Hello, friends, I Pushp Pulkit the admin of this blog welcomes you to my blog. Today we will talk about what is cement and their types.
What is Cement
Cement constitutes of calcareous (from calcium carbonate) and argillaceous (from clay) compounds. Cement is a bonding adhesive material which binds brick to brick in the wall and in concrete to bind with aggregate and bars.
Composition of cement
Calcareous compounds(from calcium carbonate)
- Lime – CaO – 62-65%
- Silica – SiO2- 17-25%
- Alumina- Al3O3 – 3-8%
- Calcium Sulphate – CaSO4 – 3-4%
- Iron Oxide – Fe2O3 – 3-4%
- Magnesia – MgO – 1-3%
- Sulphur- S – 1-3%
- Alkalies – Na2O, K2O – 0.2-1%
Functions of different constituents of cement
- It imparts strength and soundness to cement. If it in excess it causes unsoundness causing the cement to expand and disintegration. If there is a deficiency it decreases the strength of cement and causes it to set.
- Soundness is the property by virtue of which Lime reacts with water resulting in the formation of compound slake. Lime whose volume is more than that of the CaO and causes the expansion.
When water is added in cement it reacts with ingredients of cement chemically resulting in the formation of complex chemical compounds Bogues compounds which are not formed simultaneously (Hydration).
2. Tri- Calcium Aluminate
- It is formed within 24hrs of the addition of water in the cement and is responsible for the maximum amount of heat of hydration(exothermic reaction – hydration of cement ).
3. Tetra-Calcium Alumino Ferrite
- It is also formed within 24hrs from the addition of water in cement.
- The amount of heat evolved during the formation of this compound initially is comparatively more, which goes on decreasing with time.
4. Tri-Calcium Silicate
- It is formed within a week or so of the addition of water in cement.
- It is responsible for the development of early strength in the cement in its initial stages. (e.g. road construction, cold weather areas, prefabricated construction).
In cold weather area, rate of hydration is less due to less temperature thus C3S content is increased.
- Manufacturing of sleepers bridge girders and speedy construction require early development of strength i.e high C3S content.
5. Di-Calcium Silicate
- It is formed very slowly after the addition of water in cement may require a year or so for its formation.
- The progressive strength development of cement is characterised by C2S.
- The strength of concrete is used for design and analysis concrete structures is characteristic strength (28 days) not ultimate strength.
- All hydraulic structures (dam, bridges) which take a longer duration of completion and demand high strength in later stages, utilise cement with high C2S content.
- The heat of hydration of cement for 7 days is approx- 89 to 90 cal/gm and during 28 days is 90 to 100 cal/gm.
- For manufacturing low heat portland cement C3A and C3S content is reduced preferably.
- About 23% water by wt. of cement is required to carry out complete hydration. About 15% of water added to cement is imbibed within the voids of cement particles. Hence effectively 38% of water is required (by wt. ) to carry out complete hydration of cement.
Manufacturing Of Cement
Manufacturing of cement includes 3 Basic operations.
- Mixing of ingredients.
Manufacturing of cement can be done by any of the following two methods:-
- Dry mixing(Recent)
- Wet mixing
- Calcareous compounds + Argillaceous compounds
- Crushing (25 mm)
- Fine grinding in Boll/ tube mill
- storage channels
- Mixing of raw ingredients in corrected proportions
- Pre-heating of ingredients at 800 celsius by exhaust gases
- Fed to rotary kiln for burning
- Formation of clinkers
- Fine grinding of clinkers in the tube mills
- cement silos
- packaging plant
Pre-heating of ingredients reduces fuel for burning, thus process is economical.
Types of Cement
1.Rapid hardening cement
- It is a type of cement which shows higher rate of development of strength and must not be confused with quick setting cement that only sets quickly.
- The strength of cement at the age of 3 days is the same as strength at 7 days of OPC.
- This role of gain of strength in rapid hardening cement gradually subsidize and at the age of 90 days, the strength of rapid hardening cement is the same as that of OPC.
- This cement is produced by fine grinding the cement in increasing the proportion of C3S and reducing proportion C2S.
- use – cold weather construction, prefabricated work, emergency road, repair works, reuse of formwork for speedy construction.
2. Extra rapid hardening cement
- This cement is manufactured by intergrinding rapid hardening cement clinkers with calcium chloride, the proportion of ionic should not be greater than 2% by wt. of cement.
- This cement should be mixed, transported, placed, compact and finished within 20 min of its formation.
- This cement has approx. 20 -25% higher strength than rapid hardening cement at the age of 1-2 days 10-15% higher strength at the age of 7 days.
- This rate of gain of strength decreases with time and strength of this cement at the age of 90 days is same as that of OPC.
- This cement finds its application where rapid hardening cement is used.
3. Sulphate resisting cement
- OPC is highly susceptible to the attack of sulphate, especially to that of magnesium sulphate that reacts with both calcium hydroxide to form calcium sulphate and with calcium aluminate to form calcium sulphoaluminate, the volume of which is approximately 227% more than original constituent.
- This cement is manufactured by reducing proportions of C3A and C4AF such that C3A should not be greater than 5%.
- This type of cement finds its application in foundation work, sewage treatment work, marine structures, construction of pipes in marshy areas.
Application of bituminous plant or construction of brickwork surrounding foundation to avoid contact between soil and foundation.
It develops strength slowly and has a low heat of hydration.
4. Super sulphated cement
- This type of cement is manufactured by intergrinding granulated blast furnace slag 15% hard burnt gypsum and 5% cement clinker. (blast furnace slag: leftover after burning of cement)
- Its cost is very less as cement clinker amount is very less.
- This cement finds its applications in all the structures which sulphate resisting cement is used.
- Due to high fineness, it has less permeability i.e it is used in watertight hydraulic structures.
5. Portland slag cement
- This cement is manufactured by intergrinding granulated blast furnace slag with gypsum and cement clinkers in a definite proportion.
- This cement offers higher resistance against chlorides on sulphates attack.
- It posses higher water tightness due to its reduced permeability that is achieved by the refinement of pore structures (smaller sized voids).
6. Quick Setting Cement
- This cement is produced by adding a small quantity of Aluminium sulphate, fine grinding the cement and reducing the proportion of gypsum.
- This cement is generally used in grouting operations and underwater concreting.
7. Low heat cement
- This cement is produced by reducing proportions of C3A and C3S and increasing proportions of C2S.
- The cement shows a slow rate of development of strength.
- The heat of hydration of cement during 7 days is limited to 65 cal/gm and during 28 days is limited to 75cal/gm.
- This type of cement is generally used where mass concreting is to be done. eg- dam construction.
8. Portland Pozzolana cement
- This cement is manufactured by intergrinding cement clinkers with 10-25% pozzolanic materials. Pozzolanic materials are essentially siliceous and aluminous compounds which in themselves do not possess any cementitious property but when finally divided in the presence of water reacts with calcium hydroxide formed doing hydration of cement resulting in the formation of a compound which posses cementitious property.
- As cement is replaced by pozzolanic material its cost is reduced but it develops its strength slowly.