CEMENT

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

Подпись: Hydration The reaction between the cement and water is an exothermic (meaning ‘giving off heat’) chemical reaction known as ‘hydration’, which results in the setting of the cement paste. Concrete, and mortars for that matter, set as a result of the completion of the hydration process and not simply by drying out.

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’.

CEMENT

Fig. 1 Ordinary Portland cement.

Подпись: Fig. 2 A 25-kg bag of Ordinary Portland cement.
Подпись: Cement Hazards Cement is classified as an irritant, having the potential to cause severe cases of dermatitis and burning of the skin. Great care must therefore be taken when handling cement (and the materials made from it) in order to avoid contact with the skin or eyes and breathing in cement dust. The use of PPE (personal protective equipment) is essential.

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.