Hydration of Cement
The reaction with water is termed “hydration”. This involves many different reactions, often occurring at the same time. As the reactions proceed, the products of the hydration process gradually bond together the individual sand and gravel particles, and other components of the concrete, to form a solid mass.
In the anhydrous state, four main types of minerals are normally present: alite, Belite, aluminate (C3A) and a ferrite phase (C4AF). For more information on the composition of clinker, see the clinker pages. Also present are small amounts of clinker sulfate (sulfates of sodium, potassium and calcium) and also gypsum, which was added when the clinker was ground up to produce the familiar grey powder.
When water is added, the reactions which occur are mostly exothermic, that is, the reactions generate heat. We can get an indication of the rate at which the minerals are reacting by monitoring the rate at which heat is evolved using a technique called conduction calorimetry.
TYPES OF CEMENT
It is obtained by finely pulverizing clinker produced by calcination the properly proportioned mixture of argillaceous and calcareous materials. It is commonly known as Ordinary Cement.
Ordinary Portland cement has been classified as
- OPC- 33 grade (IS: 269-1989)
- OPC- 43 grade (IS: 8112-1989)
- OPC- 53 grade (IS: 12269-1987)
The number 33, 43 and 53 correspond to 28 days Compressive strength of cement. Higher the strength of Portland cement higher the rate of heat development during hydration of cement.
Cement of special characteristics for required performance in a given environment are manufactured by altering by either chemical composition of OPC, by using additives, or by using different raw materials.
RAPID HARDENING CEMENT (IS :8041)
This cement is similar to OPC, except that it has more C3S and less C2S. Due to which it is also known as high early strength Portland cement.
It is used in a situation where rapid development of strength is required.
This cement cannot be used for mass concreting because due to the large quantity of heat of hydration develops.
It has same strength in 1 day with an ordinary cement attained in a 3 day with a same water cement ratio.
It is subjected to large shrinkage and water requirement for workability is more.
It is suitable for repair of roads, bridges and in structure where load is applied in a short period of time.
HIGH ALUMINA CEMENT (IS: 6452)
The raw material used for its manufacture consists of 40% bauxite, 40% lime and 15% iron oxide with a little % of ferric oxide and silica, magnesia etc. ground finely at a very high temperature.
It has a very high early compressive strength and has high heat of hydration.
It should not be used in a place where temperature exceeds 18 degree Celsius and it is extremely resistant to action of fire, chemical attack, sea water etc.
LOW HEAT PORTLAND CEMENT (IS: 12600)
It is a Portland cement with relatively lower contents of C3S and C2S and more content of C2S.
This cement possesses less compressive strength having initial setting time not less than 1 hour and final setting time not more than 10 hours.
Rate of development of strength is slow but the ultimate strength is same.
This is desirable in mass concreting of gravity dam
Low heat is evolved helpful in preventing shrinkage at high temperature.
PORTLAND PUZZOLANA CEMENT (IS: 1489 Part)
Manufactured by grinding Portland cement clinker and pozzolana.
Pozzolana has no cementing properties itself but has the property of combining with lime to produce a stable lime pozzolana which has definite cementitious properties.
These has low rate of development of strength but ultimate strength is same.
Drying shrinkage should not be more than 0.15%
It has low heat evolution and is used in the places of mass concrete such as dams.
QUICK SETTING PORTLAND CEMENT
In the manufacture of this cement, gypsum content is reduced to get the quick setting properly. Also, a small amount of aluminum sulphate is added.
It sets quickly but does not harden quickly.
Initial setting time 5 minutes whereas final setting time is 30 minutes.
It is used when concrete is to be laid under water or in running water.