•    Concrete is a composite construction
material composed of cement and other cementitious materials such as fly ash and slag cement, aggregate (generally a coarse aggregate made of gravels or crushed rocks such as limestone, or granite, plus a fine aggregate such as sand), water, and chemical admixtures.

•    Recently, the use of recycled materials as concrete ingredients has been gaining popularity because of increasingly stringent environmental legislation. The most conspicuous of these is fly ash, a by-product of coal-fired power plants. This significantly reduces the amount of quarrying and landfill space required, and, as it acts as a cement replacement, reduces the amount of cement required.

•    Concrete is strong in compression, as the aggregate efficiently carries the compression load. However, it is weak in tension. Reinforced concrete solves these problems by adding either steel reinforcing bars, steel fibres, glass fibre, or plastic fibre to carry tensile loads.

•    Chemical admixtures can be added to the concrete to give it certain characteristics not obtainable with plain concrete mixes. In normal use, admixture dosages are less than 5% by mass of cement, and are added to the concrete at the time of. Types include:
Accelerators speed up the hydration (hardening) of the concrete. However, use of chlorides may cause corrosion in steel reinforcing.
Retarders slow the hydration of concrete, and are used in large or difficult pours where partial setting before the pour is complete is undesirable.
Air entrainments add tiny air bubbles to the concrete, which will reduce damage in freezing temperatures thereby increasing the concrete's durability. However, entrained air is a trade-off with strength, as each 1% of air may result in 5% decrease in compressive strength.
Plasticizers increase the workability of fresh concrete, allowing it to be placed more easily, with less effort. Plasticizers can be used to reduce the water content of a concrete while maintaining workability, improving its strength and durability characteristics.
Pigments can be used to change the colour of concrete, for aesthetics.
Corrosion inhibitors are used to minimize the corrosion of steel and steel bars in concrete.
Bonding agents are used to create a bond between old and new concrete.

•    Mineral admixtures are inorganic very fine-grained materials that can be added to the concrete mix to improve its properties or as a replacement of cement. They include:
Fly ash: A by product of coal fired electric generating plants, it is used to partially replace cement. The properties of fly ash depend on the type of coal burnt.

Ground granulated blast furnace slag: A by-product of steel production is used to partially replace cement.
Silica fume: A by-product of the production of silicon and alloys. Silica fume is used to increase strength and durability of concrete, but generally requires the use of superplasticisers for workability

High reactivity Metakaolin: Metakaolin produces concrete with strength and durability similar to concrete made with silica fume. While silica fume is usually dark gray or black in colour, high reactivity metakaolin is usually bright white in colour, making it the preferred choice for architectural concrete where appearance is important.