25-08-2017, 09:32 PM
CHEMISTRY INVESTIGATRY
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INDRODUCTION
Cement is essentially a finely ground mixture of calcium silicates (3Cao. SiO2) and aluminates (3Cao. Al2O3) which sets to a hard mass treated with water. This property makes cement capable of joining rigid masses like bricks, stones, tiles etc. into coherent structures. The cements have property of setting and hardening under water due to certain physicochemical process and are, therefore, called hydraulic cements. During setting of cement, the physical changes taking place are gel formation and crystallization and chemical changes are hydration and hydrolysis.
The process of solidification of cement paste involves: (i) setting, and (ii) hardening.
Setting is stiffening of the original plastic mass into initial gel formation. After setting, hardening starts due to gradual start of crystallization in the interior of the mass. The strength developed by cement at any time depends upon the amount of gel formed and the extent of crystallization. A mixture of cement, sand, small pieces of stone (gravel) and water is known as concrete and sets to an extremely hard structure.
When cement is used for construction purposes, it is always mixed with sand and little water to make a pasty material called mortar. Here cement or lime forms the binding material and function of sand is to prevent shrinking and cracking and to increase the bulk, thereby reducing the cost of the mortar. When cement is used as the binding material it is called cement mortar and when lime is used as the binder it is called lime-mortar. Sand in addition to its other functions also increases the adhesive qualities of the binding material.
Effect of quality of sand on setting of cement mortar.
Sand obtained from different sources has different qualities.
For example, sea sand obtained from sea contains some unwanted salts and retards the setting of cement and is unsuitable for making mortar. On the other hand, pit sand obtained from pits in the soil and river sand obtained from river bed are considered excellent for preparing mortar and concrete.
CONCLUSION :-
The strength of the slab increases with
increase in the setting time allowed