Rate Of Hydration
The reaction of compound C3A with water is very fast and is responsible for flash setting of cement (stiffening without strength development) and thus it will prevent the hydration of C3S and C2S. However, calcium sulphate (CaSO4) present in the clinker dissolves immediately in water and forms insoluble calcium sulphoaluminate. It deposits on the surface of C3A forming a colloidal membrane and consequently retards the hydration of C3A. The amount of CaSO4 is adjusted to leave a little excess of C3A to hydrate directly. This membrane in the process breaks because of the pressure of the compounds formed during hydration and then again C3A becomes active in the reaction.
The hardening of C3S can be said to be catalyzed by C3A and C3S becomes solely responsible for gain of strength up to 28 days by growth and interlocking of C-S-H gel. The increase in strength at later age is due to hydration of C2S.
Notes: 1. The development of strength of the four principal compounds of cement with age is shown in Fig. 21
2.The rate of heat evolution of the compounds if equal amount of each is considered will be in the following descending order:
C3A,C3S, C4AF, C2S
3.The rate of hydration is increased by an increase in fineness of cement. However, total heat evolved is the same. The rate of hydration of the principal compounds is shown in Fig. 5.2 and will be in the following descending order:
C4AF, C3A, C3S, C2S
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