What is Cement?
Cement is a mixture of compounds, consisting mainly of silicates and aluminates of calcium, formed out of calcium oxide, silica, aluminium oxide and iron oxide. Cement is manufactured by burning a mixture of limestone and clay at high temperatures in a kiln, and then finely grinding the resulting clinker along with gypsum. The end product thus obtained is called Ordinary Portland Cement (OPC). In India, OPC is manufactured in three grades, viz. 33 grade, 43 grade and 53 grade, the numbers indicating the compressive strength obtained after 28 days, when tested as per the stipulated procedure. Apart from OPC, there are several other types of cement, most of them meant for special purposes, e.g. sulphate resistant cement, coloured cement, oil well cement, etc. However, there are some general purpose cements, the commonest one being Portland Pozzolana Cement (PPC).
After food and clothing, shelter is the next priority item for humans. Since its evolution, mankind had been pursuing a relentless search for viable building materials for securing a stable shelter. The history of mankind traced through its ancient civilizations and the track record of the past two millennia will show that Man had been using different types of materials for putting up dwelling to provide him shelter from sun, rain and wind and a home for his family.
The building materials used from Stone Age to the Bronze Age in the progressive march of human civilisation ranged from stone or wood, cemented with mud or any other naturally occurring cementing materials (volcanic ash - pozzolan of Italy, natural tuff- Trass of Germany, diatomaceous earth and many others in different countries), to semi-processed materials like lime, burnt clay. With the march of civilization, better binding materials like plastic clay, lime in combination with natural gypsum or in combination with sand (lime mortar), burnt clay ( surkhi), burnt gypsum (plaster of Paris) and powdered naturally occurring rocks like volcanic ash came to be used in different places.
But all these binding materials were not adequate to provide high strength and long-term durability in constructions. The invention of Portland cement brought about a landmark change and provided a satisfactory answer to mankind's quest for a strong and durable binder for constructions. The patent on Portland Cement by Joseph Aspadin in 1824 and subsequent developments have resulted in the cements as we know today. Indeed from the latter half of the 19 thcentury, Portland cement has emerged as a leading binding material and continues to enjoy its pre-eminent position amongst the various cementing materials to this day.
Cement ranks second in volume among the industrial products manufactured in the world. The presence of Portland cement as binding material led to the development of plain cement concrete (PCC) and subsequently reinforced cement concrete (RCC). It now became possible to construct high-rise buildings, sky scrapers, large dams, reservoirs with less consumption of building materials and much higher strength The use of RCC became very popular from the beginning of 20th century. The advent of concrete, especially reinforced concrete, significantly replaced traditional construction materials, such as steel, stone, wood and bricks. This had made concrete the most widely used man-made product and second only to water as the world's most heavily consumed substance.
The widespread use of concrete boosted cement demand spectacularly throughout the world during the last one hundred years. This in turn led to innovations in the manufacturing technology, storage, handling and distribution techniques, not to speak of the utilisation of cement, thus giving birth to the modern cement and construction industries.
Is there a substitute for Cement ?
Despite innumerable technological advances in the past century in all spheres of human activity, including civil engineering, architecture and construction technology, cement retained its quintessential role as the ubiquitous binding material for all sorts of constructions. In fact, it is playing a progressively larger role in many other spheres of human habitat. There are a host of reasons in favour of cement's irreplaceable role in contemporary human society.
1. Unlike all other non-brittle materials for construction (wood, steel, aluminium, etc) cement is a low-cost but high-performance product.
2. It can be used as a binder with almost any hard material.
3. It can be used both as a building block (hardened cement mix) or as a binder of building components (bricks, stone blocks, sand, rock fragments or any other hard material).
4. Most building materials are prone to decay and loss of property with time, whereas properly made and cast cement concrete gains strength progressively with ageing.
5. With substitution of very small quantities of cement by other reinforcing materials (steel, polyester, varied sorts of fibers) or chemicals (epoxy resins, plasticisers) its binding properties can be increased manifold to satisfy the performance needs of construction for different purposes (High rise buildings, Towers, Concrete roads, Railway sleepers, Dams, Reservoirs, Canal lining, etc)
6. Cement and its derivative concrete have turned out to be an excellent conduit for recycling varied types of industrial, agro-industrial and metallurgical wastes (flyash, blast furnace slag, rice husk ash, etc) providing thereby support to environmental protection.
7. Cement manufacturing process can absorb a host of hazardous, obnoxious and toxic wastes (from petroleum refining, pharmaceutical, pesticides industries) through their effective incineration in the cement kiln, providing dual benefits of energy conservation and waste recycling inter alia environmental protection.
Consumer Interest and cement use?
With the advances in manufacturing technology and construction techniques, the production of cement is getting more and more sophisticated, while the use of cement is becoming more versatile. Indeed Indian cement industry produces today a range of six (main) varieties and three grades of cement, besides a host of special cements earmarked for use for specific purposes. So much so, it has now become imperative for the consumer, in his own interest, to avoid confusion and mistake, to have an overall knowledge of these cements including their properties and application and also some basic idea about cement manufacture. This will help the consumer to make an informed choice both from functional utility and economic points of view. This site is an attempt at enabling the consumer with the essential guidance in selecting the right type of cement suiting his purpose, its proper storing and handling and check-testing of cement for quality reliability. Our primary objective is that the customer should get the value for the money spent in using cement and should get the maximum performance out of it.
Let us now start our journey to the world of cement. Before delving into various issues with cement, as a customer, your first brush with cement is the bag of cement you are going to purchase from the market. You will naturally like to understand what is written on it. Let us start with the cement bag, what does the bag say?
Information available on a Standard Cement Bag ?
The following information is normally printed on a standard cement bag, which may be taken note of at the time of purchasing or receiving cement bags:
• Type of cement ? Colour of lettering on the bag is Black for OPC, Red for PPC and Orange for PSC
• IS Certification Mark, Bureau of Indian Standards (BIS) Website ? Net mass of cement
• Week and year of packing
• Manufacturer's name, address and registered Trade Mark, if any.
• Maximum Retail Price (MRP)
What a consumer should check while buying cement ?
A legend of the details is given alongside. Small consumer mostly ignore the information and prefer to go by the advice of the mason or the contractor. In most cases of house constructions, the contractor decides the type and quantity of cement without the owner's knowledge. What happens if a wrong type, adulterated or underweight bags of cement, or a very old consignment is purchased? The fallout may not be evident for years, if not a decade. But then, it may be too late for remedy!
All major manufacturers of cement, who are members of the Cement Manufacturers' Association (CMA) comply with the requirements of correct weight of the bag and the quality of cement it contains. Each bag of cement compulsorily conforms to the provisions laid down by the Department of Weights and Measures,and also complies with the quality specifications laid down by the Bureau of Indian Standards (BIS), and checked at regular intervals. Any deviation is mostly due to what happens after the bagged cement leaves the manufacturer's premises.
Storing Cement in the right way ?
Cement purchased in bulk and stored for long on-going construction needs proper care in preservation. Cement tends to readily absorb moisture from the surroundings, not only in the form of free water but moisture from the air as well, and react with it chemically. Its binding property and strength depend upon its capacity for this chemical reaction, which is irreversible. The strength of cement, when used, would show adverse effect to the extent that such chemical reaction would already have taken place. Hence, it is imperative to protect it from dampness before actual use. Cement strength deteriorates with passage of time by absorbing moisture directly or indirectly as well as the absorption of the pollutants present in the environment.
For protection of cement against deterioration and ensuring that it retains its freshness, it should be stored in a manner that no dampness or moisture is allowed to reach it either from the ground, walls or from the environment. This becomes particularly important during the humid season and in coastal regions.
Important Guidelines for Cement storage ?
Waterproof warehouse ? A must
The first requisite for storing bagged cement is a building/shed which is completely weatherproof. Apart from this, the cement storage shed should also have some other more important requisites, as under:
Important requisites of a cement storage shed
1.The walls must be of waterproof concrete or masonry construction. (If concrete blocks are not available or cannot be made, brick masonry plastered with cement-sand plaster on both faces may be used.)
2.The roof must preferably be of reinforced concrete construction overlaid with a waterproofing course. AC/GI sheet or tiled roof construction may be used provided they are waterproof.
3.The floor must be raised by at least 45 cm above the ground level to prevent any inflow of water. The flooring may consist of a 15-cm thick concrete slab or layer of dry bricks laid in two courses over a layer of earth consolidated to a thickness of 15 cm above the ground level. Although not shown on the drawing, the ground is drained away from the building to prevent accumulation of rainwater in its vicinity. All these precautions ensure that the floor will remain absolutely dry.
4.For further protection, cement bags should be stacked at least 10-20 cm clear above the floor by providing wooden battens and planking arrangement (Fig.9.1). For saving timber, concrete may be used.
5.For full storage, the plinth should be high enough for a lorry to back conveniently to the door so that the chassis and the building floors are almost at the same level, thus making loading and unloading of bags very easy.
6.Windows provided, if any, should be very few in number and of small size, and normally kept tightly closed to prevent entry of atmospheric moisture from outside. The door should also be air-tight.
7.A newly constructed godown should not be used for storage of cement unless its interior is thoroughly dry.
Proper storing and stacking of cement bags
Once a good weatherproof building is available, certain essential precautions should be taken in storing the bags to prevent them from any possible contact with moisture and to ensure systematic functioning of the warehouse.
Essential precautions for stacking of cement bags:
1.No cement bags should be stacked in contact with an external wall. A clear space of at least 60 cm should be left between the exterior wall and the stacks.
2.Likewise, bags should be piled off-the-floor upon wooden planks. If, however, the floor is a well-constructed dry concrete floor, the bags can be placed directly on it after spreading tarpaulin or polythene sheet.
3.Cement bags should be placed closely together in the stack to reduce circulation of air as much as possible.
4.Cement bags should not be stacked more than 10-bags high to avoid lumping or ?warehouse pack? under pressure. (This can usually be corrected by rolling the bags on the floor). Should the stack be higher, arrange the bags in header- and-stretcher fashion, i.e. alternately length-wise and cross-wise, to achieve interlocking between them and reducing the danger of toppling over. The arrangement of two stacks with a height of seven bags and ten bags respectively.
5.For extra safety during rainy season, the stacks of cement bags should be enclosed completely in polythene sheets (at least 700 micron thick) or similar material if it is anticipated that cement would not be required for a prolonged period. This can be achieved by making a large loose sack of the polythene sheet and arranging cement bags within it with flaps of the sheet closing on the top of the pile. Care should be taken to ensure that the polythene sheet is not damaged any time while in use.
Temporary storage at site
Sometimes cement requirement for a day or two may have to be stored at site in the open. In such cases cement bags should be laid on a dry platform made of wooden planks resting over brick-masonry, concrete, dry sand or aggregates raised about 15 cm above the ground level. The number of bags should be kept to a minimum, preferably just sufficient for the day's consumption. The stack must be kept fully covered with tarpaulin or polythene sheet and protected against atmospheric moisture. The covering sheets must overlap each other properly.
Temporary storage in the open should be avoided in damp weather
Don'ts in storage of Bagged Cement.
1.Don't store the bags in a building where the walls, roof, and floor are not completely weatherproof.
2.Don't store the bags in a new warehouse until the interiors have thoroughly dried out.
3.Don't be content with badly fitting windows and doors, but see that they are kept tightly shut.
4.Don't stack the bags against the wall. Similarly, pile off-the-floor on wooden planks except in the case of a dry concrete floor.
5.Don't forget to pile bags close together.
6.Don't pile more than 10-bags high, and arrange the bags in header-and-stretcher fashion.
7.Don't disturb the stored cement until it is to be taken out for use.
8.Don't take out bags from one tier only. Step back two or three tiers.
9.Don't keep dead storage, that is, a stack which remains in place for a long time while other consignments of cement come in and go out.
10.Don't stack bags on the ground for temporary storage at works site. Pile up on a raised dry platform and cover with tarpaulins.
If these few simple precautions are taken in the storage of cement, the cement will stay as fresh when taken out for use as when it arrived from the works.
Bulk cement should be stored in watertight concrete or steel bins or silos. Dry low-pressure aeration or vibration should be used in bins or silos to make the cement flow better and avoid bridging. Due to fluffing of cement, silos may hold only about 80 per cent of the rated capacity.
Proper Cement Content in Concrete ?
Cement is an intermediate product while the actual construction products are concrete or mortar. The quality and quantity of cement significantly affect the ultimate performance of concrete and the durability of the structure. Keeping this fact in view, the Bureau of Indian Standards (BIS) has issued a revised Code IS:456-2000 for plain and reinforced concrete in which a separate chapter has been devoted to durability of concrete. The different grade designations of concrete and proportions of ingredients specified for each grade are given in this Code and are briefly indicated in Table-11.1 below.
These proportions are for nominal mixes only
Different grades of concrete and their recommended mix proportions (Based on IS: 456-2000)
concrete mixes above M-20, the mix design should be adopted as per approved procedure
Cement content in concrete as prescribed under BIS revised code IS: 456-2000.
Minimum cement content ?
Code has classified environments to which concrete will be exposed into five levels of severity, namely, mild, moderate, severe, very severe and extreme. Depending upon the exposure conditions, the minimum cement content and the minimum grade of concrete have been specified in the Code. Irrespective of the grade and type of cement, the minimum cement content specified in the Code must be used while producing concrete for a particular environmental condition.
Maximum cement content ?
456-2000 also stipulates that cement content (not including fly ash and ground granulated blast furnace slag) in excess of 450 kg/m3 should not be used in concrete. In special cases, where due consideration has been given in design to the increased risk of cracking due to drying shrinkage, etc., a higher proportion of cement can be used but precautionary measures have to be taken before doing so. Excess cement over the actual requirement is harmful as it is likely to induce micro-cracks in concrete. These micro-cracks, on the one hand, reduce the compressive strength of concrete and are a potential cause for reducing the durability of the structure, on the other
Varieties of Cement and Indian Standards Specifications on Cements ?
India is at present producing a range of six main varieties of cement like Ordinary Portland Cement (OPC), Portland Pozzolana Cement (PPC), Portland Blastfurnace Slag Cement (PSC), Sulphate Resisting Cement (SRC), Oil Well Cement and White Cement. Under OPC again it is producing high compressive strength grades, e.g., ?43? (43 MPa * at 28 days) and ?53? (53 MPa* at 28 days), to meet special requirements like prestressed concrete, precast products, besides 53S-43S (earlier IRS-T 40) for railway-sleepers.
Indian Standards Specifications for cements
Cements produced in India compulsorily conform to Indian Standard specifications (Table 12.1) issued by the Bureau of Indian Standards (BIS), the national body for the formulation and implementation of Indian Standards (IS). It is a statutory requirement that each cement bag must conform to BIS specifications for the type and grade printed on it.
Indian Standards Specifications on Cements
HOW TO GET THE BEST FROM CEMENT USE - Sustainable Construction is a Primary Pre-requirement for Sustainable Development
India has been successful in meeting the food and clothing requirements of its vast population; however, the problem of providing ?shelter to all? is defying solutions. While there has been an impressive growth in the total housing stock from 65 million in 1947 to 187.05 million in 2001, a large gap still exists between the demand and supply of housing units. It is estimated that the present housing shortage is about 22.44 million units in urban areas and 15 million units in rural areas.
The Government of India has taken a number of laudable initiatives to ease the housing shortage. While the National Housing Policy-1994 and the National Housing and Habitat Policy-1998 provide the broad guidelines, the government has announced a number of fiscal and other incentives from time to time to encourage accelerated growth in this sector. The Urban Renewal Mission is one eminent among them.
Studies conducted show that about 400 million people will migrate to the cities. Today about 48 per cent of the population lives in cities world over and this will touch 75 per cent in the future. The Government will not be able to provide all the infrastructure needs of a city. Hence there has to be greater public-private partnership in city management.
The residential housing construction in India is dominated by labour-intensive activities. This underlines the need to educate all those involved in concrete-related housing construction in the country. This site is a small attempt in this direction. Besides briefly explaining the properties of concrete and its ingredients, its mix proportions, more emphasis is placed on the construction aspects involving batching, mixing, placing, compaction and curing of concrete. In addition, consider the fact that nearly 60 percent of the area in the country lies in the moderate-to-severe seismic zones, Guidelines for Earthquake-resistant Buildings is therefore introduced by BIS