Table of Content for Cement-
- Composition of Cement
- Bogue’s Compounds
- Site Testing
- Lab Testing
The property of hardening when mixed with water makes the cement a popular construction material. The presently used form of cement started developing after William Aspdin accidentally discovered the calcium silicates. On hardening, it appeared similar to a stone found in Isle of Portland, England. Thus, the name Portland Cement came into existence. OPC is the most basic form of cement. The properties of cement are strength, setting time, soundness, etc. The properties can be altered as desired by changing the percentage composition of the limestone, alumina, and silica.
The cement is used with sand, and aggregate to make the concrete. On mixing water to the above-said mixture, concrete is produced. The properties of concrete can be modified as required by changing the proportion of its elements. Further, admixtures can also be used for controlling the properties of the mix.
Types of Cement-
The properties of various types of cement differ from each other.
- Ordinary Portland Cement (OPC)- It is prepared by calcining limestone with silica and alumina.
- Pozzolana Portland Cement (PPC)- It is prepared by grinding a 10-15% fly-ash with the OPC.
- Rapid Hardening- It is used where early strength of concrete is required. It is finer than the OPC.
- Extra Rapid Hardening – It is prepared by adding accelerators such as calcium carbide in a small amount. The rate of gain of strength is higher.
- Quick Setting- It is used where a quick setting is required. It has a lower setting time than OPC.
- Portland Slag- It is prepared by grinding OPC with the slag.
- Sulphate Resisting- It is used for the concrete that is to be placed in contact with sulphate containing aggressive soil.
- Low Heat- It is used for mass-concreting works such as dams etc. It generates lesser heat of hydration. The rate of gain of strength is also lower.
- High Alumina- It is made up of calcium aluminate unlike the calcium silicate as in OPC.
- Hydrophobic- It is prepared by grinding OPC clinker with the film-forming substance. It is used when cement is to be stored in high humidity areas.
- White- It has very small or no percentage of iron oxide.
- Low Alkali- It has alkali content lesser than 0.6%. It is used where aggregate has silica avoid the alkali-aggregate reaction.
- Coloured- It is prepared by grinding 5-10% of suitable pigments with white or grey cement.
Composition of Cement-
The cement composition can be broadly classified into two heads-
- Calcareous- This term refers to the Calcium. This head is represented by the lime.
- Argillaceous- This term refers to the Alumina. This head is represented by the clay.
The cement composition should also satisfy the following criteria-
The chemical composition is as follows-
Lime- It imparts strength and soundness. The excess of lime makes the cement unsound, increases expansion, and disintegration. The deficiency of lime leads to lower strength and quick setting.
Silica- It imparts strength. The excess of silica increases the strength and setting time.
Alumina- It imparts strength. The excess of the alumina increases the strength and setting time. It also aids in reducing the clinkerisation temperature.
Gypsum- It is present in the form of calcium sulphate. It is added to the clinker when grinding to increase the initial setting time.
Iron Oxide- It imparts strength and hardness to the cement. The grey colour is due to iron oxide.
Magnesia- It imparts strength and yellowish tint. The excess of magnesia makes the cement unsound.
Sulphur- It imparts soundness.
Alkalis- Alkalis cause efflorescence and staining on the finished surface after hydration. They cause white-grey spots on reacting with water.
The calcareous and argillaceous constituent fuse together to make four types of compounds known as Bogue’s Compounds. These compounds govern the properties of the cement. The silicate compounds formed are mainly responsible for the strength. The alumina and iron compounds contribute little to the strength development.
Upon hydration, the silicate compounds form an amorphous gel of calcium silicate hydrate and calcium hydroxide. This gel is called C-S-H gel. The C-S-H gel binds the cement particles together and provides strength.
Tricalcium Aluminate, C3A- It is formed within 24-hours of the hydration. It gives maximum heat of hydration.
Tetra Calcium Alumino Ferrite, C4AF- It is formed within 24-hours of the hydration. The heat of hydration is comparatively higher in the initial stage of hydration and decreases substantially.
Tricalcium Silicate, C3A- It is formed till a week of the hydration. It provides strength in the first week after the hydration.
Dicalcium Silicate- It is formed very slowly and the strength developed due to it may take a year. Thus, it can be said that C2S responsible for the progressive strength in the later stages.
Order of Heat of Hydration- C3A > C3S > C4AF > C2S
The testing of cement can be conducted on site and as well as in the laboratory.
- It should not be older than three months.
- The colour should be uniform.
- The lumps should not be present.
- On inserting a hand inside the cement bag, it should feel cold.
- On throwing a hand full of cement into the bucket of water, it should sink.
- Fineness Test- By Sieve & air permeability test.
- Standard Consistency Test- By Vicat apparatus.
- Setting Time Test- By Vicat apparatus
- Soundness Test- By Le-Chatelier apparatus
- Strength Test- By compression testing machine
- Chemical Composition Test
For more on Bogue’s Compounds- http://shodhganga.inflibnet.ac.in/jspui/bitstream/10603/142503/9/09_chapter%203.pdf