Hydration of Portland Cement

Portland cement is a hydraulic cement, hence it derives its strength from chemical reactions between the cement and water. The process is known as hydration.

Cement consists of the following major compounds (see composition of cement):

Chemical reactions during hydration
When water is added to cement, the following series of reactions occur: The garnets only take up space and do not in any way contribute to the strength of the cement paste.

The hardened cement paste
Hardened paste consists of the following:

Ettringite                                     - 15 to 20%
Calcium silicate hydrates, CSH     - 50 to 60%
Calcium hydroxide (lime)             - 20 to 25%
Voids     - 5 to 6% (in the form of capillary voids and entrapped and entrained air)
It can therefore be seen that each of the compounds in cement has a role to play in the hydration process. By changing the proportion of each of the constituent compounds in the cement (and other factors such as grain size), it is possible to make different types of cement to suit several construction needs and environment.

Sidney Mindess & J. Francis Young (1981): Concrete, Prentice-Hall, Inc., Englewood Cliffs, NJ, pp. 671.

Steve Kosmatka & William Panarese (1988): Design and Control of Concrete Mixes, Portland Cement Association, Skokie, Ill. pp. 205.

Michael Mamlouk & John Zaniewski (1999): Materials for Civil and Construction Engineers, Addison Wesley Longman, Inc.,