3. Portland cement
A powdered materials which initially have plastic flow when mixed with water or
other liquid, but has the property of setting to a hard solid structure in several
hours with varying degree of strength and bonding properties.
Portland cement is a mixture of following compounds:
Dicalcium silicate, tricalcium silicate, tricalcium aluminate, tetracalcium
alumino ferrite, MgO, CaO.
4. History
In 1824, Portland cement was invented and named by Joseph Aspidin .
Portland cement: Portland rock (stone)
Natural cement: mixture of burnt silicate and lime
5. Types of Portland cement
Depending upon the rate of setting ,heat evolution and strength characterstics,
Portland cement is of various types;
Type 1. regular Portland cements.
Type 2. modified Portland cements.
Type 3. high early strength Portland cements.
Type 4. low heat Portland cements.
Type 5. sulphate resisting Portland cements.
6. Other types of cement
Acid resisting cement- capable of resisting corrosion by acids in the post
hardening stage. It is used in concrete and reinforced concrete structures to be
exposed to corrosive action of mineral and organic acids.
White cement- the color of the ordinary Portland cement is greyish-black due to
presence of iron oxide present. If iron oxide can be avoided by suitable selection of
raw materials and produce white cement.
Colored cement- obtained by mixing white or grey Portland cement with suitable
colored pigments. For getting blue, green, black and red cements, the pigments that
are respectively cobalt blue, chromium oxide, carbon black and iron oxide.
7. Raw materials for the manufacture of
cement
Limestone: CaO, Fe2O3
Clay: SiO2, Al2O3
The raw materials include
a) Calcareous materials: e.g limestone (containing 65-80% CaCO3), calcium
carbonate sludge, marl, chalk and alkali waste.
b) Siliceous or Argillaceous materials: e.g silica, iron oxide and alumina.
8. Cement manufacturing process
Two process:
1.Wet process: older and used mostly in INDIA & EUROPE
2. Dry process: invented in America and used there
The choice between two process is usually govern by the following factors
1.Physical condition of available raw materials.
2.Climate surrounding the plate of manufacture
3.Cost of fuel.
9. If limestone and clay are soft, climate is fairly moist, and the fuel is cheap, the wet
process is preferred.
The dry process is employed, if the limestone and clay are hard.
Wet process is more common and is almost universally employed for the manufacture of
cement.
Dry process is slow and cement produced is of low quality.
In wet process, the fuel consumption is higher, process is comparatively faster, the
cement produced is of superior quality.
11. Reaction at the kiln
Evaporation zone (100-500C): To remove crystal water.
Calcining water (600-900C): To break Calcium carbonate to form calcium oxide and
carbon dioxide.
Reaction zone (900-1400C): C2S, C3S, C3A, C4AF.
12. Reactions:
CaCO3 = CaO + CO2
2CaO+SiO2 = 2CaOSiO2
3CaO+SiO2 = 3CaO.SiO2
3CaO+Al2O3 = 3CaO.Al2O3
4CaO+Fe2O3+Al2O3 = 4CaO.Fe2O3.Al2O3
Generally (2-3%) Gypsum is also added to the clinker before grinding. The function of
gypsum in cement is to slow down the time of setting.
C3A + 6H2O = C3A.6H2O + Heat
13. Function of cement manufacturing
constituents
Lime(CaO)- is the principal constituents of cement. Excess of lime and lesser than
needed both reduce the strength of cement.
Silica(SiO2)- imparts strength to cement.
Alumina(Al2O3)- increase the rate of setting but excess of Alumina weakens the
strength of cement.
Gypsum(CaSO4.2H2O)- retards the rate of setting of cement and actually enhances the
initial setting time of cement.
Iron oxide(Fe2O3)- gives colour, strength and hardness to cement
SO3- in excess reduces the soundness of cement.
Alkalies- should be present in small amounts. In excess, they cause the cement to
become efflorescent.
14. Setting of cement
The various compounds that contributes to the heat of hardening are:
C3A>C3S>C4AF>C2S
Strength contribution of various compounds in Portland cement is ,
1 day C3A>C3S>C4AF>C2S
3 days C3A>C3S>C4AF>C2S
28 days C3A>C3S>C4AF>C2S
2 years C2S>C3S>C4AF>C3A
15. Uses of cement
Cement is one of the most important building materials. It is used
1. In the construction of roads, buildings, dams, bridges.
2. In the form of a paste with sand and water.
3. In reinforced concrete (being very hard and strong is used in the construction of bridges
and roofs).
4. In the production of mortar in masonry.
