Concrete is a composite; that is, it is composed of several materials held together in one mass. In the case of concrete, it is aggregates which are bound into a mass by cement paste. Concrete has been termed a pseudosolid because, although resembling one, it does not behave as a true solid. The aggregate is used to reduce the amount of cement paste required because cement is the most expensive ingredient of concrete. Economy dictates that in most work the aggregate should compose the greatest possible volume of the mix. Proper aggregate gradation assures this maximum aggregate content in concrete together with such valuable attributes as good workability and easy compaction. When cement and water hydrate or react with one another, the product is the cement paste which binds the aggregate together into a workable construction material. All cement paste is composed of water, unhydrated cement and the products of hydration, the latter being the only elements which contribute to structural strength. There are several products of cement hydration, but by far the most important is cement gel. Portland cement is composed mainly of four chemical compounds. During hydration these chemicals work together and form cement gel.