Limestone is an important raw material in industry.
It is crushed and heated with clay (or shale) in rotating kilns to make cement. The cement is used to make concrete, when added to sand and stones (crushed rock), then mixed with water.
Sand, limestone, and sodium carbonate (soda) are heated together to make glass. Powdered limestone is also used to neutralize acidic lakes, soils, and gases from industrial processes.
Cement is the most important material used in the building industry. We use this pale grey powder to make concrete and mortar. Before we find out more about these materials, let's see how cement made. Limestone is crushed up with clay (or shale) and heated in large rotating kilns, as shown in Fig.1 below.
Figure 1. A rotating kiln in a cement factory
A little calcium sulfate powder is added to the cement to stop it setting as soon as water is mixed in.
You might have seen cement factories. You can recognize them by the light-coloured dust that covers the site. If the dust is allowed to escape into the surrounding countryside, it can cover the leaves of plants, making photosynthesis difficult. The grinding machinery that crushes the limestone is also noisy.
Cement powder is mixed with sand and small stones to make concrete (the most widely used building material in the world). When mixed with water, the concrete sets to form a hard, rock-like material.
Sometimes, we strengthen the concrete with iron rods. This reinforced concrete is made by pouring the wet mixture into moulds with the iron already in place. The setting is a slow chemical reaction that can take years to finish completely, although it is hard enough to use in a few days.
The reaction that takes place is complex. We can think of it as crystal 'fingers' of calcium hydroxide spreading through the concrete, holding the sand and small stones in place.
Glass has been made and used for a very long time. The earliest glass object has been dated at about 4500 BC. Glass was probably discovered by chance in the sand beneath some ancient fire. Sand is the main raw material we use to make glass. The actual mixture used varies, but a typical mix for an inexpensive glass (called soda-lime glass) is shown in Fig.2 below:
Figure 2. Raw materials used to make glass
Recycled glass (cullet) is becoming increasingly important and some glass mixtures contain up to 30 per cent cullet.
Over the years, scientists have experimented with the mix of materials used in glass and have invented glasses with many different properties.
Extraction of iron
Limestone is part of the mixture added to the top a blast furnace in the extraction of iron from its ore. (You can find out details of this process in the unit Extraction of Iron.) In Fig.3 below, you can look more closely at the role limestone plays in removing acidic impurities from the iron ore in the furnace.
Figure 3. Limestone in the blast furnace
The liquid slag is tapped off and cooled. It is used in the construction industry for making roads.