Cement

Cement is a special powder.

It acts like glue for building. We mix it with sand and small rocks. This makes a strong mix called concrete. Most concrete is used to build things. It is used more than almost anything else on Earth. Some cement can even get hard under water. This is because it reacts with water to get strong. It helps us make big walls and roads.

It is a very important part of our world.

Cement is a special powder used for building. It acts like a binder. A binder is a substance that holds other things together.

People rarely use cement alone. They mix it with sand and gravel. This mixture is called concrete. Concrete is used more than almost any other material on Earth.

There are two main kinds of cement. The most common kind is hydraulic cement. It gets hard through a process called hydration. This means it reacts with water to set and harden. Because of this, it can even set under water.

Ancient Romans used this kind of cement. They mixed lime with volcanic ash.

Another kind is non-hydraulic cement. This kind does not set under water. It only hardens as it dries in the air. It reacts with carbon dioxide in the air to get hard. This is called carbonation.

Making cement uses a lot of power. We must heat raw materials in a hot kiln. This process lets out a lot of gas into the air. In 2021, the world made about 4.4 billion tonnes of cement.

Cement is a special substance used for building. It acts as a binder. A binder is something that holds other materials together. People rarely use cement by itself. Instead, they mix it with sand and gravel. This mixture is called aggregate. When cement is mixed with this aggregate, it becomes concrete. Concrete is used more than almost any other material on Earth. Only water is used more often by people on our planet.

There are two main ways cement works. The most common way is called hydraulic cement. This type hardens through a process called hydration. This means the dry ingredients react with water to set and get hard. Because of this reaction, hydraulic cement can even set under water. This makes it very useful for building in wet places. The reaction creates mineral hydrates that do not dissolve easily in water. This protects the hardened material from being washed away.

A second type is called non-hydraulic cement. This kind is less common. It cannot set under water or in wet conditions. Instead, it sets as it dries in the air. It reacts with carbon dioxide in the air to get hard. This specific way of hardening is called carbonation. Because it needs to touch the air to work, it is not good for underwater use.

People have used cement for a very long time. Ancient Romans were famous for using hydraulic cement. They mixed lime with volcanic ash from a place called Pozzuoli. This ash is known as pozzolana. This special mixture allowed them to build huge things like the Pantheon dome. They even used it to build long aqueducts to move water. Later, in the 1750s, an engineer named John Smeaton studied how to make better mortar. He found that clay in limestone helped cement set.

Making cement today is a huge job. In 2021, the world made about 4.4 billion tonnes of cement. China makes about half of all the cement in the world. India and Vietnam are also very large producers. Making cement requires heating raw materials in a very hot kiln. This process releases a lot of gas into the air. In 2018, cement production was responsible for nearly 8% of global emissions. However, concrete can slowly reabsorb some of that gas from the air.

Cement is a chemical binder used extensively in the construction industry. A binder is a substance that sets, hardens, and adheres to other materials to hold them together. In most building projects, cement is not used alone. Instead, it is mixed with sand and gravel, which are known as aggregates. When cement is mixed with fine aggregate, it creates mortar for masonry. When mixed with larger sand and gravel, it produces concrete. Concrete is the most widely used material on Earth. It is second only to water in terms of how much humans consume globally.

There are two primary categories of cement based on how they harden: hydraulic and non-hydraulic. Hydraulic cement is the most common type used today. It hardens through a process called hydration. This occurs when the dry ingredients undergo a chemical reaction with water. This reaction creates mineral hydrates that do not dissolve easily in water. Because of this, hydraulic cement can set in wet conditions or even underwater. This ability also protects the hardened material from being damaged by chemical attacks.

To understand hydraulic cement, one must look at the chemistry of clinker. Clinker is the material produced during the high-temperature sintering process in a kiln. Hydraulic cement is made from a mixture of silicates and oxides. There are four main mineral phases in clinker, often described using cement chemist notation. These include alite (C3S), belite (C2S), tricalcium aluminate (C3A), and brownmillerite (C4AF). The silicates are responsible for the mechanical strength of the cement. Meanwhile, the tricalcium aluminate and brownmillerite help form a liquid phase during the heating process.

The production of these minerals involves several specific chemical steps. First, limestone, which is calcium carbonate, is burned in a kiln. This is called a calcination reaction, which removes the carbon and produces lime, or calcium oxide. This step is a major source of global carbon dioxide emissions. The lime then reacts with silicon dioxide to create dicalcium silicate and tricalcium silicate. Additionally, lime reacts with aluminium oxide to form tricalcium aluminate. Finally, calcium oxide, aluminium oxide, and ferric oxide react together to produce brownmillerite.

Non-hydraulic cement is a less common alternative that functions differently. It does not require water to set and cannot be used underwater. Instead, it sets as it dries by reacting with carbon dioxide in the atmosphere. This process is known as carbonation. For example, slaked lime is made by mixing calcium oxide with water to create calcium hydroxide. Once the excess water evaporates, the carbonation process begins. Because the concentration of carbon dioxide in the air is relatively low, this reaction is quite slow.

Humans have used various forms of cement for thousands of years. The Babylonians and Assyrians used bitumen, a type of asphalt, to bind bricks. Ancient Egyptians used a mortar made of sand and burnt gypsum. The Greeks and Romans made massive leaps in technology by developing hydraulic mixtures. The Romans used volcanic ash from Pozzuoli, called pozzolana, mixed with lime. This allowed them to build massive structures like the Pantheon dome and long aqueducts. Even in Mesoamerica, pre-Columbian builders used a pozzolanic cement made of volcanic ash and lime to build in El Tajin.

In the 18th and 19th centuries, engineers formalized the science of cement. John Smeaton studied how the clay content in limestone affected the ability of lime to set underwater. Later, James Parker patented "Roman cement" in 1796, which was made by burning clay-rich nodules. Eventually, Portland cement became the industry standard in the 1850s. Today, cement production is a massive global industry. In 2021, world production reached approximately 4.4 billion tonnes per year. China produces about half of this total, followed by India and Vietnam.

While cement is essential, its production has a significant environmental impact. In 2018, cement production was responsible for nearly 8% of global emissions. These emissions come from heating raw materials in kilns and the calcination process itself. However, there is a natural counter-process. As hydrated products like concrete age, they gradually reabsorb carbon dioxide from the atmosphere. This process, called carbonation, compensates for approximately 30% of the initial emissions produced during manufacturing.