Concrete...

is basically a mixture of two components: aggregates and paste. The aggregate component is normally comprised of sand and gravel or crushed stone. The paste component is normally comprised of cementing materials, (portland cement with or without supplementary cementing materials), water, and air. The paste, acting like a glue, hardens due to a chemical reaction between the cement and water and binds the aggregates together into a rock-like mass - which is known as concrete. The quality of the concrete depends upon the quality of the paste and aggregate, and the bond between the two. In properly made concrete, each and every particle of aggregate is completely coated with paste and all of the spaces between aggregate particles are completely filled with paste. The less water used, the better the quality of the concrete provided the mixture can be consolidated properly.