Q: What is blended cement?
A: 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: What is setting of cement?
A: 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 the ‘setting’ of cement.
Q: What is initial and final setting time of cement?
A: 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 are determined by Vicat needle penetration resistance.
Q: What are the different grades of Ordinary Portland Cement (OPC)?
A: 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 50cm2 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:
Q: Is there a universal international specification for Portland cement?
A: Each country has its own standard for Portland cement, so there is no universal international standard.
The United States uses the specification prepared by the American Society for Testing and Materials-ASTM C-150 Standard Specification for Portland cement. There are a few other countries that also have adopted this as their standard, however, there are countless other specifications. Unfortunately, they do not use the same criteria for measuring properties and defining physical characteristics so they are virtually ‘non-translatable’. The European Cement Association located in Brussels, Belgium, published a book titled ‘Cement Standards of the World’.
Q: Are there different types of Portland cement?
A: Though all Portland cement is basically the same, eight types of cement are manufactured to meet different physical and chemical requirements for specific applications:
Type I is a general purpose Portland cement suitable for most uses.
Type II is used for structures in water or soil containing moderate amounts of sulphate, or when heat build-up is a concern.
Type III cement provides high strength at an early state, usually in a week or less.
Type IV moderates heat generated by hydration that is used for massive concrete structures such as dams.
Type V cement resists chemical attack by soil and water high in sulphates.
Types IA, IIA and IIIA, are cements used to make air-entrained concrete. They have the same properties as types I, II and III, except that they have small quantities of air-entrained materials combined with them.
White Portland cement is made from raw materials containing little or no iron or manganese, the substances that give conventional cement its grey colour.
Q: What are the advantages of using Portland pozzolana cement over OPC?
A: 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: Does the shade of cement affect quality?
A: No. The quality of cement depends upon the raw materials used and the quality control measures adopted during its manufacture, not on the shade of the cement. The cement gets its colour from the nature and colour of raw materials used, which will vary from factory to factory and may even differ in different batches produced in a factory. Further, the colour of the finished concrete is affected by the colour of the aggregates. Preference for any cement on the basis of colour alone is technically misplaced.
Q: Why is the colour of PPC sometimes different from OPC?
A: The colour of PPC comes from the colour of the pozzolanic material used. PPC containing fly ash as a pozzolana will invariably be a slightly different colour than OPC.
Q: What is the effect of long storage periods on cement?
A: 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. The quality of cement deteriorates 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?
A: 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, do not 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.|
Q: How to identify the time for which the cement was stored before use?
A: On the cement bag, week number, month and year of manufacturing are being mentioned and this can be checked before use.
Q: How can concrete be damaged?
A: Fire, aggregate expansion, seawater, bacterial corrosion, leaching, physical damage and chemical damage (from carbonation, chlorides, sulphates and distilled water) can damage concrete.
Q: How long must concrete be kept moist (cured)?
A: The period of time needed to properly cure concrete depends upon the type of cement, mixture, proportions, required strength, size and shape of the structure, weather conditions and future exposure conditions. It can range from 3 hours to 3 weeks. Typically, a concrete driveway or sidewalk should be cured for a minimum of 3 days and up to 7 days if possible.