The chemical reaction which takes place between cement and water is known as hydration of cement. This reaction is exothermic in nature, which means heat is released during the reaction, and the evolved heat is called as heat of hydration.
Typically, cement produces 89-90cal/gm of heat in 7 days and 90-100cal/gm in 28 days. Hydration is not an instantaneous process, and it may take years for the complete hydration of cement particles.
There are basically four types of compounds present in cement, namely C3A, C4AF, C3S, and C2S, popularly known as Bogue’s compounds, which take part in the chemical reaction.
Hydration of these compounds leads to the formation of certain products, which in turn leads to setting and hardening of cement and, as a result, transformation into a solidified strong mass.
Major Products of Hydration
1. Calcium Silicate Hydrate
As soon as water is added to cement particles, C3S and C2S react with water to form calcium silicate hydrate (C-S-H) and calcium hydroxide (Ca(OH)2).
The chemical reaction is as shown below:
2C3S + 6H ----- C3S2H3 + 3Ca(OH)2
2C2S + 4H ----- C3S2H3 + Ca(OH)2
The product C-S-H is also known as Tobermorite gel.
The term C-S-H is hyphenated because there is no well-defined ratio of CaO and SiO2. Its composition in the hydrated product ranges from 50-60%. This is the most important of all the products and is responsible for all the good properties of concrete i.e. strength, durability, etc.
It has been found that the hydration of C3S produces lesser C-S-H than Ca(OH)2 as compared to the hydration of C2S. Further, the quality and density of C-S-H produced by C3S is inferior to C2S.
2. Calcium Hydroxide
Ca(OH)2 is another product of hydration of C3S and C2S. It constitutes 20 to 25% of the solidified volume of hydrated cement paste. Calcium hydroxide helps in maintaining a pH value of 13 around the reinforcement, which acts as a passive protective layer preventing the corrosion of reinforcement. This is the only advantage of the presence of calcium hydroxide in concrete mass.
It leads to loss of durability of concrete due to the following three reasons:
- Ca(OH)2 is easily soluble in water and gets leached out, making concrete mass porous, resulting in low strength and durability.
- Ca(OH)2 reacts with sulfates present in water or soil and forms CaSO4, which in turn reacts with C3A and forms higher volume products, which produces cracks and leads to deterioration of concrete. This phenomenon is also called as Sulphate Attack.
- Ca(OH)2 reacts with CO2 present in the atmosphere and forms CaCO3. Initially, the reaction transpires on the surface of concrete but gradually penetrates into the mass. If the concrete mass is a little porous and reduces the pH value of the passive protective layer, it makes the reinforcement susceptible to corrosion. This type of deterioration is called carbonation of concrete.
Thus, it can be seen that Ca(OH)2 is mostly undesirable in concrete, but due to its ability to protect the reinforcement, its value can’t be completely undermined.
3. Calcium Aluminate Hydrate
C3A and C4AF are the aluminates present in cement and after hydration, turn into Calcium Aluminate Hydrate. C3AH6 and C3FH6 are relatively stable compounds of hydration of C3A and C4AF, respectively.
These hydration products do not impart any strength or unique property to concrete; instead, their presence is harmful to the concrete, particularly in cases where concrete is prone to sulfate attack.
4. Ettringite
It is well known that due to the rapid hydration of C3A and C4AF, flash setting of cement occurs, which may lead to the development of cracks in concrete. In order to prevent this phenomenon, gypsum (CaSO4) is added during the manufacturing of cement, which decreases the solubility of C3A and C4AF and controls flash setting behavior.
This sulfate from gypsum reacts with C3A and C4AF and forms calcium aluminate sulfate, also popularly known as ettringite.
It should be noted that this compound is being formed before the concrete hardens, and hence this primary ettringite formation doesn’t lead to any harmful effects on concrete. The same compound becomes deleterious to concrete if it is formed after the concrete has hardened.
This is popularly known as Delayed Ettringite Formation (DEF), which leads to the development of microcracks in concrete, making it porous and less durable.
FAQs on Hydration Products of Cement
The chemical reaction which takes place between cement and water is known as hydration of cement. This reaction is exothermic in nature, which means heat is released during the reaction, and the evolved heat is called as heat of hydration.
There are basically four types of compounds present in cement, namely C3A, C4AF, C3S, and C2S, popularly known as Bogue’s compounds, which take part in the chemical reaction.
The sulfate present in the gypsum reacts with C3A and C4AF and forms calcium aluminate sulfate also popularly known as ettringite.
Read More:
1. Low Heat Cement- Composition, Properties, Uses and Advantages
2. Heat Resistant Concrete or Refractory Concrete -Installation and Applications