Concrete is defined as "a composite material that consists essentially of a binding medium within which are embedded particles or fragments of aggregate; in hydraulic-cement concrete, the binder is formed from a mixture of hydraulic cement and water."  Hydraulic-cement concretes are those most widely used in the United States and worldwide.  Hydraulic cement is defined as "a cement that sets and hardens by chemical interaction with water and that is capable of doing so under water."  Portland cement is the most important hydraulic cement.  It is produced by pulverizing portland cement clinker, consisting essentially of hydraulic calcium silicates, usually by intergrinding with small amounts of calcium sulfate compounds to control reaction rates.  It may be used in combination with one or more supplementary cementitious materials such as fly ash, ground granulated blast furnace slag, silica fume or calcined clay.

Aggregate is defined as "granular material, such as sand, gravel, crushed stone, or iron blast-furnace slag, used with a cementing medium to form hydraulic-cement concrete or mortar."

Typical hydraulic-cement concretes have volume fractions of aggregate that range approximately from 0.7 to 0.8.  The remaining volume is occupied initially by a matrix of fresh cement paste consisting of water, cementitious materials, and chemical admixtures, that also encloses air voids.  While the aggregates occupy most of the volume, they are relatively inert and intended to be stable.  It is the cement paste matrix that undergoes the remarkable transformation from nearly-fluid paste to rock-hard solid, transforms plastic concrete into an apparent monolith, and controls many important engineering properties of hardened concretes.