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 C2S) and water. Thefactors 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 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–SH 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.
Rate of hydration.
The reaction of compound C3A with water is very fast and is responsible for flash setting of cement (stiffening without strength development) and thus it will prevent the hydration of C3S and C2S. However, calcium sulphate (CaSO4) present in the clinker dissolves immediately in water and forms insoluble calcium sulphoaluminate.
It deposits on the surface of C3A forming a colloidal membrane and consequently retards the hydration of C3A. The amount of CaSO4 is adjusted to leave a little excess of C3A to hydrate directly. This membrane in the process breaks because of the pressure of the compounds formed during hydration and then again C3A becomes active in the reaction. The hardening of C3S can be said to be catalyzed by C3A and C3S becomes solely responsible for gain of strength up to 28 days by growth and interlocking of C-S-H gel. The increase in strength at later age is due to hydration of C2S.
Water requirement for hydration
This water combines chemically with the cement compounds and is known as bound water. Some quantity of water, about 15 per cent by weight of cement, is required to fill the cement gel pores and is known as gel water. Therefore, a total of 38 per cent of water by weight of cement is required to complete the chemical reaction. The general belief that a water/cement ratio less than 0.38 should not be used in concrete because for the process of hydration, the gel pores should saturated – is not valid.
This is because as even if excess water is present, complete hydration of cement never takes place due to deposition of hydration products. As a matter of fact water/cement ratio less than 0.38 is very common for high strength concretes. If excess water is present, it will lead to capillary cavities.
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