Portland cement is composed of calcium silicates and aluminate and aluminoferrite. It is obtained by blending predetermined proportions limestone clay and other minerals in small quantities which is pulverized and heated at high temperature – around 1400 deg centigrade to produce ‘clinker’. The clinker is then ground with small quantities of gypsum to produce a fine powder called Ordinary Portland Cement (OPC). When mixed with water, sand and stone, it combines slowly with the water to form a hard mass called concrete.
Cement is a hygroscopic material meaning that it absorbs moisture. In presence of moisture it undergoes chemical reaction termed as hydration. Therefore cement remains in good condition as long as it does not come in contact with moisture. If cement is more than three months old then it should be tested for its strength before being taken into use.
Ordinary Portland Cement (OPC) is produced by 95% of clinker with addition of 5% of Gypsum. It is highly used in concrete work.
Portland Pozzolana Cement (PPC) is obtained by either intergrading a pozzolanic material with clinker and gypsum, or by blending ground Pozzolana with Portland cement. Nowadays good quality fly ash is available from Thermal Power Plants, which are processed and used in manufacturing of PPC. PPC is produce by 70% of Clinker, 5% of gypsum and remaining 25% of fly ash.
Initial set is when the cement paste loses its plasticity and stiffens considerably. Final set is the point when the paste hardens and can sustain some minor load. Both are arbitrary points and these are determined by Vicat needle penetration resistance.
Slow or fast setting normally depends on the nature of cement. It could also be due to extraneous factors not related to the cement. The ambient conditions play an important role. In hot weather, the setting is faster, in cold weather, setting is delayed.
Cement which is in the form of a fine powder has a tendency to absorb moisture present in the atmosphere. When it absorbs moisture it hydrates, and when subsequently used does not contribute to the strength development. Jute bags (gunny bags) in which cement is bagged are neither airtight nor damp-proof and do not prevent absorption of moisture. Cement deteriorates in quality on long storage. Cement bagged in woven polythene bags or paper bags are not likely to deteriorate to the extent mentioned above. The loss of strength also depends on the condition of the godown. It is advisable to use cement within three months of its bagging, or to test the cement for its strength if stored for longer periods. Hence cement bought first should be used first.
Precautions that must be taken in the storage of Portland cement are given below in a series of DON’Ts.
(i)Do not store bags in a building or a godown in which the walls, roof and floor are not completely weatherproof.
(ii)Do not store bags in a new warehouse until the interior has thoroughly dried out.
(iii)Do not be content with badly fitting windows and doors, make sure they fit properly and ensure that they are kept shut.
(iv)Do not stack bags against the wall. Similarly, don’t pile them on the floor unless it is a dry concrete floor. If not, bags should be stacked on wooden planks or sleepers.
(v)Do not forget to pile the bags close together.
(vi)Do not pile more than 15 bags high and arrange the bags in a header-and-stretcher fashion.
(vii)Do not disturb the stored cement until it is to be taken out for use.
(viii)Do not take out bags from one tier only. Step back two or three tiers.
(ix)Do not keep dead storage. The principle of first-in first-out should be followed in removing bags.
(x)Do not stack bags on the ground for temporary storage at work site. Pile them on a raised, dry platform and cover with tarpaulin or polythene sheet.
On the cement bag, week number, month and year of manufacturing are being mentioned and this can be checked before use.