The hydration of cement is an exothermic reaction that is carried out when cement comes in contact with water. The reaction will result in several by-products that will contribute to the strength of the concrete or mortar formed.
The chemical reaction that takes place between the cement and water is referred as hydration of the cement. The hydration reaction is an exothermic reaction.
The cement hydration will liberate a considerable amount of heat. This is called as Heat of liberation or Heat of Hydration. The mixing of cement with water will result in rapid evolution of heat that will last for few minutes. This evolution of heat is probably due to the reaction of a solution of aluminates and the sulfates. This rapid heat and reaction are depressed by the addition of gypsum. The Early heat of hydration is due to the hydration of C3S.
The rate of development of heat is greatly influenced by the fineness of the cement. The normal cement generally produces 89-90J/g in 7 days and 100J/g in 28 days.
The hydration process is not instantaneous. The reaction is faster in the early stages and will go decreasing with the time period. It is observed that the complete hydration is possible only for cement particles that have a particle size smaller than 50 microns. With the increase in the fineness, there is an increase in the rate of hydration. The table-1 below shows the bogues compounds present in cement material that is involved in the hydration process.
|Table.1: Bogues Compounds|
Heat of Hydration of Cement
Hydration is an exothermic process, that will liberate heat through chemical reactions. The process of hydration can be easily followed by monitoring the production of heat that accompanies the reaction. This is done by civic mortar from a batch of concrete and weighing it into a bottle which is placed into an insulated container. A thermistor is then impeded into the fresh mortar. The output of the thermistor is recorded by the computer. The result is plotted as a curve of temperature v/s the time. The area under the major peak can be related to the early strength development.
Stages of Hydration of Cement
The five stages involved in the hydration of cement are explained with respect to the figure-2. There are Five stages of cement hydration:
- Initial Hydrolysis
- Induction Period or the Dormant Period
- Steady State
|Fig.2.The Heat of Hydration - Calorimetry curve.|
Stage 1- Initial Hydrolysis: The initial dissolution of cement will result in the sort release of heat shown by the first peak in the calorimetry curve.
Stage 2 - Induction or Dormant Period: After the initial dissolution process, the hydration products are precipitated on the surface of each cement particle. The layer acts as a protective barrier and temporarily delays the dissolution of the particle. This slows down the reaction for a period of several hours. This is called as the Dormant Period.
The existence of the dormant period allows the concrete to be transported to the construction site and placed and finished in the forms.
Stage 3- Hydration Acceleration: The end of the dormant period shows the beginning of the setting at which time the cement starts to react more rapidly with water. This will result in the formation of new hydration products.
Stage 4- Hydration Deceleration: This period will undergo formation of hydration products but the rate of reaction and the dissolution is very controlled and slow.
Stage 5 - Steady State: This is the stage that is for a longer period that is equal to the age of the structure. The hydration reaction carried out throughout this period at a very slow rate.
Hydration of Bogues Compounds
Here the reaction of bogues compounds during different stages as explained before is briefly explained.
- Tricalcium aluminate (C3A) : Celit is the quickest one to react when the water is added to the cement. This is celit which is responsible for the flash setting (which is stiffening without the development of strength) thus prevents the hydration of Dicalcium silicate (C2S) and Tricalcium silicate (C3S). The hardening of concrete is generally sped up by this compound. The increase of this content will help in the manufacture of Quick Setting Cement. The heat of hydration is 865J/Cal.
- Tricalcium silicate (C3S): This is also called as Alite. This is one that readily reacts with the water and results in more heat of hydration. This is also responsible for the early strength of the concrete. The cement that has more C3S content is good for cold weathering concrete. Alite is responsible for 7-day strength and the hardness. The heat of hydration is 500J/Cal. This will produce a higher quantity of Ca(OH)2 or calcium hydroxide.
- Dicalcium Silicate (C2S) : This compound will be undergoing reaction slowly. This is hence the factor responsible for the progressive strength of concrete. This is also called as Belit. It will impart strength to the chemical attack. The heat of hydration is 260J/Cal. Higher amount C-S-H is produced by this compound than C3S. This demands less amount of water for hydration than C3S.
- Tetracalcium Aluminoferrite (C4AF): This is called as Felit. The heat of hydration is 420J/Cal. It has the poorest cementing value.
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