Hydration of Cement
- The chemical reactions that take place between cement and water are referred to as the hydration of cement.
The hydration of cement can be visualized in two ways viz. through solution” and “solid-state” type of mechanisms.
Heat Of Hydration
- The reaction of cement with water is exothermic. it liberates a considerable quantity of heat and this liberated heat is called the heat of hydration.
The hydration process is not an instantaneous one. The reaction is faster in the early periods and continues indefinitely at a decreasing rate.
During hydration, C3S and C2S react with water, and calcium silicate hydrate (C-S-H) is formed along with calcium hydroxide (Ca(OH)2.
Calcium silicate hydrate is the most important product of hydration and it determines the good properties of concrete.
2(3Cao SiO2 + 6H2O —–> 3Cao. 2SiO 2H2O + 3Ca(OH)2
or it can be written as:
2C3S + 6H —-> C3S2H3 + 3Ca(OH)2
100 + 24 —–> 75 +49
The corresponding weights involved are
2(2Cao SiO2) + 4H20 ——-> 3CaO 2SiO2 3H,0 + Ca(OH)2
or it can be written as:
2C2S + 4H → C3S2H3 + Ca(OH)2
100+21 → 99 +22
The corresponding weights involved are:
2C3S + 6H ——> C2S2H3 + 3Ca(OH)2
2C2S+ 4H ——> C3S2H3+ Ca(OH)2
Chlorides ( Cl-)
2000 mg/l for plain concrete work,
500mg/l for reinforced concrete work
- It can be seen from the above reactions that C.S produces a comparatively less quantity of calcium silicate hydrate and more quantity of calcium hydroxide than that formed in the hydration of C2S.
- Calcium hydroxide is not a desirable product in the concrete mass as it is soluble in water and gets leached out thereby making the concrete porous, particularly in hydraulic structures.
- C2S reacts rather slowly and is responsible for the strength of concrete at a later stage. It produces less heat of hydration.
- The lack of durability of concrete is on account of the presence of calcium hydroxide.
- The calcium hydroxide also reacts with sulphates present in soils or water to form calcium sulphate which further reacts with C.A and cause deterioration of concrete. This is known as sulphate attack.
- The only advantage of calcium hydroxide is that being alkaline in nature it maintains a pH value of around 13 in the concrete which resists the corrosion of reinforcements.
- From the viewpoint of hydration, it is convenient to discuss C-A and CAF together because the products formed in the presence of gypsum are similar. Gypsum and alkalies go into solution quickly and the solubility is depressed. Depending upon the concentration of aluminate and sulphate ions in the solution, the precipitating crystalline product is either calcium aluminate trisulphate hydrate or calcium aluminate monosulphate hydrate The calcium aluminate trisulphate hydrate is known as Ettringite.
- It has been estimated that on an average 23% of water by weight of cement is required for chemical reaction with Portland cement compounds. This 23% of water chemically combines with cement, and therefore it is called as bound water.
- A certain quantity of water is imbibed within the gel pores. This water is known as gel water. The bound water and gel water are complementary to each other.
- It has been estimated that about 15% water by weight of cement is required to fill up the gel pores.
- Therefore, a total of 38% of water by weight of cement is required for the complete chemical reactions and occupy the space within gel pores.
- If water equal to 38% by weight of cement is only used then it can be noticed that the resultant paste will undergo full hydration and no extra water will be available for the formation of undesirable capillary cavities.
- If more than 38% of water is used, then excess water will cause undesirable capillary cavities which ultimately reduces the strength of the cement concrete.