COMPOSITION OF ORDINARY PORTLAND CEMENT
This is the cement used for normal concrete construction. It has adhesive and cohesive properties so as to render and to form good bond with other material. It solidifies when mixed with water. It is the most active binding medium and is perhaps the most scientifically controlled component in concrete. Cement is obtained by burning a mixture of the following materials in a define proportion :
- Siliceous materials
- Argilleceus materials
- Calcareous materials
The temperature of which the mixture must be burnt is about 1400°C. The clinker so is cooled and powdered to the required fineness. The product so obtained is called cement.
COMPOSITION
Lime (CaO) | 60 to 67% |
Silica (SiO2) | 17 to 25% |
Alumina (Al2O3) | 3 to 8% |
Iron oxide (Fe2O3) | 0.5 to 6% |
Magnesia (MgO) | 0.1 to 4% |
Sulphur trioxide (SO3) | 1 to 3% |
Soda and/or Potash (Na2O+K2O) | 0.5 to 1.3% |
The above constituent mixing together to form raw material undergo a chemical reaction during burning and fusion, the compounds formed in the burning process have the properties of setting and hardening in the presence of water. They are known as Bogue compounds which are given below :
Compound | Abbreviated designation |
Tricalcium silicate (3CaO.SiO2) | C3S |
Dicalcium silicate (2CaO.SiO2) | C2S |
Tricalcium aluminate (3CaO.Al2O3) | C3A |
Tetracalcium aluminoferrite (4CaO.Al2O3.Fe2O3) | C4AF |
The properties of Portland cement varies markedly with the proportions of the above four compounds, reflecting substantial difference between their individual behaviour.
FUNCTION OF FOLLOWING COMPOUNDS
Tricalcium Silicate
- Tricalcium silicate is supposed to be the best cementing material and is well burnt cement. It is about 25-50% (normally about 40 per cent) of cement.
- It renders the clinker easier to grind, increases resistance to freezing and thawing, hydrates rapidly generating high heat and develops an early hardness and strength.
- However, raising of C3S content beyond the specified limits increases the heat of hydration and solubility of cement in water.
- The hydrolysis of C3S is mainly responsible for 7 day strength and hardness.
- The heat of hydration is 500 J/g.
Dicalcium Silicate
- Dicalcium silicate is about 25-40% (normally about 32 per cent) of cement. It hydrates and hardens slowly and takes long time to add to the strength (after a year or more).
- It imparts resistance to chemical attack.
- Raising of C2S content renders clinker harder to grind, reduces early strength, decreases resistance to freezing and thawing at early ages and decreases heat of hydration.
- The heat of hydration is 260 J/g.
Tricalcium Aluminate
- Tricalcium aluminate is about 5-11% (normally about 10.5 per cent) of cement.
- It rapidly reacts with water and is responsible for flash set of finely grounded clinker. The rapidity of action is regulated by the addition of 2-3% of gypsum at the time of grinding cement.
- Tricalcium aluminate is responsible for the initial set, high heat of hydration and has greater tendency to volume changes causing cracking.
- Raising the C3A content reduces the setting time, weakens resistance to sulphate attack and lowers the ultimate strength, heat of hydration and contraction during air hardening.
- The heat of hydration of 865 J/g.
Tetracalcium Alumino Ferrite
- Tetacalcium allumino ferrite is about 8–14% (normally about 9 per cent) of cement.
- It is responsible for flash set but generates less heat.
- It has poorest cementing value.
- Raising the C4AF content reduces the strength slightly.
- The heat of hydration is 420 J/g.
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