Apr 2017 PORTLAND CEMENT: HOW IT’S MADE
Portland cement is well-known and most widely used in the construction industry but how is it made? It was developed from natural cement made in Britain from the middle of the 18th century. Its name is derived from its similarity to Portland stone, a type of building stone quarried on the Isle of Portland in Dorset, England.
The development of modern Portland cement began in 1756 when John Smeaton experimented with combinations of different limestones and additives relating to the construction of a lighthouse known as Smeaton’s Tower.
Most people are aware of and familiar with the normal, grey cement known as Portland cement and below is the process of making the most common type of cement used today.
Firstly, to make Portland cement, you need to mine or extract the raw materials from the quarry:
- Argillaceous shales (shales are a fine, clastic sedimentary rock made up of a mix of clay minerals and other minerals such as quartz. Argillaceous minerals mean the minerals contain substantial amounts of clay-like components and may appear silver)
- Schists (schists are coarse, metamorphic rock made up of layers of minerals and can easily be split into sheets or flakes)
- Calcareous nodules (calcareous means a component containing lime or being chalk-like and calcareous nodules are small, irregular lumps made up of calcium carbonate)
- Iron-bearing aluminium-silicates (A chemical compound made up of aluminium, silicon and oxygen found in certain minerals)
The main raw materials are usually mined on site and the other minor raw materials can be brought in from other quarries. Mining limestone requires drilling and blasting techniques, but technology ensures that vibration, dust and noise are kept low.
Depending on the size of the mined raw materials, they might be crushed before being stored until they are required.
The next step is to crush these raw materials into a very fine powder. This is then blended in the correct proportions. This mix is called the kiln feed as it is sent to be heated in a rotary kiln. The kiln reaches temperatures up to about 1 500˚C.
The rotary kiln is a tube (which can be up to 200m long and 6m in diameter) with a flame at the end. The kiln feed enters the rotary kiln from the cool side and moves towards the end with the flame. At the end of the kiln, the feed falls out and cools down again.
The kiln creates the material known as clinker. The intense heat causes chemical and physical reactions that melt the kiln feed into clinker. The clinker is usually rounded nodules between 1mm and 25mm across.
Next, the clinker is cooled with air and the coolers create clinker minerals which define the performance of the cement. After it has cooled, the clinker can be stored in a clinker store, or it can move on to the cement mill.
At the cement mill, the cooled clinker is ground into a fine powder to become Portland Cement. Small amounts of gypsum (a type of calcium sulphate) are also usually ground up with the clinker. This helps to control the setting properties of the cement when mixed with water.
Finally, the cement is checked to ensure it meets the standard and is stored in silos. It is then sent to be packed in bags.
Sephaku Cement has two cement plants namely, Aganang in Lichtenburg, Delmas milling plant in Mpumalanga, as well as a fly ash plant in Kendal.
Contact Sephaku Cement to learn more about our production of Portland cement, the different types of cement as well as our concrete and mortar options. Sephaku Cement wants to help you build strong with excellent quality products.