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Hydration Products of Cement | Chemical Reaction | Significance

The reaction of cement when mixed with water is called as hydration. The main oxide and the Bouges compounds of the cement are the major ingredients which will influence the rate and the mechanism of the cement hydration and hence the hydration products.








Hydration Products of Cement

Fig.1. Hydration Products of Cement

The products of cement hydration have a very low solubility in water. The progress of the cement hydration can be determined by different means, like by the:


  1. Measurement of amount of Ca(OH)2 in the cement paste
  2. Measurement of Heat evolution during the Hydration Process
  3. Measurement of amount of unhydrated cement present
  4. Measurement of amount chemically combined water
Read >> Hydration of Cement - Stages of Hydration Reaction

During the Hydration of the cement, the major Bouges compounds C3S and C2S  are responsible for the setting and the hardening of the cement. In the presence of water, the silicates and aluminates form products of hydration that will result in a hard mass over a period of time. This hard mass is called as the Hydrated Cement Paste (HCP).

                  Read >> Structure of Hydrated Cement Paste (HCP)

Hydration of Tricalcium silicate (C3S)


The chemical reaction of C3S with water can be expressed as:


C3S + water C-S-H + C-H + Heat
i.e. 2(3CaO.SiO2) + 6H2O  → 3CaO. 2SiO2. 3H2O + 3Ca (OH)2 + Heat
       
The equation implies that the the hydration reaction of  C3S with water will give Calcium Silicate Hydrate (C-S-H) and Calcium Hydroxide.


  • Calcium Silicate Hydrate ( C-S-H): The C-S-H will constitute 50 to 60% of the solids in the cement paste. This will form a continuous binding matrix. It is amorphous and fibrous and hence will have larger surface area. This product is an important factor for the development of strength for the cement mix.
  • Calcium Hydroxide (Ca(OH)2): This product will constitute 20% of the solids in the paste. This takes existences in the form of thick crystalline hexagonal plates and is embedded in the C-S-H matrix. It is not a desirable product in concrete as it is soluble in water and will get leached out. Its growth will fill the pore spaces. This product does not have significant role in strength development. Its leaching effect will cause white patches and efflorescence.                  


Hydration of Dicalcium Silicate (C2S)


The hydration of  C2S is similar to that of  C3S. The same products C-S-H and C-H are also produced during the hydration of  C2S. C2S reacts slowly and generates less heat. This will contribute to strength in later stages only. The hydration reaction of C2S is as follows:

2C2S + 4H C-S-H + C-H 

The hydration of C2S as mentioned above will result in the very lesser amount of C-S-H compared to that produced by the hydration of  C3S.
C3S will produce comparatively lesser quantity of C-S-H than that is produced by C2S. The quality of C-S-H product that is produced by the hydration of C2S is more superior compared to that produced by C3S. The products of hydration of  C2S is denser and have a higher surface area.

Hydration of Tricalcium aluminate (C3A)


The Hydration of tricalcium aluminate will produce a substance called as ettringite. The reactions are:

2C3A + 21H → C4AH13 + C2AH8 → 2C3AH6 + 9H
On hydration of C3A, a calcium aluminate system C-A-H is formed. This cubic compound is the most probable stable compound in the reaction.

The reaction of pure C3A with water is very fast and this may lead to flash setting. In order to prevent this flash setting, gypsum is added at the time of grinding the cement clinker. The below is the reaction of C3A with the presence of gypsum.

C3A + 32H +3CaSO4 → C3A. 3CSH32 → C6AS3H32
 C6AS3H32  =(Calcium Aluminate Tri Sulphate Hydrate or Ettringite)
Calcium Aluminate Tri Sulphate Hydrate is called as ettringite. The ettringite is a crystalline needle like substance. It constitutes about 10 to 20% of the solid content. It is a long, slender and prismatic crystal. This product is stable only in the presence of gypsum. This has a minor role in strength development but contributes highly to durability.

Hydration of Tetracalcium Aluminoferrite (C4AF)


The hydration of C4AF is similar to that of  C3A, and the same products are formed. The C4AF reacts slowly and hence generate less heat and combines well with gypsum. On hydration, the C4AF is believed to form a system of the form CaO - Fe2O3 - H2O. A hydrated calcium ferrite of the form C3FH6 is more stable. 


C4AF + 10H +2CH → C6AFH12

Where, H = H2O;  

The hydrates of C4AF show a comparatively higher resistance to the attack of the sulphates than the hydrates of the calcium aluminate. 


Summary


The hydration of the cement occurs at the surface of the grains. All the compounds react simultaneously; a compound reaction takes place.

The hydration rate has the following hierachy.


C3A → C3S → C4AF → C2S


Ettringite is formed first, followed by C-H and C-S-H. There is no change in volume of the cement paste as a result of hydration. After hydration, the paste composition consists of both the solid and water phases. The bogues compounds react to form C-S-H, C-H, ettringite, monosulphate, aluminate and heat.

The calcium silicates contributes to the strength development and calcium aluminate relates to setting. The behaviour of bogues compound in cement hydration can be written as in Table-1.



Table.1


Name
Reaction Time
Strength Development
Setting Time
Heat Evolution
C3S
Medium
High
Low
High
C2S
Slow
Low/high
Low
Low
C3A
Fast
Low
High
Very High
C4AF
Medium
Low
Medium
Medium











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