Cement is generally regarded as the most important binding material used in the construction process and is used for the manufacture of mortar and concrete. The use of cement is so common that it is the second most consumed substance in the world after water! The most commonly used is Ordinary Portland cement (OPC), so called
because, in its solid state, its grey colour is much like that of natural Portland stone. The fact that Portland cements form a solid when mixed with water means they are often referred to as ‘hydraulic cements’.
Fig. 1 Ordinary Portland cement.
Cement is made from chalk (calcium carbonate) excavated from natural chalk deposits, which is then heated in a rotating cement kiln at temperatures of up to 1450 degrees centigrade. The resultant clinker is then ground to a fine powder, which is then packaged in 25-kg bags.
Although Ordinary Portland cement is the most commonly used for concrete and general construction purposes, there are various other types with slightly altered chemical
compositions for specialized circumstances. These include rapid-hardening cement, sulphate-resisting cement, special cements for working in cold weather, and so on.
Being hydraulic, bags of cement must be protected from damp before use. Preferably, bags should be stored clear of the ground on a wooden pallet in a well-ventilated, rain-proof shed. Bags of cement should be stacked flat, no more than five bags high, otherwise the bags at the bottom will set due to the pressure exerted by the bags above. ‘Pressure setting’, as it is called, is sometimes known as ‘warehouse setting’.
Even when stored in ideal conditions, cement still has a ‘shelf life’ and will lose around 20 per cent of its strength over a period of a couple of months. Accordingly, care must be taken to ensure that cement is used in the same sequence in which it was delivered – in other words, old bags first! Under no circumstances should cement that has been exposed to moisture and contains lumps be used as it will produce a weak and less durable mix.