Concrete is a composite material, which is made from a mixture of cement, aggregate (sand or gravel), water and sometimes admixtures in required proportions. It is one of the most important and useful materials for construction work.
When all the ingredients (cement, aggregate, water) are mixed in the required proportions, the cement and water start a reaction with each other to bind themselves into a hardens mass. This hardens rock-like mass is known as concrete.
Study of concrete materials :
Portland cement is the most common type of cement in general usage. It is a basic ingredient of concrete, mortar and many plasters. It consists of a mixture of calcium silicates , aluminates and ferrites – compounds which combine calcium, silicon, aluminum and iron in forms which will react with water. Portland cement and similar materials are made by heating limestone with clay and grinding this product with a source of sulfate .
Combining water with a cementitious material forms a cement paste by the process of hydration. The cement paste glues the aggregate together, fills voids within it, and makes it flow more freely.
Fine and coarse aggregates make up the bulk of a concrete mixture. Sand, natural gravel, and crushed stone are used mainly for this purpose. Recycled aggregates are increasingly used as partial replacements for natural aggregates, while a number of manufactured aggregates, including air-cooled blast furnace slag and bottom ash are also permitted.
Chemical admixtures are materials in the form of powder or fluids that are added to the concrete to give it certain characteristics not obtainable with plain concrete mixes. In normal use, admixture dosages are less than 5% by mass of cement and are added to the concrete at the time of batching/mixing.