Hydration Of Cement
The chemical reaction between cement and water is known as hydration of cement. The reaction takes place between the active components of cement (C4AF, C3A, C3S and C 2S) and water. The factors responsible for the physical properties of concrete are the extent of hydration of cement and the resultant microstructure of the hydrated cement. When the cement comes in contact with water, the hydration products start depositing on the outer periphery of the nucleus of hydrated cement. This reaction proceeds slowly for 2-5 hours and is called induction or dormant period. As the hydration proceeds, the deposit of hydration products on the original cement grain makes the diffusion of water to unhydrated nucleus more and more difficult, consequently reducing the rate of hydration with time. At any stage of hydration, the cement paste consists of gel (a fine-grained product of hydration having large surface area collectively), the unreacted cement, calcium hydroxide, water and some minor compounds.
The crystals of the various resulting compounds gradually fill the space originally occupied by water, resulting in the stiffening of the mass and subsequent development of the strength. The reactions of the compounds and their products are as follows:
C3S + H2O C-S-H* + Ca (OH)2
C2S + H2O C-S-H + Ca (OH)2
C3A + H2O C3AH6
C3A + H2O + CaSO4 CA C S Calcium sulpho-aluminate
C4AF + H2O C3AH6 + CFH
The product C-S-H gel represents the calcium silicate hydrate also known as tobermorite gel which is the gel structure. The hydrated crystals are extremely small, fibrous, platey or tubular in shape varying from less than 2 mm to 10 mm or more. The C-S-H phase makes up 50-60% of the volume of solids in a completely hyderated Portland cement paste and is, therefore, the most important in determining the properties of the paste. The proposed surface area for C-S- H is of the order of 100-700 m2/g and the solid to solid distance being about 18 Å. The Ca(OH) 2 liberated during the silicate phase crystallizes in the available free space. The calcium hydroxide crystals also known as portlandite consists of 20-25% volume of the solids in the hydrated paste. These have lower surface area and their strength contributing potential is limited. The gel must be saturated with water if hydration is to continue. The calcium hydroxide crystals formed in the process dissolve in water providing hydroxyl (OH-) ions, which are important for the protection of reinforcement in concrete. As hydration proceeds, the two crystal types become more heavily interlocked increasing the strength, though the main cementing action is provided by the gel which occupies two-thirds of the total mass of hydrate.
Notes : 1. It has been found that hydration of C 3S produces lesser calcium silicate hydrate and more Ca(OH)2 as compared to the hydration of C2S. Since Ca(OH)2 is soluble in water and leaches out making the concrete porous, particularly in hydraulic structures, a cement with small percentage of C3S and more C2S is recommended for use in hydraulic structures.
2.It is particularly important to note that the setting (the change of cement paste from plastic to stiff solid state) and hardening (gain of strength with hydration is a chemical reaction, wherein water plays an important role, and is not just a matter of drying out. Infact, setting and hardening stop as soon as the concrete becomes dry.