Hydration of cement | Definition - Learncivil


HYDRATION OF CEMENT

The chemical reaction between cement and water is called hydration or cement hydration. When water is added to cement, the ingredients of cement react chemically with water and forms various complicated chemical compounds. The setting and hardening of cement is due to hydration and hydrolysis of its constituents.

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Only a small proportion of the added water is actually required for the chemical hydration of
the cementitious constituents to hydrated calcium silicates. The additional water is needed to ensure
the workability of the mix when aggregates are added, so that concrete, for example, can be
successfully placed within formwork containing steel reinforcement. Water in excess of that required
for hydration will ultimately evaporate, leaving the capillary pores in the concrete and mortar products.

Typically, an increase in void space of 1% reduces crushing strength by 6%. It is therefore necessary
to control carefully the water content of the mix by reference to the water/cement ratio. A minimum
water/cement ratio of 0.23 is required to hydrate all the cement, although, as the cement powder is
hydrated, it expands, and thus a ratio of 0.36 represents the point at which cement gel fills all the
water space. However, a water/cement ratio of 0.42 more realistically represents the minimum water content to achieve full hydration without the necessity for further water to be absorbed during the curing process.

Hydration of cement
hydration with time

The heat of hydration of cement:

The heats of hydration of cement constituents decide the formulation of cement for different
applications as well as useful in preventing the water in cement paste from freezing in winter and in accelerating the setting and hardening processes. If heat liberated is not dissipated rapidly particularly in large constructions like dam serious stress cracking may occur.


SETTING AND HARDENING OF CEMENT:

The cement clinkers (which are formed when calcareous and argillaceous raw materials are
mixed and burned in rotary kilns) consist of the following major compounds:

Tricalcium silicate (3Cao Sio)
Dicalcium silicate (2Cao sio)
Tricalcium aluminate (3CaO ALO)
Tetracalcium aluminum
ferrite (4CaO AIO, Fe, 0,)

The four compounds mentioned above have different functions to perform with respect to the setting of cement and gain of strength of cement.

  • Tricalcium Silicate (C3S):

 Hardens rapidly and is largely responsible for the initial set and early strength. In general, the early strength of Portland cement concrete is higher with increased Percentages of C3S.

  • Dicalcium Silicate (C2,S): 

Hardens slowly and contributes largely to strength increases at ages beyond 7 days.

  • Tricalcium Aluminate (C3,A): 


Liberates a large amount of heat during the first few days of hardening and together with CS and CS may somewhat increase the early strength of the hardening cement. It affects setting time.

  • Tetracalcium Aluminoferrite (C4AF): 

Contributes very slightly to strength gain. However, Acts as a flux during manufacturing. Contributes to the color effects that make cement gray.


The phenomenon by virtue of which the plastic cement changes into a solid mass is known as
the setting of cement.

The setting is the stiffening of the cement paste which commences immediately the cement is mixed with water. Because the major cementitious constituents set at different rates it is
convenient to refer to the initial set and final set. Typically, the initial set, or the formation of a plastic gel,
occurs after one hour and final set, or the formation of a rigid gel, within 10 hours. The setting
process is controlled by the quantity of gypsum added to the cement in the final stages of production.

The phenomenon by virtue of which the cement paste sets and develops strength is known as the hardening of the cement.

Hardening is the gradual gain in strength of the set cement paste. It is a
process that continues, over periods of days, months and years. The rate of hardening is governed partially by the particle-size distribution of the cement powder. Finely ground cement hydrates more rapidly, and therefore begins to set and harden more quickly. Furthermore, the relative proportions of tricalcium silicate and dicalcium silicate have a significant effect on the rate of hardening.

Conclusion: I hereby confirm that you have understood the importance of hydration of cement it's setting and hardening properties.

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