Cement

With external renders, two types of cement are used. OPC (Ordinary Portland Cement) is 75% limestone and 25% cement and grey. White Portland Cement is 75% limestone and 25% white china clay and is white. WPC is used when a lighter coloured render is needed.Cement sets due to a chemical reaction so temperature effects the time it takes to set. If the cement is in a warm environment, it will set quicker than in a cold environment. The final set time should be no more than 10 hours.
Special purpose cements are sometimes used. For example, sulphate-resisting portland cement will be used instead of normal portland cement if there is sulphate present .

Lime is normally used in external plastering on old buildings.

A cement is a binder, a substance that sets and hardens and can bind other materials together. The word "cement" traces to the Romans, who used the term opus caementicium to describe masonry resembling modern concrete that was made from crushed rock with burnt lime as binder. The volcanic ash and pulverized brick supplements that were added to the burnt lime, to obtain a hydraulic binder, were later referred to as cementum, cimentum, cäment, and cement.

Cements used in construction can be characterized as being either hydraulic or non-hydraulic, depending upon the ability of the cement to be used in the presence of water.

Non-hydraulic cement will not set in wet conditions or underwater, rather it sets as it dries and reacts with carbon dioxide in the air. It can be attacked by some aggressive chemicals after setting.

Hydraulic cement is made by replacing some of the cement in a mix with activated aluminium silicates, pozzolanas, such as fly ash. The chemical reaction results in hydrates that are not very water-soluble and so are quite durable in water and safe from chemical attack. This allows setting in wet condition or underwater and further protects the hardened material from chemical attack (e.g., Portland cement).

The chemical process for hydraulic cement found by ancient Romans used volcanic ash (activated aluminium silicates). Presently cheaper than volcanic ash, fly ash from power stations, recovered as a pollution control measure, or other waste or by products are used as pozzolanas with plain cement to produce hydraulic cement. Pozzolanas can constitute up to 40% of Portland cement.

The most important uses of cement are as a component in the production of mortar in masonry/brickwork, and of concrete, a combination of cement and an aggregate to form a strong building material.