Of the large variety of cements available today, ordinary portland cement (OPC) is the most common, and usually the type referred to when speaking of cement. It is the fine, grey powder that can be mixed with sand, gravel and water to produce a strong and long lasting mortar or concrete.
Portland cement was developed in the 1 9th century and was so named because it resembled a popular building stone quarried in Portland, England. It has since been associated with high strength and durability, and has consequently become one of the most prestigious building materials.
Cement is usually produced in large centralized plants, which incur high capital costs and long transportation distances to most building sites. In most developing countries, production capacities are far below the demand and also on account of losses and deterioration in transports and storage, cement is generally associated with high costs and short supplies.
In order to improve the situation, efforts have been concentrated on the development of small-scale cement plants (also called "mini-cement" plants), particularly in China and India.
Large-scale cement production
• About 95 % of the world's cement is being produced in rotary kilos with daily outputs ranging between 300 and more than 5000 tonnes.
• Limestone (calcium carbonate) and clay (silica, alumina and iron oxide) are ground and mixed with water to form a slurry, which is fed into the upper end of the slightly inclined, refractory lined rotating furnace, which can be more than 100 m long. Hot air of temperatures between 1300° and 1400° C is blown in at the lower end, drying the slurry, which is then sintered and fused into hard balls known as clinker. These drop out of the kiln, are cooled and interground in a ball-mill with about 3 % gypsum to retard the setting of the cement. The finer it is ground, the higher is the rate of the setting and strength development reactions.
• The wet process, described here, has largely been superceded by the dry process, which needs less energy to dry the raw material feed.
• OPC is sold in 50 kg bags, preferably heavy quality multi-ply paper bags. However, in some countries (eg India) reusable jute bags are used, leading to great wastage and difficulties in maintaining quality control.
Small-scale cement production
• This production method utilizes small vertical shaft kilns, a technology that accounts for more than half of China's annual cement production.
• The kiln feed is made of crushed limestone, clay and coal, which are proportioned and finely interground in a ball mill and then made into nodules in a disc nodulizer.
• The nodules are fed into the top conical portion of the kiln, in which the rising preheated air causes the fuel in the nodules to ignite, forming clinker.
• The clinker nodules gradually drop into the cylindrical portion, where it is cooled by the air introduced from below.
• A rotary grate discharges the clinker, which is then interground with gypsum in a ball-mill. Since the nodules are porous, less energy is required for grinding.
• Daily outputs of a vertical shaft kiln can range between 2 and 30 tonnes of ordinary portland cement.
• Numerous varieties of cement are produced by altering the types and proportions of the raw materials to be calcined, or by blending or intergrinding portland cement with other materials. A few common types are:
• Rapid hardening portland cement (more finely ground than OPC; ultimate strength same as OPC).
• Sulphate resisting portland cement (made by adjusting the chemical composition of the raw mix).
• Portland-pozzolana cements (made by blending or intergrinding a pozzolana, eg rice husk ash or fly ash, in proportions of 15 to 40 % by weight, thus saving on cement and improving some of its properties).
• Portland blastfurnace cements (made by blending ground granulated blast furnace slag, thus achieving slower hardening and sulphate resistance).
• Magnesium oxychloride or sorer cement (obtained by calcining magnesium carbonate, achieving much higher strengths than OPC, but is attacked by water).
• High alumina cement (obtained by calcining limestone and bauxite, achieving high early strengths, optimum sulphate resistance, good acid resistance, and heat resistance up to 1300°C; but 3 times the cost of OPC and not suitable for structural concrete).
Hydration of cement
• Water reacts on the surface of the cement grains and diffuses inwards to reach unreacted cement. Therefore, the finer the grains the quicker the reaction.
• The water in the capillary space between the grains is filled with products of the hydration process. The more water used, the larger is the space that needs to be filled, and if there are insufficient hydration products, capillary pores remain, which weaken the cement. Hence, the correct water-cement ratio is important for strength development.
• During hydration, lime is set free. This hardens (by combining with CO2) very slowly and expands in doing so, causing cracking and failure of concrete. By adding a pozzolana, it forms a hydraulic binder, which sets and hardens like cement.
• Setting (which means stiffening) takes place within 45 minutes, but hardening (which means useful strength development) takes several weeks. Specifications are, therefore, based on strengths achieved after 28 days.
• Because they set quickly, cement mixes have to be used as soon as possible.
• In hot climates, cements dry out too quickly and must be kept wet for at least two weeks.
• Cement is used as a binder for several inorganic and organic materials, eg soil-cement, sand-cement blocks, cement-bonded fibre boards.
• It is primarily used together with sand and gravel (and reinforcements) to produce (reinforced) concrete.
• It is used with sand and chicken-wire mesh (or fibres) to produce ferrocement (or fibre concrete).
• Mortars and plaster are made from cement and sand, often mixed with lime for better workability. With a very fine sand it is used for screeding.
• A paint can be made from cement mixed with excess water.
• Cements can achieve extremely high strengths, generally remain unaffected by water, and do not significantly swell and shrink.
• Cements are resistant to fire and biological hazards, if kept clean.
• Cement constructions have a high prestige value.
• With regard to decentralized, small-scale cement production, the advantages are: low capital investment; use of cheaper quality coke or coal; lower transportation costs, due to shorter distances to consumer; lower technical sophistication, thus providing job opportunities even for unskilled labour; adaptability to market demands; capability of using different raw materials and producing a variety of cementitious products; increase of supporting industries around the plant.
• In most developing countries, cement is still too expensive for the majority of the population, and usually in short supply.
• Storage requires great care to avoid premature setting.
• Cracks occur in hot dry conditions due to rapid setting or due to temperature fluctuations.
• Sulphates and salts can cause rapid deterioration.
• Due to the high reputation of cement, it is often used to make over-strong mortars which cause brittleness, or porous mortars which lack durability.
• Increase of supplies and reduction of costs are possible by introducing decentralized, small-scale cement plants.
• Improved bagging and storage methods in dry conditions, but also quick turnover can avoid wastage through premature setting.
• Proper wet curing avoids cracking, and special cements are used to avoid damage by sulphates and salts.
• Unnecessary and wrong usage of cement can be reduced by increased dissemination of information and increased use of lime, eg to improve the quality of cement mixes.
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