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

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 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 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 a larger surface area. This product is an important factor in the development of the strength of the cement mix.
• Calcium Hydroxide (Ca(OH)₂): This product will constitute 20% of the solids in the paste. This takes existence 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 a 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 a comparatively lesser quantity of C-S-H than that is produced by C₂S. The quality of the C-S-H product that is produced by the hydration of C₂S is superior compared to that produced by C₃S. The products of hydration of  C₂S are 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. 

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 hierarchy.

C₃A → C₃S → C₄AF → C₂S

Ettringite is formed first, followed by C-H and C-S-H. There is no change in the 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, mono-sulfate, aluminate, and heat.

The calcium silicates contribute to the strength development and calcium aluminate relates to setting. The behavior of the bogues compound in cement hydration can be written 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

Read More: Structure of Hydrated Cement Paste (HCP)