Composition of Concrete
The composition of concrete is determined initially during mixing and finally during placing of fresh concrete. The type of structure being built as well as the method of construction determine how the concrete is placed and therefore the composition of the concrete mix (the mix design).
Cement
Portland cement is the most common type of cement in general usage, as it is a basic ingredient of concrete, mortar and plaster. An English engineer named Joseph Aspdin patented Portland cement in 1824, and it was named after the limestone cliffs on the Isle of Portland in England because of the similarity of its color to the stone quarried from Portland. It consists of a mixture of oxides of calcium, silicon and aluminium. Portland cement and similar materials are made by heating limestone (a source of calcium) with clay and grinding this product (called clinker), with a source of sulfate (most commonly gypsum). The resulting powder, when mixed with water, will become a hydrated solid over time.
High-temperature applications, such as masonry ovens and the like, generally require the use of a refractory cement; concretes based on Portland cement can be damaged or destroyed by high heat, whereas refractory concretes can absorb the heat better with less degradation.
Water
Water suitable for human or animal consumption can be used for the manufacture of concrete. The w/c ratio (mass ratio of water to cement) is the key factor that determines the strength of concrete. It is also a key factor in the viscosity of wet concrete, which directly affects its workability during placement. A lower w/c ratio will yield a concrete which is stronger but all else being equal, more difficult to work and described as having a low slump and high viscosity. A higher w/c ratio yields a concrete with a lower strength. Cement paste is the material formed by combination of water and cementitious materials - that part of the concrete which is not aggregate or reinforcing. The workability is affected by both the w/c ratio and the amount of cement paste in the overall mix, as well as the physical characteristics of the aggregates.
-- What you also find are aggregates, admixtures and additions --