Q. What is portland cement?
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 1500 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.
Q. Is there any shelf life of cement?
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.
Q. What are Supplementary Cementations Materials (SCM)?
Supplementary Cementations Materials (SCM) like silica fumes, meta-kaolin, fly ash, slag are the materials which improve the properties of concrete and enhance its durability, by reducing pore size in concrete through better particle distribution and through increased packing density of the concrete.
Q. How fineness of cement affect strength gain?
Fineness defines the surface area of cement particles present in per unit weight, which implies that more fineness means more particles in unit weight. This enhances the reaction rate which in turn will result in faster gain of strength at earlier stages.
Q. What is blended cement?
Blended cement is obtained by either intergrinding pozzolanic material or slag with clinker along with Gypsum or by blending ground pozzolana or slag with Portland cement. They are also termed as composite cements.
Q. How does blended cement affect durability?
Pozzolana combines with lime and alkalies in the cement and when water is mixed forms compounds which contribute to strength, impermeability and sulphate resistance It also contributes to workability, reduced bleeding and controls destructive expansion from alkali-aggregate reaction. Leaching of free lime is also reduced.
Q. What is setting of cement?
When water is mixed with cement, the paste so formed remains pliable and plastic for a short time. During this period it is possible to disturb the paste and remit it without any deleterious effects. As the reaction between water and cement continues, the paste loses its plasticity. This early period in the hardening of cement is referred to as 'setting' of cement.
Q. What is initial and final setting time of cement?
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.
Q. What are the reasons for slow or fast setting of concrete or mortar?
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 Some types of salts, chemicals, clay, etc if inadvertently get mixed with the sand, aggregate and water could accelerate or delay the setting of concrete.
Q. What are the different grades of Ordinary Portland Cement (OPC)?
The Bureau of Indian Standards (BIS) has classified OPC in three different grades The classification is mainly based on the compressive strength of cement-sand mortar cubes of face area 50 cm2 composed of 1 part of cement to 3 parts of standard sand by weight with a water-cement ratio arrived at by a specified procedure. The grades are:
1. 33 grade
2. 43 grade
3. 53 grade
The grade number indicates the minimum compressive strength of cement sand mortar in N/mm2 at 28 days, as tested by above mentioned procedure.
Q. What is Portland pozzolana cement?
Portland Pozzolana Cement (PPC) is obtained by either intergrinding 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.
Q. What are the advantages of using Portland pozzolana cement over OPC?
Pozzolana combines with lime and alkali in cement when water is added and forms compounds which contribute to strength, impermeability and sulphate resistance It also contributes to workability, reduced bleeding and controls destructive expansion from alkali-aggregate reaction. It reduces heat of hydration thereby controlling temperature differentials, which causes thermal strain and resultant cracking n mass concrete structures like dams.
Q. Why is the colour of PPC sometimes different from OPC?
The colour of PPC comes from the colour of the pozzolanic material used. PPC containing fly ash as a pozzolana will invariably be slightly different colour than the OPC.
Q. Does the shade of cement affect quality?
No. The quality of cement depends upon the raw materials used and the quality control measures adopted during its manufacture, and not on the shade of the cement. The cement gets its colour from the nature and colour of raw materials used, which will be different from factory to factory, and may even differ in the different batches of cement produced in a factory. Further, the colour of the finished concrete is affected also by the colour of the aggregates, and to a lesser extent by the colour of the cement. Preference for any cement on the basis of colour alone is technically misplaced.
Q. What is slag?
Slag is a non-metallic product consisting essentially of glass containing silicates, alumino-silicates of lime and other bases and is obtained as a by-product in the manufacture of pig iron in a blast furnace or electric furnace. Granulated slag is used in the manufacture of Portland Slag Cement (PSC).
Q. How is PSC made?
PSC is made by intergrinding clinker, granulated blast furnace slag and gypsum, or by blending ground slag with Portland cement.
Q. Where can PSC be used?
Slag cement can be used for all plain and reinforced concrete constructions, mass concreting structures such as dams, reservoirs, swimming pools, river embankments, bridge piers, etc. It is used with advantage where low heat of hydration and resistance to alkali-silica reaction are desired, for structures in aggressive environments where chemical and mildly acidic waters are encountered (where the use of OPC is not recommended), for marine constructions, dykes, wharves, etc where sulphatic water is encountered. In short, PSC can be used wherever OPC is used.
Q. What is the effect of long storage periods on cement?
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.
Q. How should cement be stored?
Precautions that must be taken in the storage of Portland cement are given below in a series of DON'Ts.
1. Do not store bags in a building or a godown in which the walls, roof and floor are not completely weatherproof.
2. Do not store bags in a new warehouse until the interior has thoroughly dried out.
3. Do not be content with badly fitting windows and doors, make sure they fit properly and ensure that they are kept shut.
4. 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.
5. Do not forget to pile the bags close together
6. Do not pile more than 15 bags high and arrange the bags in a header-and-stretcher fashion.
7. Do not disturb the stored cement until it is to be taken out for use.
8. Do not take out bags from one tier only. Step back two or three tiers.
9. Do not keep dead storage. The principle of first-in first-out should be followed in removing bags.
10. 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.
Q. How to identify the time for which the cement was stored before use?
On the cement bag, week number, month and year of manufacturing are being mentioned and this can be checked before use.