The Chemical Composition of Concrete

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Concrete is an important part of modern infrastructure. It is used in buildings, roads, bridges and dams. It is known for its high compressive strength and versatility, which makes it an ideal material for the basis of most structures.

Concrete is actually a mixture of cement (the binder), water and some form of aggregate (the filler). This means that concrete is a composite material. In addition to this, cement is also a compound material, as it is a mixture of limestone and clay. It is made by burning the two compounds together at extremely high temperatures ranging from 1400 - 1600°C.

While there is a range of cements available on the market - in addition to new research into sustainable alternatives - the most popular type of cement is known as Portland cement. Portland cement uses crushed CaCO3 (also known as limestone), mixed with clay, sand and iron ore to form a homogeneous powder.

This powder is heated to the high temperatures discussed previously. To achieve these temperatures, the mixture is poured into kilns which consist of long steel cylinders that are rotated on an incline. Depending on the size of the kiln, the materials can take up to 2 hours to pass slowly through the cylinder. The slow process allows the different elements of the material to react. The reaction of these materials involves the following processes:

  1. Evaporation – the first stage of the process is the loss of water from the mixture due to evaporation.
  2. Calcination – decomposition occurs in the dry mixture due to the loss of water and carbon dioxide.
  3. Clinkering – the mixture then undergoes a transformation in which calcium silicates are formed. These are pieces that are the size of marbles.
  4. Cooling – once the mixture leaves the kiln, it is allowed to cool to working temperatures.

The cooled clinker is then ground once more, and a compound known as gypsum is added to the mixture. This is in order to regulate the setting of the mixture. In Portland cement, 5% of its chemical composition is the gypsum mineral.

The major compounds that make up Portland cement are tricalcium silicate, dicalcium silicate, tricalcium aluminate, tetracalcium aluminoferrite and gypsum. Once this process is complete, the cement is packaged and stored for use in concrete at a later date.

Concrete can be created on site with the use of a rotating metal drum, known aptly as a cement mixture. The cement is rehydrated with water to make a thick consistency and large or fine aggregate is added depending on its intended use.

Aggregates are an important part of the concrete mixture as they determine the desired characteristics of the concrete. All aggregates are known to be chemically inert but vary in shapes, sizes and materials. The most commonly used are a mixture of fine sand and coarse stone. They also make up the largest portion of the concrete’s material composition, ideally between 70-80% of the volume.  Concrete then must be vibrated, in order to release any air bubbles which may compromise the structural integrity of the material. Once poured, concrete needs at least 28 days to cure to full strength.

References

  • Bye, G. C. Portland Cement: Composition, Production and Properties. Pergamon Press, NY, 1983.
  • Hewlett, P. C., and Young, J. F. "Physico-Chemical Interactions Between Chemical Admixtures and Portland Cement," Journal of Materials Education. Vol. 9, No. 4, 1987.
  • Roy, D. Instructional Modules in Cement Science. Pennsylvania State University, PA, 1985.

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Isabelle Robinson

Written by

Isabelle Robinson

Isabelle Robinson is a freelance writer for a variety of AZoNetwork sites and is based in the UK. She graduated from Heriot-Watt University in 2015 with a BEng (Hons) degree in Civil Engineering. She also recently achieved an MSc degree, with merit, in Structural Engineering at the University of Salford.

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