Cement and additions
Safety Note - Most cements when mixed with water are highly alkaline and can produce alkali burns to unprotected skin. Avoid skin contact with fresh wet concrete.
Cement when mixed with water will form a paste that hardens into a strong rigid material. It is this paste that fills the voids between aggregate particles and binds them all together to form concrete. This hardening process (which occurs by a chemical reaction with water) is known as hydration.
When water is added to cement, hydration products grow outwards around a cement particle as it takes up water. This zone of expanding hydration products will intersect and bind with those from other cement particles and firmly encase aggregate particles thus ‘binding’ the concrete together.
Cement will hydrate under water. If there is an inadequate amounts of water available hydration will stop and the concrete may be of poor quality. The process of keeping water in concrete to facilitate full hydration is known as curing.
Until recently, the term ´cement´ was an abbreviation for Portland cement. However, its meaning has now expanded to cover a greatly increased product range, in which Portland cement is combined with other constituents such as fly ash, also known as pulverised fuel ash (pfa) and ground granulated blast-furnace slag (ggbs).
Pozzolans are natural or industrially produced materials that react with the lime released from the hydration of Portland cement. Natural pozzollans occur in mainland Europe and other parts of the world and have been used in concrete since Roman times. Industrial pozzolans are normally the by-products of other processes and materials of this type include pulverised fuel ash, ground, granulated blastfurnace slag, microsilica and metakaolin. They are widely used in the United Kingdom and elsewhere to improve the properties of concrete, usually by eliminating free lime and converting it into stable cementitious products.