Cement tests
1. Fineness test
The fineness of cement has an important bearing on the rate of hydration and hence on the rate of gain of strength and also on the rate of evolution of heat. Finer cement offers a greater surface area for hydration and hence faster the development of strength, fig. below. Different cements are ground to different fineness. The disadvantages of fine grinding is that it is susceptible to air and early hydration. Increase in fineness of cement is also found to increase the drying shrinkage of concrete.
Fineness is necessary to cover surfaces of the fine aggregate particles at better manner – leading to better adhesion and cohesion between cement mortar constituents.
An increase in the fineness of cement, both consistency of cement and workability of concrete increase. Increase in consistency means increase in water demand to get cement paste of standard consistency while increase in workability means increase in flow ability at the same water/cement ratio. In other words decrease in water demand to get the same workable concrete.
The disadvantage of high fineness, include:
- The cost of grinding to a higher fineness is considerable.
- The finer the cement the more rapidly it deteriorates on exposure to the atmosphere during bad storage.
- Finer cement increases the surface area of its alkalis – leads to stronger reaction with alkali- reactive aggregate – cracks and deterioration of concrete.
- Finer cement exhibits a higher shrinkage and a greater proneness to cracking.
- An increase in fineness increases the amount of gypsum required for proper retardation because, in finer cement, more C3A is available for early hydration (due to the increase of its
surface area).
Fineness of cement is tested in two ways: (a) By sieving.
(b) By determination of specific surface (total surface area of all the particles in one gram of
cement) by air-permeability apparatus. Expressed as cm2/gm or m2/kg. Generally Blaine Air permeability apparatus is used.
a) By sieving:
Weigh correctly 100 grams of cement and take it on a standard IS Sieve No. 9 (90 microns). Break down the air-set lumps in the sample with fingers. Continuously sieve the sample giving circular and vertical motion for a period of 15 minutes. Mechanical sieving devices may also be used. Weigh the residue left on the sieve. This weight shall not exceed 10% for ordinary cement. Sieve test is rarely used due to clogging sieves opens with cements particles.
b) By Blaine test
Principle of this method is in observing the time taken for a fixed quantity of air to flow through compacted cement bed of specified dimension and porosity.
Volume of cement bed in the cell can be found by it with mercury before and after putting cement on it then determine the difference between two weights, this difference divided by mercury density in room temperature to obtain volume of cement bed. Repeat this for twice at least and use the average value of volume in following equations, also the weight of cement used in cell must be with porosity equal to 0.5 ± 0.0005.
W= 3.15 V ( 1 – e )
Which: 3.15 = specific gravity of cement
W= weight of cement
V = volume of cement bed e = porosity of cement bed
Place the perforated disc on the ledge at the bottom of the cell and place on it a new filter paper disc. Place the sample of standard cement weighted (W) with knowing surface area, in the cell. Place a second new filter paper disc on the levelled cement. Insert the plunger and press it gently but firmly until the lower face of the cap is in contact with the cell.
Note//For more details download the attached file.
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