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In the manufacture of cement definite proportions of argillaceous and calcareous materials are burnt at a temperature of
It is necessary while manufacturing cement to maintain a temperature around 1400° Celsius in the “Rotary Kiln” for proper production of cement. Hence, nearest option is 1450°C.
The percentage of gypsum added to the clinker during manufacturing process is
Gypsum controls the rate of hardening of the cement. During the manufacturing process, a small amount of gypsum during the final grinding process helps to control the setting time of cement.
Flash set of cement would lead to premature stiffening of cement and hence it is undesirable for construction work.
Gypsum is added to the cement after cooling of the clinkers in around 3 to 5%.
The most suitable option among all of the above is 2.5 to 3.5%
The setting and hardening of cement after addition of water is due to
Setting is due to hydration of C3A and hardening is due to hydration of C3S and C2S known as Bogue Compounds of Cement.
Gypsum is a retarder and delays the setting time.
The tricalcium aluminate compound present in cement
The compound C3A characteristically reacts fast with water and may lead to an immediate stiffening of paste, and this process is termed flash set. It provides weak resistance against sulphate attack and its contribution to the development of strength of cement is less significant than that of silicates. In addition, the C3A phase is responsible for highest heat of evolution.
The contribution compounds of cement in decreasing order of rate of hydration are
C3A having a faster rate of reaction accompanied by greater heat evolution develops early strength. On the other hand C2S hydrates and hardens slowly and provides much of the ultimate strength.
When water is added to the cement
When Portland cement is blended with water, heat will be generated. This heat is named the heat of hydration, and it is the product of the exothermic chemical reaction between cement and water.
Initial setting time is maximum for
Initial setting time mainly depends on the Bogue component C3A (Tri Calcium Aluminate). This component is responsible for setting of cement, however, it also increases heat of hydration produced.
Thus, low-heat Portland pozzolana cement has low concentration of C3A leading to high initial setting time.
In medium-strength concrete the water-cement ratio should not be less than
Rich: 1 part cement, 2 parts sand, 3 parts coarse aggregate. A rich mix is used for concrete roads and waterproof structures.
Standard: 1 part cement, 2 parts sand, 4 parts coarse aggregate. A standard mix is used for reinforced work floors, roofs, columns, arches, sewers, tanks etc.
Medium: 1 part cement, 2 1/2 parts sand, 5 parts coarse aggregate. A medium mix is used for foundations, walls, abutments, piers, etc. For a medium strength concrete the water-cement ration should not be less than 0.4.
Lean: 1 part cement, 3 parts sand, 6 parts coarse aggregate. A lean mix is used for all mass concrete work, large foundations, backing for stone masonry, etc.
An excess of free lime in portland cement
Unsoundness in cement is due to excess of lime, excess of magnesia or excessive proportion of sulphates. Le chatelier test detects unsoundness due to free lime only. Autoclave test is used to detect unsoundness due to magnesia and lime.
In the air permeability test, the specific surface (in mm2/g) is of the order of
Air permeability method is used to find the fineness of cement. The cement is generally of order of 225000 to 350000 mm2/g.
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