Cement
Cement is manufactured through a closely controlled chemical combination of calcium, silicon, aluminum, iron and other ingredients. Common materials used to manufacture cement include limestone, shells, and chalk or marl combined with shale, clay, slate, blast furnace slag, silica sand, and iron ore.{ The most important raw materials for making cement are limestone, clay, and marl. These are extracted from quarries by blasting or by ripping using heavy machinery.}
Ingredients of cement
The main ingredients used in the manufacturing of cement are lime, silica, alumina and other oxides. A brief discussion about the contribution of different ingredients in cement is below:
Lime
About 60 - 65 % of cement constitutes of lime which contributes to the strength. Proper care should be taken in proportion to make the cement sound and strong, as an excess of lime makes the cement unsound and causes the cement to expand and disintegrate. In case, if lime is deficient, the strength of the cement is reduced and sets quickly.
Silica
Silica consists of about 17 - 25 % in cement, which also contributes to the strength of cement due to the formation of dicalcium and tricalcium silicates. Excess presence of silica increases the cement strength but prolongs the setting time of cement.
Alumina
The percentage of alumina is 3.5 - 9.0 in cement, which imparts quick-setting quality to the cement. Surplus amount of alumina presence in cement leads to the reduction of strength.
Iron Oxide
Iron oxide is present about 0.5 - 6.0 % in cement, which provides colour, hardness and strength to the cement.
Magnesium Oxide
The correct quantity of magnesium oxide imparts hardness and colour to the cement, and the required amount is about 0.5 - 4.0 %. Presence of excess quantity seriously affects the soundness of cement.
Sulphur Trioxide
It is present in small quantities of about 1 - 2.0 % to make the cement sound. Excess of sulphur trioxide causes the cement to become unsound.
Alkalis
Alkalis like soda or potash should be present only in small quantities up to 0.4 - 1.3 %. Excess quantity of these materials cause efflorescence and straining while using in concrete or mortar.
Top 10 Requirements for Quality Cement:
It should possess a uniform colour.
It should be free from lumps.
It should be smooth as it should get sink completely if thrown in the water.
The ratio of percentage of alumina to that of iron oxide should not be less than 0.66 in cement.
The total sulphur content in cement should not be greater than 2.75%, and Magnesia content should not exceed 5% by weight.
Insoluble residue in cement should not exceed 1.5% by weight.
When heated, cement should not lose its weight more than 4%.
The specific surface of cement as from the fineness test should not be less than 2,250 mm² / gm.
The initial setting time of cement should be about 30 minutes and the final setting time should be about 10 hours.
Cement should not expand more than 10 mm in a soundness test.
Different Types Of Cement
Ordinary Portland Cement (OPC)
Portland Pozzolana Cement (PPC)
Rapid Hardening Cement.
Extra Rapid Hardening Cement
Low Heat Cement.
Sulfates Resisting Cement.
Quick Setting Cement
Blast Furnace Slag Cement
Portland cement
Is used in concrete for bridges, walls, culverts, floors, pavements, sidewalks, pipe, railway structures, reinforced concrete buildings, tanks and reservoirs, as well as for masonry units and other precast products.
Some harmful constituents of cement:
The presences of the following two oxides adversely affect the quality of cement.
Alkali oxides K2O and NaO2.
Magnesium oxide MgO.
If the amount of alkali oxides exceeds 1 percent, it leads to the failure of concrete made from that cement. Similarly, if the content of magnesium oxide exceeds 5 percent, it causes cracks after mortar or concrete hardens.
It is due to the fact that magnesium oxide, burned at a temperature of about 1500 °C, Slakes very slowly, when mixed with water.
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