The Setting of Cement.
The conversion of a plastic paste of cement to a non-plastic and rigid mass is known as Setting of cement.
And the time required for this process to take place is known as the setting time of cement.
This change from a plastic mass to a non-plastic mass may take any time between a few minutes to an hour or more.
The setting of cement is rather a complicated chemical phenomenon and is believed to take place in following three stages:
(i) Hydrolysis and hydration stage:
It is the first stage and starts Immediately after addition of water.
In this process the four compounds of cement (C3S, C2S, 3CA1, 4CAFe) get hydrated.
The first compound also undergoes hydrolysis.
As a result complex hydro silicates are formed.
(ii) Colloidisation stage:
In this stage, the intricate system of hydro silicate crystals formed as above separates out in the form of a gel which gets gradually thickened and acts as glue around other particles.
Thereby initiating the setting of the cement.
During this stage, the mortar (cement-water-sand paste) becomes fully saturated and can take no more water.
(iii) Crystallization stage:
In this stage, most of the components get transformed from a gel or colloidal state to a crystalline state.
The least stable colloids of tri-calcium hydro aluminate and calcium hydroxide are first to pass into a stable crystal phase.
Calcium hydro silicate gel also hardens almost simultaneously.
This nearly simultaneous development of crystals and hardening of gel results into a strong and inter-grown mass of crystals and gels.
Hardening of cement is different than the setting of cement.
It may be defined as the capacity of the set cement to withstand loads without disintegrating or cracking.
Thus whereas cement may require only a few minutes and hours to set, it may become sufficiently hard only after a considerable time, measured in days, weeks, months and even in years.
Setting Time of Cement.
Some types of cement set quickly, within a few minutes whereas others may take comparatively longer time.
However, the initial setting time of cement is 30 minutes for ordinary Portlandcement.
And the final setting time of cement is 600 minutes or 10 hours on average for ordinary Portland cement.
Moreover, the setting is not an abrupt process which may complete immediately after its start; it is rather a progressive phenomenon, which has a beginning, full development, and an end.
It is, on this latter basis, the setting of cement is distinguished into an initial and final setting, qualified by the time required in each case.
The time required for setting of cement is greatly influenced by the following factors:
(i) Temperature at which cement paste is allowed to set; very cold temperatures retard the time of setting.
(ii) Percentage of water mixed to the cement (water-cement ratio) in making the paste; it must be the right quantity for complete hydration and crystal formation.
Less water will result in incomplete hydration; extra-quantity will bleed out after the cement is set.
(iii) The humidity of the atmosphere in which setting is allowed to take place. It acts through temperature and moisture conditions.
It is, therefore, essential that all the mists for setting time of cement are performed at Standard conditions of temperature and humidity and on cement pastes of known consistency.
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