Content of this article:
Concrete making materials Cement: Ordinary portland cement, Rapid hardening Portland cement, Low heat Portland cement, Portland slag cement, Sulfate resisting cement, Portland pozzolana cement, white cement, Aggregate and Water
Concrete Making Materials
Cement is the only chemically active ingredient of concrete which shows binding properties after reacting with water. It consist of silicates and aluminates of calcium which form a hardened mass after mixing with water. This type of cement is also known as hydraulic cement. There are many types of cements available in market, some of which are explained below :
(i) Ordinary Portland Cement
Ordinary portland cement (OPC) is commonly used in construction. The Bureau of Indian Standard has classified OPC in three grades. This classification is based on the compressive strength of cement-sand mortar cubes. The face area of these cubes shall be 50 cm2. The cube is made of 1 part of cement to 3 parts of standard sand by weight, with a specified water cement ratio. The grades of OPC cement as per this classification are following :
(i) 33 grade
(ii) 43 grade
(iii) 53 grade.
Here the grade number indicates the minimum compressive strength of cement-sand mortar cubes in N/mm2 at 28 days.
It is preferable to measure cement in terms of its weight, not volume because volume of cement changes with the environment conditions. In our country, cement is supplied in bags of 50 kg and its volume is equal to 34.5 litres. The maximum cement content used for R.C.C. construction is 450kg/m3 as per IS 456:2000.
In addition to above, BIS has further classified cement into following types depending on the 28 days compressive strength.
|Type||Compressive Strength (N/mm2 or MPa)|
|A||32.5 – 37.5|
|B||37.5 – 42.5|
|C||42.5 – 47.5|
|D||47.5 – 52.5|
|E||52.5 – 57.5|
|F||57.5 – 62.5|
|G||62.5 – 67.5|
(ii) Rapid Hardening Portland Cement
Rapid hardening cement is finely ground and has more percentage of tricalcium silicate and dicalcium aluminate, which results in rapid hardening and quick development of the strength. This type of cement is used in places where faster construction works are to be done, so that the form work can be removed early. For example, in road works etc. Rapid hardening portland cement should confirm to IS 8041:1990.
(iii) Low Heat Portland Cement
Low heat portland cement has lower percentage of tricalcium silicate and dicalcium aluminate, which results in slow hydration of cement and hence low heat of hydration is evolved. This type of cement is used in mass concrete works, such as gravity dams, retaining wall, bridge abutments etc., where large amount of concrete may result in very high heat of hydration. High heat of hydration can lead to severe cracking of the concrete. This cement should confirm to IS 12600:1989.
(iv) Portland Slag Cement
Blast furnace slag is the waste product of Pig Iron industry. This slag is mixed with the cement clinker and ground properly to obtain portland slag cement. The slage imparts high resistance against sulphate attack to the cement. Therefore, it is widely used in places where chances of sulphate attack are there, such as under soil construction works (subjected to ground water exposure) and near the industries emitting waste containing sulphates. It should confirm to IS 455:1989.
(v) Sulphate Resisting Cement
Sulphate resisting cement has very low percentage of tricalcium aluminate and is ground much finer than ordinary portland cement. This cement has very high sulphate resistance and is widely used for under soil/ground water construction works.
(vi) Portland Pozzolana Cement
Portland Pozzolana cement contain pozzolanic material like burnt clay or flyash. This cement is obtained by grinding pozzolana and cement clinker together. The pozzolana have no cementitious properties themselves but when they combine with free lime they produce a compound having cementitious properties. Portland Pozzolana cement is cheaper and has a lower rate of development of strength as compared to OPC. Portland Pozzolana cement is being used widely now for all general construction works.
(vii) White Cement
This cement is obtained by controlling the percentage of iron oxide while manufacturing cement, by using China clay, chalk and lime stone as raw materials. This cement is manufactured by heating on a very low flame by using oil fuel in place of coal so that there are no chances of contamination with coal ash. Coloured pigments are added to the white cement inorder to obtain cement of desired colour. This cement is used for all aesthetic works.
The bulk of concrete is made up of aggregates. Aggregates are inert material or chemically inactive material like crushed rock, sand, broken bicks, gravel etc. Aggregate, less than 4.75 mm size are known as fine aggregate and which are more than 4.75 mm size are known as coarse aggregate. Coarse aggregate gives strength to concrete and fine aggregate acts as a filler between the coarse aggregate and provides workability to the concrete mix. The aggregate should be hard, strong, durable and well graded to form a compact and dense mass of concrete. More about aggregate can be read from any book on “Concrete Technology”.
Water is required for chemical reaction of cement and also for providing workability to concrete. The water used for mixing and curing of concrete should be clean and free from injurious amount of acids, alkalies, salts or other chemical substance, which can harm concrete or steel. BIS 456:2000 gives the permissible limits of impurities present in water.
Permissible Limits of Impurities in Water
|Chlorides (Cl–)||2000 for plain concrete works|
|Sulphates (SO3)||400 and 500 mg/l for RCC works|
|Suspended solids||2000 mg/l|