Why is cement a powder form but hard as stone when mixed with water?

Cement is one of the most useful materials in the construction industry. Cement itself is a soft powder, but when mixed with water, it hardens and can combine with sand or gravel to become a very solid object.

Cement is the main component of mortar and concrete.
Masonry mortar is a mixture of cement, sand, and water.
Concrete is a type of mortar mixed with gravel.

Why Does Cement Powder Harden When It Comes in Contact with Water?

Cement is one of the important materials in the construction industry.

Currently, cement is produced by heating limestone, clay, or mineral slag at high temperatures. These materials must be heated together until they form a sintered product resembling glass, which is then crushed into powder.

When water is added to cement powder, a very complex chemical reaction occurs, resulting in the formation of an artificial stone that is insoluble in water and has high abrasion resistance.

Cement reacts with water to produce compounds such as calcium silicate hydrate, ettringite, and calcium aluminate silicate monohydrate. These compounds are solid and have a crystalline structure. They bond together, causing the cement to harden.

So, what kind of chemical reaction occurs? How does cement harden? This is a question that chemists have yet to find a precise answer to.

Cement primarily consists of four compounds. It is believed that after water is added, these compounds transform into crystals. These crystals interlink, forming a type of hard stone.

Cement was created by Joseph Aspdin (an Englishman) in 1824. Prior to that, mankind had no concept of cement; constructions were typically made of wood or mud.

During ancient Roman times, people used volcanic ash and lime mixed with water as a binding agent, enhancing the sturdiness of buildings. However, at that time, people did not understand why this mixture had such an effect. The term cement (in English) originates from the Latin term “opus caementicium” (the binding agent of the Romans).

By the early 19th century, through research, Joseph Aspdin discovered that when lime, clay, and slag were mixed and fired in a specific ratio, the resulting product could be hardened directly by adding water after grinding, creating a very strong structure. He named this product “Portland cement” due to the unusual resemblance of the material to Portland stone, a type of limestone used for construction in England.

After Joseph Aspdin applied for a patent, cement began to be produced and used in large quantities. This can be considered one of the most important inventions, making a significant contribution to humanity.

The main materials for cement production are limestone, clay, and fine sand.

How is Cement Produced?

Ordinary cement is generally Portland cement, with the primary raw materials being limestone, clay, and fine sand. Limestone mainly provides calcium carbonate, while clay and fine sand provide silica. To save time and transportation costs, cement plants are often built right next to open-pit limestone quarries.

Limestone is extracted using blasting technology. The mined stones are very large, so crushers are needed to break them down. The crushed limestone is then mixed with clay, sand, and other materials in specific ratios, ground together, and fired. The temperature inside the kiln can reach up to 1500°C. The firing process is key to the formation of cement. Through firing, the raw materials undergo a series of chemical reactions to create a binding material that can be hardened by adding water.

In 2010, the global cement production was 3.3 billion tons. The three countries that produce the most cement in the world are also the three most populous countries on the planet: China, India, and the United States, accounting for more than half of the total cement production worldwide. These three countries also lead the world in cement reserves, holding nearly half of the total reserves globally.