Physical Requirements of Cement:
Fineness
The degree of grinding of cement is called fineness. The fineness of cement is a measure of the size of particles of cement. For a given weight of cement the surface area is more for finer cement than for coarser cement. Finer the cement the rate of hydration is more, since more surface area is available for chemical reaction. This output in greater strength development. If it is fine beyond a certain limit its cementative property reduces due to pre- hydration by atmospheric moisture. Fineness is measured in terms of percentage weight retained on IS sieve No. 9 after 15 minutes sieving. It may be measured by air permeability method in cm2/gm of cement.
Setting Time
The phenomenon by virtue of which the plastic cement paste changes into hard mass is called as setting of cement. The setting time at which the cement paste looses its plasticity after the addition of water is known as initial setting time. The time corresponding to the paste becoming a hard mass is known as final setting time. The transportation, mixing, placing and compaction of concrete must be finished within the initial setting time of cement.
Soundness
The phenomenon by virtue of which cement does not undergo large change in volume when treated with water is called as soundness. Unsound cement will disintegrate due to volumetric changes caused by the presence of free lime and magnesia in cement.
Compressive Strength
The quality of cement for its strength in compression is judged by finding the compressive strength of cement sand mortar. For this purpose, cement sand mortar in the ratio of 1: 3 (one part of cement and three parts of standard sand) is used and cubes of 7.06 cm × 7.06 cm × 7.06 cm are prepared and cured.
Heat of Hydration
The setting of cement is due to chemical reaction between cement and water called as hydration. Therefore, during the process of setting, cement chemically reacts with water and heat is generated. This heat is called as heat of hydration since it affects the rate of hydration, which is directly proportional to the generation of heat. Evolution of heat of hydration is advantageous for normal concrete work and it is disadvantageous for concrete work of massive nature like dam.
Specific Gravity
The specific gravity of Portland cement is about 3.15. Specific gravity is not an indication of the quality of cement. It is used in calculation of mix proportions.