemble the desirable rocks quarried near the town of Portland, England. Portland cement is made by crushing up limestone and then mixing it with a certain percentage of shale or clay to provide the silicates. So what I found was that the concrete mix with sand and gravel was the strongest, followed by the cement with sand, and then the pure cement was the weakest.
Ground up into a fine powder and heated to very high temperatures, clinker is produced. It is suspected that cement chemists in the past may have produced clinker by accident when they overcooked their lime mixtures, but since it is so hard to grind down, it was considered waste. However, the cement that it produces is far superior to most other chemistries, which is why it is so commonly used today.
The most common compound inside Portland cement is tricalcium silicate. Surprisingly, it is three times as dense as water, so one can float just up to their waist.
The other components needed to make concrete are the aggregates, sand, and gravel, which are blasted out of a quarry and ground up to particular sizes. There are very strict requirements for the sizes and shapes of the aggregate, as it will affect the strength of the concrete.
When comparing the strength of pure cement, cement with sand, and cement with sand and gravel, the concrete mix with sand and gravel was found to be the strongest, followed by the cement with sand, and then the pure cement was the weakest. If it’s too dry, it won’t stick together and it won’t be strong enough.And if it’s too wet, it’s gonna be too weak.So it’s important to get the right consistency to make sure the concrete’s strong and durable.
I was surprised to see that all the cylinders broke under about the same pressure, regardless of the amount of cement used. This showed that cement is the most expensive part of concrete, but reducing it to 30% in the mix can still give the same strength characteristics. Additionally, Roman concrete was actually less well-mixed than modern concrete, and had the advantage of being self-healing. However, modern concrete can be adjusted with chemicals like superplasticizers to ensure the right consistency without affecting the strength, and this is checked with a slump test. It is important to get the right consistency to make sure the concrete is strong and durable. Doing your best to stay in the concrete is a difficult task - the buoyancy of the mixture prevents it from happening. But how does concrete harden? The dry mixture of gravel, sand, and cement powder is mixed with water, which starts to dissolve the cement powder grains and release calcium hydroxide ions into the solution. This allows crystals to form of calcium silicate hydrates, plus other hydrate minerals, and as these crystals grow and become interlocking, the concrete hardens. The water is essential to the process, as it becomes part of the solid concrete material. In order to slow the concrete down from setting, you can add regular pop like Coke - the sugars inside this coke will prevent the setting up process from happening and buy you a few hours. Lastly, the core component of cement and concrete is limestone, which is formed from the skeletons and shells of ancient sea organisms that died millions of years ago. So when you look at a city skyline, you’re really seeing ancient marine life - skyscrapers are made of seashells! Derek exclaims: “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? So 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. That is 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.**