Rock cycle index

The Rock cycle

Rock formation

Erosion and weathering

Identification of rocks



Crystal structures of rocks








Cement is our most important building material. It surrounds us most of the time : at home, school, etc. Nevertheless, many people don't know what it is made of.

Cement is produced out of limestone and clay which are extracted from quarries.

When clay (alumino silicates) is submitted to high temperatures, the onset of the process of cementation sticks all its grains together, and the result is a hard material, a rock.

When the cementing material is silica (silicon dioxide), a very hard rock is produced. The silicon-oxygen is one of the strongest chemical bonds known. Other materials play this role as well. Rocks that are cemented by limestone are not as hard and durable.

Thise process occurs underneath the earth's crust, where the temperatures are high enough. However, we can try to mimic these conditions of extreme heat in an oven. When you produce bricks, clay is fired in an oven,usually called kiln. This process will cause cementation and a solid brick will result. Latin american natives heated clay by exposing it to sunlight only. These process results in a relatively weak brick (adobe), which meets their needs.

In more extreme weathers like in Britain, you need more solid bricks (less porosity, waterproof) . We can see how this basic needs triggered the development of science in countries like England, Germany, Netherlands etc).

Many ancient civilizations managed to produce bricks. Cement, however, was not so widespread. That is because it is a more technologically demanding business

To produce cement, clay and limestone are heated in a kiln. The result is a mixture of hot lumps, which are transformed in powder.

Curiously, the cementation process doesn't occur in the kiln, and the resultant material (cement) will wait for water so that it solidifies. Water causes the onset of cementation. The presence of limestone stops the clay from cementing in the kiln. This way, you have a more versatile material which can be used to bind stones, sand, and bricks together, only when you add water.

The mixture of cement, sand and small stones is called concrete. It is analogous to a conglomerate, which is a kind of sedimentary rock. The forces that hold the grains together resist to high forces of compression . However, they are weak with respect to forces on the opposite direction (stretching forces) so that it has a low tensile strength.

Nowadays we also use steel bars inside the beams of concrete , which gives it much more resistance to traction (reinforced concrete).