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Fineness of cement

The hydration of cement starts at the surface of cement particles. Therefore the surface area is important in determining the amount of area available for hydration. Increasing the fineness of cement will increase the rate of hydration and the rate of gaining strength. However, the cost of grinding of cement and other effects on properties such as workability of fresh concrete and long term properties should be considered to select the appropriate fineness value for cement. 

Fineness is a critical property of cement. High fineness cement will rapidly hydrate, which can generate a high heat of hydration. The high heat of hydration can adversely affect the fresh concrete by causing tensile stress more than the strength of fresh concrete, which can create cracks. Also, the fineness of cement should not be low because this will result in a low rate of hydration and a low rate of gaining strength. Therefore it is essential to determine the specific surface (m2/kg) for cement. The specific surface of cement can be determined by sedimentation or elutriation. These methods are based on stoke's law. The velocity of fall under specific gravity for different spherical particles in a fluid. Also, it can be determined by the air permeability test. The fineness for cement will be determined by measuring the drop of pressure when dry air flows through a bed of cement. The porosity and thickness of the cement bed should be known. The surface area per the unit mass can be related to the permeability of the bed. Air permeability test modified, in which a known volume of air passes at an average pressure. Then, the time taken by the air to flow is measured. Finally, the specific gravity is calculated. 

Figure 1


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