Cement

Introduction

Cemet is finely ground, usually grey coloured mineral powder. when mixed with water, cement act as a glue to bind together the sand gravel and crushed stone to from concrete, the most widely used construction material in the world.
Cement is the general term given to the powdered materials which initially have plastic flow when mixed with water or other liquid, but has the property of setting to a hard solid structure in several hours with vary degree of strength and bonding properties.
cement, plaster of paris, and lime, which, as a group, are produced in extremely large quantities. The characteristic feature of these materials is that when mixed with water, they form a paste that subsequently sets and hardens. This trait is especially useful in that solid and rigid structures having just about any shape may be expeditiously formed. Also, some of these materials act as a bonding phase that chemically binds particulate aggregates into a single cohesive structure. Under these circumstances, the role of the cement is similar to that of the glassy bonding phase that forms when clay products and some refractory bricks are fired. One important difference, however, is that the cementitious bond develops at room temperature.
Of this group of materials, portland cement is consumed in the largest tonnages. It is produced by grinding and intimately mixing clay and lime-bearing minerals in the proper proportions and then heating the mixture to about 1400℃ (2550F) in a rotary kiln;this process,sometimes called calcination, produces physical and chemical changes in the raw materials. The resulting “clinker” product is then ground into a very fine powder to which is added a small amount of gypsum (CaSO₄-2H₂O) to retard the setting process. This product is portland cement. The properties of portland cement, including setting time and final strength, to a large degree depend on its composition.
Several different constituents are found in portland cement, the principal ones being tricalcium silicate (3CaO-SiO₂) and dicalcium silicate (2CaO-SiO₂). The setting and hardening of this material result from relatively complicated hydration reactions that occur among the various cement constituents and the water that is added.
For example, one hydration reaction involving dicalcium silicate is as follows:

2CaO-SiO₂ + xH₂O S 2CaO-SiO₂ -xH₂O
where x is variable and depends on how much water is available. These hydrated products are in the form of complex gels or crystalline substances that form the cementitious bond. Hydration reactions begin just as soon as water is added to the cement. These are first manifested as setting (i.e., the stiffening of the once-plastic paste), which takes place soon after mixing, usually within several hours. Hardening of the mass follows as a result of further hydration, a relatively slow process that may continue for as long as several years. It should be emphasized that the process by which cement hardens is not one of drying, but rather of hydration in which water actually participates in a chemical bonding reaction.
Portland cement is termed a hydraulic cement because its hardness develops by chemical reactions with water. It is used primarily in mortar and concrete to bind aggregates of inert particles (sand and/or gravel) into a cohesive mass; these are considered to be composite materials. Other cement materials, such as lime, are nonhydraulic; that is, compounds other than water (e.g., CO₂) are involved in the hardening reaction.