1: Chemical Structure

Concrete is the most widely abundant engineering material in the world today in terms of volume used.

Concrete is not officially a ceramic but is rather a composite made up of sand, aggregate, and cement. With the addition of water the cement reacts to form a ceramic like structure around the sand and aggregate particles. The strength of the concrete is gained from a reaction between the water and the cement.

Since the nineteenth century the most commonly used form of cement has been Portland cement.

The Portland cement binder comes as a very fine powder and is composed of various ratios of 3CaO - Al2O3 - 2CaO - SiO2,3CaO - SiO2 and other minerals. For simplicity we will use the following reduced nomenclature.

  • C = CaO (lime)
  • A = Al2O3 (alumina)
  • S = SiO2 (silica)
  • H = H2O (water)

The cement is prepared by firing a controlled mixture of chalk (CaCO3) and clay in a kiln at 1500°C. This process provides a combination of three products given above, ie. C3A, C2S, and C3S.

Upon the addition of water the first reaction to occur is the hydration of the C3A. This reaction occurs within the first four hours and causes the cement to set.

The second and third reactions are much slower taking approximately 10 hours to commence and up to 100 days to complete. They are, however, the main reactions that cause the cement to harden. The reactions are the hydration of C2S and C3S to tobomorite gel, the principal gel that makes up 70% of the structure.

In some cold countries the evolution of heat from the reaction process is beneficial because its stops the water in the cement from freezing. However, in the construction of large structures, such as dams, the evolution of heat can be a problem. In these cases, cooling pipes are placed through the structure and left there as a form of reinforcement afterwards.