Materials used to make concrete have an effect on how easily the concrete can be transported, placed and finished. Material properties also affect how fast the concrete gains strength, what strength it will ultimately reach and how well it performs in service. Portland cement, aggregates and admixtures are discussed in this article.
The primary raw materials for portland cement are limestone and clay or shale which contain calcium, silica, iron and alumina. Portland cement is produced in five basic types, each serving a different purpose. Type I is a general purpose cement and is by far the most commonly used. Type II is also a general purpose cement, but with some special characteristics. This cement is moderately resistant to sulfate attack. It is often specified for concrete used in sewage treatment plants and for structures exposed to seawater. Type III cement is used where high early strength is beneficial. Types IV and V cements are the least commonly used and are intended for special purposes. Type IV, a low heat of hydration cement, is primarily used in massive structures such as dams. Type V is used in concrete exposed to soils or groundwaters that have a high sulfate content, generally in the southwestern part of the United States.
Water is one of the four basic ingredients of concrete, along with portland cement, sand (fine aggregate) and gravel or crushed stone (coarse aggregate). The water and cement form a cement paste that coats the aggregate. The quality of the cement paste has the major effect on the quality of the hardened concrete. Adding water dilutes the cement paste and weakens it. The ratio of water to cement (w/c ratio) determines the quality of the paste and, to a large extent, controls the quality of concrete.
Aggregate plays no role in the chemical reactions that cause the cement paste to harden, serving instead as an inexpensive filler material that makes concrete a practical, economical construction material. A well-graded aggregate will have the particle sizes well distributed from fine to coarse and will require the use of less cement than will a poorly-graded aggregate.