may have absorptions of around 5 or more it is the reacted cement and water paste in the concrete mass that has the higher absorption values the cement in concrete needs water to hydrate and harden hydration is a chemical reaction in which the major compounds in cement form chemical bonds with water molecules and become hydrates or hydration products the calcium silicates are the main part of the chemical reaction when the calcium silicates first come into contact with water the reaction occurs calcium ions are formed and the water molecules are broken down to form hydroxide ions this bond breaking also produces heat hardened concrete contains some entrapped air from the resulting chemical reaction of cement and water sometimes air entrainment admixtures are added to increase the air content of concrete for engineering purposes most exterior structures have this to reduce freeze thaw and other weathering effects freshly mixed concrete can be cast into any shape desired it is one of the most widely used construction materials in the world because of its abundance many structures have been built with it worldwide when these structures have reached their service life they are subsequently demolished this demolition serves as a source of raw.