Q.2 What is cement? Explain its chemical composition and uses. Also define concrete.
Related Questions -
Q. Write a brief note on cement indicating its properties and applications. (AKTU. 2012 - 13)
Ans. Cement is a solid structural material which is a combination of CaO, Al2O3 and SiO2. The most extensively used cementing material is Portland cement. Now-a-days cement is produced by a partial fusion of a mixture containing clay, limestone, oxides of silicon, aluminium, small quantity of iron and magnesium. When heated up to 1450*C, the mass hardens to a clinker which is a fused stony material. This material is crushed and mixed with few percent of gypsum and then is ground to a fine powder containing particles not larger than 0.001 cm.
Composition of an Ordinary Port Land Cement: -
Some other elements and glass amount to 9% by weight including about 3% Gypsum. Cement is largely used in civil construction work and is an important content of concrete. The setting of Portland cement is an exothermic reaction as a result of which the heat liberated pose problems for big structures like dams, bridges, flyovers, multistory buildings, etc. This heat can be removed by pipes which are incorporated in the structure and remain after setting as a means of reinforcement. Portland cement taken nearly 90 - 100 days to achieve its full strength and hardness.
Another category of cement is High Alumina cement which consists of CaO and Al2O3. High Alumina cement has a high setting rate. It achieves its full strength and hardness in a very few days.
Concrete: -
Concrete is a mixture of cement, sand and stones. It is widely used in civil construction works like dams, bridges, high rise buildings, etc.
In order to obtain high strength, it is important to use the correct ratio of water to cement. If the quantity of water is small, entrapped air causes pores and therefore produces weaker structure. Too much water also leaves pores. Ultrasonic vibration technique is used to compact the cement and to remove air from its pores.
Reinforced Cement Concrete: -
Concrete produces by casting over steel rods is known as reinforced cement concrete.
Advantages of RCC: -
(a) It is fire resisting.
(b) It is good in both tensile and compressive strengths.
(c) It is very durable and is not attacked by termites.
(d) Monolithic structures can be made of RCC.
(e) Water can not penetrate easily into it and for practical purposes, it is impermeable when well proportioned in composition.
(f) Different shapes of RCC members can be easily cast.
Q.3 Write short note on ceramics and its application. (AKTU. 2008 - 09, 09-10, 10 - 11)
Ans. Ceramic: -
A ceramic is an inorganic, non-metallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous (e.g., a glass). Because most common ceramics are crystalline, the definition of ceramic is often restricted to inorganic crystalline materials, as opposed to the non-crystalline glasses.
The earliest ceramics were pottery objects made from clay, either by itself or mixed with other materials. Ceramics now include domestic, industrial and building products and art objects. In the 20th century, new ceramic materials were developed for use in advanced ceramic engineering; for example, in semiconductors.
Types of Ceramic Materials: -
A ceramic material is often understood as restricted to inorganic crystalline oxide material. It is solid and inert. Ceramic materials are brittle, hard, strong in compression, weak in shearing and tension. They withstand chemical erosion that occurs in other materials subjected to acidic or caustic environment. Ceramics generally can withstand very high temperatures such as temperatures that range from 1,000 °C to 1,600 °C (1,800 °F to 3,000 °F). Exceptions include inorganic materials that do not include oxygen such as silicon carbide or silicon nitride. A glass is often not understood as a ceramic because of its amorphous (non-crystalline) character. However, glass making involves several steps of the ceramic process and its mechanical properties are similar to ceramic materials.
Traditional ceramic raw materials include clay minerals such as kaolinite, whereas more recent materials include aluminium oxide, more commonly known as alumina. The modern ceramic materials, which are classified as advanced ceramics, include silicon carbide and tungsten carbide. Both are valued for their abrasion resistance, and hence find use in applications such as the wear plates of crushing equipment in mining operations. Advanced ceramics are also used in the medicine, electrical and electronics industries.
Crystalline Ceramics: -
Crystalline ceramic materials are not amenable to a great range of processing. Methods for dealing with them tend to fall into one of two categories - either make the ceramic in the desired shape, by reaction in situ, or by “forming” powders into the desired shape, and then sintering to form a solid body. Ceramic forming techniques include shaping by hand (sometimes including a rotation process called “throwing”), slip casting, tape casting (used for making very thin ceramic capacitors, etc.), injection moulding, dry pressing, and other variations. A few methods use a hybrid between the two approaches.
Non-Crystalline Ceramics: -
Non-crystalline ceramics, being glasses, tend to be formed from melts. The glass is shaped when either fully molten, by casting, or when in a state of toffee-like viscosity, by methods such as blowing to a mold. If later heat-treatments cause this glass to become partly crystalline, the resulting material is known as a glass-ceramic.
Properties Of Ceramics: -
Ceramics exhibit the following properties
1. They have low thermal conductivity due to strong ionic-covalent bonding.
2. Ceramics are good thermal insulators and hence they have high heat resistance.
3. They have high hardness.
4. They have good creep resistance.
5. They have good chemical stability.
6. Ceramics have high elastic modulus
7. Ceramics have high compressive strength.
8. They have low toughness and low density.
9. Most ceramics have low specific gravity.
10. They have high melting temperatures.
Types of Ceramic Products: -
For convenience, ceramic products are usually divided into four sectors; these are shown below with some examples:
· Structural, including bricks, pipes, floor and roof tiles
· Refractories, such as kiln linings, gas fire radiants, steel and glass making crucibles
· Whitewares, including tableware, wall tiles, pottery products, and sanitary ware
· Technical, is also known as Engineering, Advanced, Special, and in Japan, Fine Ceramics. Such items include tiles used in the Space Shuttle program, gas burner nozzles, ballistic protection, nuclear fuel uranium oxide pellets, bio-medical implants, jet engine turbine blades, and missile nose cones. Frequently the raw materials do not include clays.
