WHAT IS MEANT BY HYDRATION OF CEMENT ?

HYDRATION OF CEMENT

The chemical reaction between cement and water is known as hydration of cement. The reaction takes place between the active components of cement and water. 

Active components are :

Tetracalcium Aluminate (C4AF)

Tricalcium Aluminate (C3A)

Tricalcium Silicate (C3S)

Dicalcium Silicate C2S)

Gypsum (CSH2)

The factors responsible for the physical properties of concrete are the extent of hydration of cement and the resultant microstructure of the hydrated cement. When the cement comes in contact with water, the hydration products start depositing on the outer periphery of the nucleus of hydrated cement. This reaction proceeds slowly for 2-5 hours and is called induction or dormant period. As the hydration proceeds, the deposit of hydration products on the original cement grain makes the diffusion of water to unhydrated nucleus more and more difficult, consequently reducing the rate of hydration with time.  At any stage of hydration, the cement paste consists of gel (a fine-grained product of hydration having large surface area collectively), the unreacted cement, calcium hydroxide, water and some minor compounds.

The crystals of the various resulting compounds gradually fill the space originally occupied by water, resulting in the stiffening of the mass and subsequent development of the strength.

The hardened cement paste :

Hardened cement 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)

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 insolubleB 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.  

              Contribution of cement compounds to strength of cement with increasing age.

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.

                                            Rate of hydration of pure cement compound