Portland cements are made by burning a mixture of calcium carbonate (limestone or chalk) and
silica (clays or shale).
As this mixture is fed through the kiln, initially the calcium carbonate decomposes into calcium oxide and carbon dioxide gas. The calcium oxide then reacts at a higher temperature with the silica to form calcium silicates. The small amount of iron and aluminium present in the clays or shale, also react with the calcium-oxide and act as fluxes permitting the effective formation of calcium silicates at lower temperatures.
Most calcium silicates are unstable; reactive with water to form stable calcium silicate hydrates. Rapid cooling of the cement clinker is essential to ‘freeze’ this unstable material and maximise
the reactive potential of the cement.
The clinker is then ground to the required fineness and gypsum (calcium sulfate) is added to control
the setting of the cement.
The four main compounds found in cement are:
- tri-calcium silicate C3S
- di-calcium silicate C2S
- tri calcium aluminate C3A
- tetra-calcium alumino ferrite C4AF
Each of these compounds react with water in differing ways:
- C3S reacts rapidly, evolving much heat to form calcium silicate hydrate (CSH). It has a high strength and is the main component of the early strength of cement hydrate.
- C2S reacts slowly, forming the same products as C3S, but because of the slow reaction the heat evolved is dissipated before significant temperature rises occur. It contributes increasingly to the strength at later ages.
- C3A reacts very rapidly, evolving much heat and with a very rapid set. This reaction must be retarded by the addition of gypsum during the grinding process, which forms a calcium sulfo-aluminate which blocks the surface of the C3A and slows down its reaction with water. C3A contributes to the very early strength of the cement hydrate, but very little to the strength at later ages.
- C4AF reacts rapidly and does not produce much heat and contributes little to strength.
Calcium silicate hydrates have a crystalline structure which, as the reaction progresses, first loosely interlock giving a stiffening and set to the concrete or mortar and then, as the process continues, fill the voids and closely combine to give strength to the material.
Setting and hardening of concrete is a chemical reaction and not a drying out process. As a result, a mix with insufficient water either initially or during the hydration process will not develop its full potential strength. Likewise, because the reaction is a chemical one, it is temperature dependant: increased temperatures will increase the rate of the reaction and a lower temperature will slow the reaction.
