Ordinary Portland cement defined as “a product obtained by finely pulverizing clinker produced by calcining to incipient fusion, an intimate and properly proportion of argillaceous and calcareous materials.
Ordinary Portland cement (OPC) is by far the most important type of cement. Joseph Aspdin took the patent of Portland cement on 21st October 1824. The fancy name of Portland was given owing to the resemblance of this hardened cement to the natural stone occurring at Portland in England.
Constituents of Ordinary Portland Cement
Ordinary Portland Cement basically constituents of two different types of compound – Argillaceous compound (like clay, cement rock, blast furnace slag, marl) & Calcareous compound (like limestone, chalk, marine shells).
Grades of Ordinary Portland Cement
There are 3 grades of Ordinary Portland Cement as per Indian Standard Code (IS code)
IS 10262 has classified the Ordinary Portland Cement (OPC) grade wise from A to F based on 28 day compressive strength as follows:
Accordingly, the 33, 43 and 53 grades of cement correspond to categories A, C and E, respectively. However, most of the 43-grade cements available in the market fall in category D and that 53-grade cement in category F.
Chemical Composition of Raw Materials of OPC The three basic constituents of Ordinary Portland Cements are lime, silica and alumina. The relative proportions of these oxide compositions are responsible for influencing the various properties of cement.
1. Lime
- Lime Imparts strength and soundness (change in volume) to the cement
- If it is in excess it makes the cement unsound, causes it to expand and disintegrate.
- If it is in deficiency it reduces the strength of the cement and causes it to set quickly.
2. Silica
- Silica also imparts strength to the cement.
- If it is in excess, it increases the strength of the cement but also increase the setting time of the cement.
3. Alumina
- Alumina imparts quick setting properties to the cement.
- It acts as a flux and help in reducing the clinkering temperature during the burning of the cement.
- If it is in excess it weakens the cement.
4. Gypsum
- It helps in increasing the initial setting time of the cement.
- It prevents flash set.
5. Iron Oxide
- It Imparts colour, strength, hardness and toughness to the cement.
- It imparts reddish brown tint in the cement.
6. Magnesia
- It also imparts colour and hardness to the cement. It gives yellow tint to the cement.
- If it is in excess it makes the cement unsound.
7. Sulphur
- It is also responsible for inducing the soundness in the cement
8. Alkali
- Presence of alkalis in cement leads to the efflorescence and staining of the structure in which the cement is used for construction.
- The alkalis accelerate the setting of cement paste.
NOTE –
- Where development of much heat of hydration is undesirable, the silica content is increased to about 21% and the alumina and iron oxide contents are limited to 6% each.
- Resistance to the action of sulphate water is increased by raising further the silica content to 24% and reducing the alumina and iron contents to 4% each.
- Small percentage of iron oxide renders the highly siliceous raw materials easier to burn. However, if these are in excess, a hard clinker, difficult to ground, is produced. When the iron oxide combines with lime and alumina to form tetra calcium alumino ferrite, it neutralizes some of the undesirable properties contributed by alumina when combined with lime alone.
Composition of Cement Clinker
Bogue Compounds – When water is added in the cement it reacts with the ingredients of the cement clinker chemically and results in the formation of complex chemical compound termed as Bogue compounds after the name of Bogue who identified them.
Le-Chatelier and Tornbohm have referred these compounds as Alite, Belite, Celite and Felite as shown in above table.
1. Tri-Calcium Silicate
- This compound is formed within a week on the addition of water in cement.
- It is responsible for the development of strength of the concrete in its initial age.
- If in any structure early development of strength is required proportion of tri-calcium silicate is increased. For ex- prefabricate concrete construction, cold weather concrete, where form work is to be removed early in speedy construction.
- Heat of hydration of tri-calcium silicate is higher in comparison to di-calcium silicate.
- The heat of hydration is 500 J/g.
2. Di-Calcium Silicate
- It is the last compound that is formed during the hydration process which may take a year or so for its formation.
- It is responsible for progressive strength of the concrete in its later stage, it means it gives ultimate strength to the structure.
- Heat of hydration is very less in this case.
- Di-calcium silicate in high proportion is used in cement in hydraulic structure (gravity dams, canals etc.), bridges, mass concreting etc.
- The heat of hydration is 260 J/g.
3. Tri-Calcium Aluminate
- It is formed within 24 hours of the addition of water in the cement.
- This compound is responsible for maximum evolution of heat of hydration.
- C3A is fast reacting with water and may lead to an immediate setting of paste and this process is termed as flash set.
- C3A provides weak resistance against sulphate attack.
- The heat of hydration is 865 J/g.
4. Tetra-Calcium Aluminate ferrite
- It is also formed within 24 hours of the addition of the water.
- This compound is also responsible for high heat of hydration.
- The heat of hydration is 420 J/g.
NOTE –
- C3S gives faster rate of reaction accompanied by greater heat evolution and gives early strength.
- On the other hand, C2S hydrate and hardens slowly and provides much of the ultimate strength.
- C3A and C3AF contributes less in strength (‹ 10 %).
Properties of Ordinary Portland Cement
