environmental and social impacts of cement industries
Cement manufacture causes environmental impacts at all stages of the process. These include emissions of airborne pollution in the form of dustandgases; noise and vibration when operating machinery and during blasting in quarries ; and limestone quarries are visible from long distances and may permanently disfigure the local environment.Equipment to reduce dust emissions during quarrying and manufacture of cement is used by several cement producing companies. Equipment to trap and separate exhaust gases are alsoincreasingly used.Cement companies like Lafarge are now considering quarry rehabilitation; they take into account biodiversityin their site selection. When the extension of a quarry threatens a valuable nature area, or a protected/rare species, they are closing part of the site and classifying it as a voluntary nature reserve(e.g., La Couronne, near Angoulme, France), or transferring the threatenedspecies to a safe location(e.g., Dundas quarry near Toronto,Canada and the Hoppegarten plant in Germany); quarry rehabilitation oftenleads to the creation of wetlands and natural reserves or leisure areas.
Cement manufacture contributes greenhouse gases directly through the production of carbon dioxide when calcium carbonate is heated ( producing lime and carbon dioxide) and indirectly through the use of energy, particularly if the energy is sourced from fossil fuels. The cement industry produces about 5% of global man-made CO2 emissions, of which 50% is from the chemical process, and 40% from burning fuel. The amount of CO2 emitted by the cement industry is nearly 900kg of CO2 for every 1000kg of cement produced. It is interesting to note that the newly developed cement types from Novacem and Eco-cement can absorb carbon dioxide from ambient air during hardening.
Fuels and raw materialsA cement plant consumes 36GJ of fuel per tonne of clinker produced, depending on the raw materials and the process used. Most cement kilns today use coal and petroleum coke as primary fuels and, to a lesser extent, natural gas and fuel oil. Selected waste and by-products with recoverable calorific value can be used as fuels in a cement kiln, replacing a portion of conventional fossil fuels, like coal, if they meet strict specifications. Selected waste and by-products containing useful minerals such as calcium, silica, alumina and iron can be used as raw materials in the kiln, replacing raw materials such as clay, shale and limestone. Because some materials have both useful mineral content and recoverable calorific value, the distinction between alternative fuels and raw materials is not always clear. For example, sewage sludge has a low but significant calorific value and burns to give ash-containing minerals useful in the clinker matrix.
Producing cement has significant positive and negative impacts at a local level. On the positive side, the cement industry may create employment and business opportunities for local people, particularly in remote locations in developing countries where there are few other opportunities for economic development. Negative impacts include disturbance to the landscape, dust and noise and disruption to local biodiversity from quarrying limestone (the raw material for cement).
cement and concrete: environmental considerations | buildinggreen
Concrete and other cementitious materials have both environmental advantages and disadvantages. This article takes a look at how these materials are made, then reviews a number of environmental considerations relating to their production, use, and eventual disposal.
Editor's Note:For more information about concrete in green building projects, we recommend our 43-page special report, "What You Need to Know About Concrete and Green Building." The report is free with a trial membership to BuildingGreen Suite. Learn more.
Illustrations courtesy of the Portland Cement Association
Cement and concrete are key components of both commercial and residential construction in North America. The cement and concrete industries are huge. There are approximately 210 cement plants in the U.S. and 4,000 to 5,000 ready mix plants (where cement is mixed with aggregate and water to produce concrete). The Portland Cement Association estimates that U.S. cement consumption has averaged between 75 and 90 million tons per year during the last decade, and projects that consumption will exceed 100 million tons per year by 1997. Worldwide, cement production totaled 1.25 billion tons in 1991, according to the U.S. Bureau of Mines.
What does this mean in terms of the environment? Are these products good or bad? As builders and designers, should we be looking for alternatives or embracing concrete over competing materials? As with most building issues, the answers are not clear-cut. Concrete and other cementitious materials have both environmental advantages and disadvantages. This article takes a look at how these materials are made, then reviews a number of environmental considerations relating to their production, use, and eventual disposal.
Cement is the key ingredient in concrete products. Comprising roughly 12% of the average residential-grade ready mix concrete, cement is the binding agent that holds sand and other aggregates together in a hard, stone-like mass. Portland cement accounts for about 95% of the cement produced in North America. It was patented in England by Joseph Aspdin in 1824 and named after a quarried stone it resembled from the Isle of Portland.
Cement production requires a source of calcium (usually limestone) and a source of silicon (such as clay or sand). Small amounts of bauxite and iron ore are added to provide specific properties. These raw materials are finely ground and mixed, then fed into a rotary cement kiln, which is the largest piece of moving industrial equipment in the world. The kiln is a long, sloping cylinder with zones that get progressively hotter up to about 2700F (1480C). The kiln rotates slowly to mix the contents moving through it. In the kiln, the raw materials undergo complex chemical and physical changes required to make them able to react together through hydration. (See illustration, pages 8-11.) The most common type of cement kiln today (accounting for 70% of plants in the U.S.) is a
calcining of limestone (calcium carbonate) into lime (calcium oxide) and carbon dioxide, which occurs in the lower-temperature portions of the kilnup to about 1650F (900C). The second reaction is the bonding of calcium oxide and silicates to form dicalcium and tricalcium silicates. Small amounts of tricalcium aluminate and tetracalcium aluminoferrite are also formed. The relative proportions of these four principal compounds determine the key properties of the resultant portland cement and the type classification (Type I, Type II, etc.). These reactions occur at very high temperatures with the ingredients in molten form. As the new compounds cool, they solidify into solid pellet form called
hydration. When water is added to the cement, it forms a slurry or gel that coats the surfaces of the aggregate and fills the voids to form the solid concrete. The properties of concrete are determined by the type of cement used, the additives, and the overall proportions of cement, aggregate, and water.
The raw materials used in cement production are widely available in great quantities. Limestone, marl, and chalk are the most common sources of calcium in cement (converted into lime through calcination). Common sources of silicon include clay, sand, and shale. Certain waste products, such as fly ash, can also be used as a silicon source. The iron and aluminum can be provided as iron ore and bauxite, but recycled metals can also be used. Finally, about 5% of cement by weight is gypsum, a common calcium- and sulfur-based mineral. It takes 3,200 to 3,500 pounds of raw materials to produce one ton (2,000 lbs.) of finished cement, according to the Environmental Research Group at the University of British Colombia (UBC).
Table 1Typical Concrete Mix
Source: Based on figures provided by the Ready Mix Concrete Association, personal communication.
Source: Based on figures provided by the Ready Mix Concrete Association, personal communication.
The water, sand, and gravel or crushed stone used in concrete production in addition to cement are also abundant (typical proportions of a residential concrete mix are shown in Table 1). With all of these raw materials, the distance and quality of the sources have a big impact on transportation energy use, water use for washing, and dust generation. Some aggregates that have been used in concrete production have turned out to be sources of radon gas. The worst problems were when uranium mine tailings were used as concrete aggregate, but some natural stone also emits radon. If concerned, you might want to have the aggregate tested for radon.
The use of fly ash from coal-fired power plants is beneficial in two ways: it can help with our solid waste problems, and it reduces overall energy use. While fly ash is sometimes used as a source of silica in cement production, a more common use is in concrete mixture as a substitute for some of the cement. Fly ash, or
pozzolan, can readily be substituted for 15% to 35% of the cement in concrete mixes, according to the U.S. EPA. For some applications fly ash content can be up to 70%. Of the 51 million tons of fly ash produced in 1991, 7.7 million tons were used in cement and concrete products, according to figures from the American Coal Ash Association. Thus, fly ash today accounts for about 9% of the cement mix in concrete.
Fly ash reacts with any free lime left after the hydration to form calcium silicate hydrate, which is similar to the tricalcium and dicalcium silicates formed in cement curing. Through this process, fly ash increases concrete strength, improves sulfate resistance, decreases permeability, reduces the water ratio required, and improves the pumpability and workability of the concrete. Western coal-fired power plants produce better fly ash for concrete than eastern plants, because of lower sulfur and lower carbon content in the ash. (Ash from incinerators cannot be used.)
There are at least a dozen companies providing fly ash to concrete producers. Talk to your concrete supplier and find out if they are willing to add fly ash to the mix. (If your local plant doesnt know where to get the fly ash, a list of companies is available from EBN.) Portland cement with fly ash added is sometimes identified with the letter
P after the type number (Type IP). The EPA requires fly ash content in concrete used in buildings that receive federal funding (for information call the EPA Procurement Guidelines Hotline at 703/941-4452). Fly ash is widely used in Europe as a major ingredient in autoclaved cellular concrete (ACC); in the U.S., North American Cellular Concrete is developing this technology (see EBN,
Other industrial waste products, including blast furnace slag, cinders, and mill scale are sometimes substituted for some of the aggregate in concrete mixes. Even recycled concrete can be crushed into aggregate that can be reused in the concrete mixthough the irregular surface of aggregate so produced is less effective than sand or crushed stone because it takes more cement slurry to fill all the nooks and crannies. In fact, using crushed concrete as an aggregate might be counterproductive by requiring extra cementby far the most energy-intensive component of concrete.
Energy consumption is the biggest environmental concern with cement and concrete production. Cement production is one of the most energy intensive of all industrial manufacturing processes. Including direct fuel use for mining and transporting raw materials, cement production takes about six million Btus for every ton of cement (Table 2). The average fuel mix for cement production in the United States is shown in Table 3. The industrys heavy reliance on coal leads to especially high emission levels of CO
The vast majority of the energy consumed in cement production is used for operating the rotary cement kilns. Newer dry-process kilns are more energy efficient than older wet-process kilns, because energy is not required for driving off moisture. In a modern dry-process kiln, a pre-heater is often used to heat the ingredients using waste heat from the exhaust gases of the kiln burners. A dry-process kiln so adapted can use up to 50% less energy than a wet-process kiln, according to UBC researchers. Some other dry-process kilns use a separate combustion vessel in which the calcining process begins before the ingredients move into the rotary kilna technique that can have even higher overall efficiency than a kiln with pre-heater.
15 Btus). This is roughly .6% of total U.S. energy use, a remarkable amount given the fact that in dollar value, cement represents only about .06% of the gross national product. Thus, cement production is approximately ten times as energy intensive as our economy in general. In some Third World countries, cement production accounts for as much as two-thirds of total energy use, according to the Worldwatch Institute.
Fuel Use for Cement Production
Table 3Winter Emissions from Major Sources, State of Washington1984
Source: Wood Smoke: Emissions, Impacts, and Reduction Strategies, Washington State Department of Ecology, December 1986
While cement manufacturing is extremely energy intensive, the very high temperatures used in a cement kiln have at least one advantage: the potential for burning hazardous waste as a fuel. Waste fuels that can be used in cement kilns include used motor oil, spent solvents, printing inks, paint residues, cleaning fluids, and scrap tires. These can be burned relatively safely because the extremely high temperatures result in very complete combustion with very low pollution emissions. (Municipal solid waste incinerators operate at considerably lower temperatures.) Indeed, for some chemicals thermal destruction in a cement kiln is the safest method of disposal. A single cement kiln can burn more than a million tires a year, according to the Portland Cement Association. Pound for pound, these tires have a higher fuel content than coal, and iron from the steel belts can be used as an ingredient in the cement manufacturing. Waste fuels comprise a significant (and growing) part of the energy mix for cement plants (see Table 3), and the Canadian Portland Cement Association estimates that waste fuel could eventually supply up to 50% of the energy.
Energy use for concrete production looks considerably better than it does for cement. Thats because the other components of concretesand, crushed stone, and waterare much less energy intensive. Including energy for hauling, sand and crushed stone have embodied energy values of about 40,000 and 100,000 Btus per ton, respectively. The cement, representing about 12% of concrete, accounts for 92% of the embodied energy, with sand representing a little under 2% and crushed stone just under 6% (see Table 2).
Use of fly ash in concrete already saves about 44 trillion Btus (.04 quads) of energy annually in the U.S. Increasing the rate of fly ash substitution from 9% to 25% would save an additional 75 trillion Btus.
Table 4. Process-related emissions from production of PVC resin
Notes: The critical volumes data refer to amounts of water or air that would be contaminated to maximum allowable levels per the Swiss ministry. These data offer a means of comparing the relative significance of each type of emission.
Source: Ttsch and Gaensslen
Notes: The critical volumes data refer to amounts of water or air that would be contaminated to maximum allowable levels per the Swiss ministry. These data offer a means of comparing the relative significance of each type of emission.
2, both cement and concrete production generate considerable quantities of air-pollutant emissions. Dust is usually the most visible of these pollutants. The U.S. EPA (cited by UBC researchers) estimates total particulate (dust) emissions of 360 pounds per ton of cement produced, the majority of which is from the cement kiln. Other sources of dust from cement production are handling raw materials, grinding cement clinker, and packaging or loading finished cement, which is ground to a very fine powderparticles as small as
The best way to deal with the dust generated in cement manufacturing would be to collect it and put it back into the process. This is done to some extent, using mechanical collectors, electric precipitators, and fabric filters (baghouses). But recycling the dust is difficult, according to UBC researchers; it first has to be treated to reduce its alkalinity. Some cement kiln dust is used for agricultural soil treatments, and the rest (of that collected) is often landfilled on site. There was investigation into the possibility of using cement kiln dust for treatment of acidified lakes in eastern Canada, but rather than simply buffering the low pH of the water, the dust chemically created a potentially harmful salt.
In addition to dust produced in cement manufacturing, dust is also generated in concrete production and transport. Common sources are sand and aggregate mining, material transfer, storage (wind erosion from piles), mixer loading, and concrete delivery (dust from unpaved roads). Dust emissions can be controlled through water sprays, enclosures, hoods, curtains, and covered chutes.
Other air pollution emissions from cement and concrete production result from fossil fuel burning for process and transportation uses. Air pollutants commonly emitted from cement manufacturing plants include sulfur dioxide (SO
3, sulfuric acid, and hydrogen sulfide) result from sulfur content of both the raw materials and the fuel (especially coal). Strategies to reduce sulfur emissions include use of low-sulfur raw materials, burning low-sulfur coal or other fuels, and collecting the sulfur emissions through state-of-the-art pollution control equipment. Interestingly, lime in the cement kiln acts as a scrubber and absorbs some sulfur.
Nitrous oxide emissions are influenced by fuel type and combustion conditions (including flame temperature, burner type, and material/exhaust gas retention in the burning zone of the kiln). Strategies to reduce nitrogen emissions include altering the burner design, modifying kiln and pre-calciner operation, using alternate fuels, and adding ammonia or urea to the process. The cement industry claims to have reduced overall pollution emissions by 90% in the last 20 years.
Another environmental issue with cement and concrete production is water pollution. The concern is the greatest at the concrete production phase. Wash-out water with high pH is the number one environmental issue for the ready mix concrete industry, according to Richard Morris of the National Ready Mix Concrete Association. Water use varies greatly at different plants, but Environment Canada estimates water use at batching plants at about 500 gallons per truck per day, and the alkalinity levels of washwater can be as high as pH 12. Highly alkaline water is toxic to fish and other aquatic life. Environment Canada has found that rainbow trout exposed to portland cement concentrations of 300, 500, and 1,000 milligrams/liter have 50% mortality times (the time required for 50% of the population in test samples to be killed) of 68, 45, and 29 minutes, respectively.
At the batch plant, washwater from equipment cleaning is often discharged into settling ponds where the solids can settle out. Most plants are required to have process water discharge permits from state, federal, or provincial environmental agencies to dispose of wastewater from these settling ponds. As long as the pH of this wastewater is lower than 12.5, it is not considered a hazardous material by U.S. law. Some returned concrete also gets put into settling ponds to wash off and recover the aggregate. On the positive side, many newer ready mix plants have greatly reduced water use in recent years because of both wastewater disposal issues and drought conditions in some parts of the country. More companies are going to completely closed-loop systems, according to Terek Kahn of the National Ready Mix Concrete
Despite the apparent significance of the wastewater concern, the National Ready Mix Concrete Association to date has not developed standards for member companies on wastewater treatment, including rinsing of trucks and chutes at the building site. John Mullarchy of the association says that procedures are developed on a company-by-company basis. In many areas, environmental regulations dictate procedures relative to wastewater treatment. In more urban areas, the on-site rinse water (for chutes) often has to be collected and treated or disposed of at the plant.
While the cement and concrete industries can help reduce some of our solid waste problems (burning hazardous waste as cement kiln fuel and using fly ash in concrete mixtures, for example), one cannot overlook the fact that concrete is the largest and most visible component of construction and demolition (C&D) waste. According to estimates presented in the AIA
Environmental Resource Guide, concrete accounts for up to 67% by weight of C&D waste (53% by volume), with only 5% currently recycled. Of the concrete that is recycled, most is used as a highway substrate or as clean fill around buildings. As more landfills close, including specialized C&D facilities, concrete disposal costs will increase and more concrete demolition debris will be reprocessed into roadbed aggregate and other such uses.
Concrete waste is also created in new construction. Partial truckloads of concrete have long been a disposal problem. Ready mix plants have come up with many innovative solutions through the years to avoid creating wastesuch as using return loads to produce concrete retaining wall blocks or highway dividers, or washing the unset concrete to recover the coarse aggregate for reuse. But recently, there have been some dramatic advances in concrete technology that are greatly reducing this waste. Concrete
When it is possible to use pre-cast concrete components instead of poured concrete, doing so may offer advantages in terms of waste generation. Material quantities can be estimated more precisely and excess material can be utilized. Plus, by carefully controlling conditions during manufacture of pre-cast concrete products, higher strengths can be achieved using less material. The Superior Wall foundation system, for example, uses only about a third as much concrete as the typical poured concrete wall it replaces. Waste water run-off can also be more carefully controlled at centralized pre-cast concrete facilities than on jobsites.
Once it has hardened, concrete is generally very safe. Traditionally, it has been one of the most inert of our building materials and, thus, very appropriate for chemically sensitive individuals. As concrete production has become higher-tech, however, that is changing. A number of chemicals are now commonly added to concrete to control setting time, plasticity, pumpability, water content, freeze-thaw resistance, strength, and color. Most concrete retarders are relatively innocuous sucrose- (sugar-) based chemicals, added in proportions of .03% to .15%. Workability agents or superplasticizers can include such chemicals as sulfonated melamine-formaldehyde and sulphonated napthalene formaldehyde condensates. Air-entraining admixtures function by incorporating air into the concrete to provide resistance to damage from freeze-thaw cycles and to improve workability. These are usually added to the cement and identified with the letter
A after the type (Type IA). These materials can include various types of inorganic salts (salts of wood resins and salts of sulphonated lignin, for example), along with more questionable chemicals such as alkyl benzene sulphonates and methyl-ester-derived cocamide diethanolamine. Fungicides, germicides, and insecticides are also added to some concrete.
Because of these chemical admixtures, todays concrete could conceivably offgas small quantities of formaldehydes and other chemicals into the indoor air. Unfortunately, it is difficult to find out from the manufacturers the actual chemicals in these admixtures. For chemically sensitive clients, it may be advisable to specify concrete with a bare minimum of admixtures, or use a sealer on the finished concrete to minimize offgassing. Asphalt-impregnated expansion joint filler, curing agents that are sometimes applied to the surface of concrete slabs to reduce water evaporation, special oils used on concrete forms, and certain sealants used for treating finished concrete slabs and walls can also cause health problems with some chemically sensitive individuals.
Finally, concrete floors and walls can cause moisture problems and lead to mold and mildew growth, which cause significant health problems in certain individuals. There are two common sources of moisture: moisture wicking through concrete from the surrounding soil; and moisture from the house that may condense on the cold surface of concrete. To eliminate the former, provide good drainage around a foundation, dampproof or waterproof the outside of the foundation walls before backfilling, provide a layer of crushed stone beneath the slab, and install a polyethylene moisture barrier under the slab (protected from the concrete with a layer of sand if possible). To reduce the likelihood of condensation on concrete surfaces, they should be insulated. In northern climates, installing a layer of rigid foam on the outside of the foundation wall and under the slab will generally keep inner surface of the concrete warm enough that condensation will not occur. With interior foundation insulation, provide a vapor barrier to keep moisture from reaching the concrete surface. In southern climates, protecting against condensation may be more difficult.
Cement and concrete are vital components in building construction today. Concrete has many environmental advantages, including durability, longevity, heat storage capability, and (in general) chemical inertness. For passive solar applications, concretes ability to function as a structural element while also providing thermal mass makes it a valuable material. In many situations concrete is superior to other materials such as wood and steel. But cement production is very energy intensivecement is among the most energy-intensive materials used in the construction industry and a major contributor to CO
2 in the atmosphere. To minimize environmental impact, therefore, we should try to reduce the quantity of concrete used in buildings, use alternative types of concrete (with fly ash, for example), and use that concrete wisely. The accompanying checklist provides practical suggestions for accomplishing these goals.
expansive cement - an overview | sciencedirect topics
Expansive cements are usually based upon Portland cement with an expansive component. They expand slightly during the first few days of hydration and can be used to compensate for the effects of shrinkage in normal Portland cement concrete, which is useful for underpinning or other types of repair work, and in the production of chemically prestressed concrete, where the expansion of the cement is utilised to stress the reinforcement.49 Such a cement has also been employed experimentally to cement over a cavernous vug in a well,1 but there has been little commercial application to well cementing operations. Expansions of up to 5% under controlled conditions can be given.
There are a number of different types of expansive cement,252 the most familiar of which are outlined below. Type K. expansive cement is commonly produced by intergrinding ordinary Portland cement clinker, an expansive clinker containing kleinite (3CaO3Al2O3 CaSO4) and either gypsum or an anhydrite gypsum mixture. As well as kleinite, the expansive clinker generally contains alite, belite, ferrite and anhydrite plus some free lime, and is made by sintering limestone, various alumina-containing materials and gypsum at temperatures not exceeding 1300C in rotary kilns.
Not all sulfoaluminate cements containing kleinite are expansive. Other properties, such as rapid hardening and high early (13days) and late (28days) compressive strength may be the desired purpose. A mixture of limestone, gypsum, bauxite, silica sand and iron-rich industrial by-product raw meal (in carefully selected proportions) which was sintered at 1280C and ground to 3700cm2/g, with no gypsum being ground in, gave such a cement.321
Type S expansive cement is made from Portland cement clinker with a high C3A content. This cement has so far found a more limited application than the other two types because of the difficulty of steadily controlling the rate of formation of ettringite from tricalcium aluminate. Ettringite forms very rapidly from C3A at early hydration times and then the reaction slows down to a very low rate. Even after 7days, the unreacted tricalcium aluminate content is still appreciable.
Calcium sulfoaluminate cements can be produced from industrial by-products such as phosphogypsum, bauxite fines, in pulverised fly ash and ground granulated blastfurnace slag producing the components of these cements, which, when mixed in the proportions C4A3S/-C2S/CS of 1.5:1:1 by weight, have given products of acceptable compressive strengths after 1day and 28days.322,323 Shrinkage-compensating cements can contain a prehydrated calcium aluminate (high-alumina) cement-based expansive additive, in which appropriate admixtures can be employed, either to reduce or to increase expansion as desired.324
Other types of expansive cement composition contain calcium oxide or even magnesium oxide as the expansive component. The calcium oxide can be obtained by sintering a limestone, clay and anhydrite mixture. This expansive phase contains alite, free lime and some residual anhydrite, and is interground with the ordinary Portland cement clinker. The calcium oxide occurs largely as inclusions in the alite grains and undergoes hydration more slowly as the alite hydrates, resulting in controlled expansive properties.
is probably due to negatively charged colloidal grains of ettringite with a high specific surface area being formed by a through-solution mechanism; they attract the polar water molecules that surround the crystals, and cause interparticle (perhaps double layer-type) repulsion, which results in overall expansion of the system. Other hypotheses consider expansion to be caused by local transformation of anhydrous phases into hydrates.49
Expansive cement can generate volume expansion in the hydration process, and it does not shrink but also expand to some extent. The use of expansive cement can overcome and improve some shortcomings of ordinary cement concrete (commonly used cement will shrink in the hardening process, which causes the structures to crack and be permeable, inappropriate for some projects), and can enhance the density of cement concrete structures and the integrity of concrete.
The major components of expansive cement include: silicate-type, aluminate-type, sulphoaluminate-type and calcium aluminoferrite-type. The expansion mechanism is the expansion of ettringite generated in cement paste. And the setting and hardening of silicate expansive cement is relatively slow; but that of the aluminate one is fast.
If the aluminate cement in silicate expansive cement is replaced by alunite, it is known as alunite expansive cement. The composition of alunite is [K2SO4Al2(SO4)34Al(OH)3], and it can generate ettringite, the best expansive cement at present.
By adjusting the coordinate proportion of the above four types can generate the expansive cement with different expansion ratios. The expansive cement can be divided into expansive cement and self-stressing cement according to different expansion ratios. The linear expansion rate of expansive cement is generally under 1%, equivalent to or slightly larger than the shrinkage rate of ordinary cement, which can conduct shrinkage compensation. Thus it is also called shrinkage-compensating cement or the non-shrinkage cement. The linear expansion ratio of self-stressing cement is generally 1%~3%, with big expansion value. If it is the reinforced concrete, the concrete will bear compressive stress in order to achieve pre-stress. When self-stress is more than or equals to 2.0MPa, it is called self-stressing cement; if self-stress is less than 2.0MPa (normally 0.5MPa), it is called expansive cement.
The self-stressing cement can be used for pressure tubes and fittings in self-stressing reinforced concrete. The expansive cement can be used for shrinkage-compensating concrete, structure joints and pipe joints, the reinforcement and repair of concrete structures, anti-seepage and plugging projects, and the fix of machine base and foot screw.
The waterproof concrete prepared by mixing with expansive cement can expand because the expansive cement generates a lot of ettringite in the process of hydration. Bound by the conditions, it can improve the pore structure of concrete and reduce pores and porosity to improve the density and impermeability of concrete.
ASTM C150 specifies five types of Portland cement; ASTM C595 or C1157 specifies eight types of blended hydraulic cement; ASTM C91 specifies three types of masonry cement, two types of plastic cement, three types of expansive cement, and a number of special Portland or blended cements for block, pipe, prestressed heat-cured concrete, and other product applications.
Type II has moderate sulfate resistance, with or without moderate heat of hydration (modified). Modified is used when optional additional requirements are invoked such as moderate heat of hydration and low alkali. The maximum content of C3A is 8%. This type of cement is specified in concrete exposed to seawater.
Type III is high early strength cement. It is produced by finer grinding of a clinker with a higher percentage of C3A and C3S. The gypsum level is increased slightly. Concrete using this cement has a 3-day compressive strength approximately equal to the 7-day compressive strength for Types I and II and a 7-day compressive strength almost equal to the 28-day compressive strength for Types I and II.
Type IV is low heat cement. Percentages of C2S and C4AF are relatively high, while C3A and C3S are low compared to other types. Heat of hydration is less than for other types, and the heat develops more slowly. Strength development is much slower, but after 12 years is higher than other cements if curing is continued. This cement is used for massive concrete structures with low surface/volume ratios. It is available only on special order for very large tonnages with long lead times, if at all. It requires much longer and more careful curing than other types.
To avoid problems associated with drying shrinkage of concrete, it would be advantageous to use cement that does not change its volume due to drying shrinkage or, sometimes, that even expands on hardening. Concrete containing such cement expands in the first few days of its life, and a form of prestress is induced by restraining the expansion with embedded steel reinforcement; steel is in tension and concrete in compression. Restraint by external means is also possible. It is to be noted that expanding cement does not prevent the development of drying shrinkage and cannot produce shrinkless concrete, as drying shrinkage occurs after moist curing has ceased, but the magnitude of expansion can be adjusted so that the expansion and subsequent shrinkage are equal and opposite . Expansive cements are used generally to minimize cracking caused by drying shrinkage in concrete slabs and pavements structures and in special circumstances, such as prevention of water leakages.
Expansive cements consist of a mixture of Portland cement, expanding agent, and stabilizer. The expanding agent is obtained by burning a mixture of gypsum, bauxite, and chalk, which form calcium sulfate and calcium aluminate (mainly C3A). In the presence of water, these compounds react to form calcium sulfoaluminate hydrate (ettringite), with an accompanying expansion of the cement paste. The stabilizer, which is blastfurnace slag, slowly takes up the excess calcium sulfate and brings expansion to an end. Whereas the formation of ettringite in mature concrete is harmful due to its association with sulfate attack and efflorescence , a controlled formation of ettringite in the early days after placing of concrete is used to obtain the shrinkage-compensating effect or to obtain an initial prestress arising from restraint by steel reinforcement.
Three main types of expansive cement can be producedK, M, and Sbut only type K is commercially available in the United States. ASTM C 84504  classifies expansive cements, collectively referred to as type E-1 according to the expansive agent used with Portland cement and calcium sulfate. In each case, the agent is a source of reactive aluminate, which combines with the sulfates of Portland cement to form expansive ettringite. Special expansive cements containing high-alumina cement can be used for situations requiring extremely high expansion .
Shrinkage-compensating concrete is the subject of ACI 223-98 , where expansion is restrained by steel reinforcement (preferably triaxial) so that compression is induced in the concrete, which offsets the tension in the steel reinforcement induced by restraint of drying shrinkage. It is also possible to use expansive cement to make self-stressing concrete, in which there is a residual compressive stress (say, up to 7MPa) after most of the drying shrinkage has occurred; hence, shrinkage cracking is prevented [1,6].
Pretensioning and post-tensioning represent two general methodologies to which most prestressing techniques belong. It was mentioned above that electrothermal prestressing is in effect a pretensioning method. Chemical prestressing, however, does not belong to any of the above. In chemical prestressing the tendons are placed untensioned in the forms and the concrete is poured. Due to the special expansive cement used, the concrete, instead of shrinking, expands after curing and during hardening. As the steel is bonded to the concrete, it stretches with it, thus undergoing tension and inducing compression in the concrete. Because stresses due to chemical prestressing are generally low, this technique has not been widely used.
There are smart materials that allow us to envision self-stressing with effective prestress levels that can be controlled much more accurately, and can be of much larger magnitude than is achieved with expansive cement matrices or electrical prestressing. Shape memory alloys (SMAs) and some special polymeric fibers possess the unique property of being able to be frozen temporarily in a particular state, then, with proper heat or radiation treatment, go back to their previous equilibrium state. Compared to electrical prestressing these materials do not need costly specialized electric equipment and do not create safety problems in the field. The treatment (heat or radiation) can be applied any time after hardening of the matrix instead of during its curing and hardening. The special reinforcement needed, such as SMA, can be factory produced, stored, shelved, placed in the composite, and triggered to recover its deformation (inducing prestressing) at any appropriate time.
Finally, it is conceivable to combine the previous two methods, i.e., to have a matrix that expands and a reinforcement that shrinks in order to produce the proper amount of self-stressing. The technique of self-stressing using shape memory materials was still at the basic research stage at the end of the 1990s (Naaman 2000, Reinhardt and Naaman 1999).
Depending on the tricalcium aluminate and gypsum content of ordinary portland cement, the maximum ettringite content of the hydrated product at early ages is approximately 15%; cementing systems containing industrial by-products, such as fly ash, with lime and gypsum may have a higher ettringite content of, for instance, 20% (Solem and McCarthy, 1992); expansive cements based on calcium aluminates could have an ettringite content of up to approximately 50%. Because of the potential for uptake of contaminants by ettringite mentioned above, use of high ettringite systems for waste management has been advocated. However, such systems can be expected to have a different pH response to acid addition than cements based primarily on CSH.
Although the pH stability of ettringite has not been specifically addressed, some studies, both in the areas of cement and concrete, and waste management, have touched on this issue. Day (1992) cites existence of a crystalline ettringite in the pH range from 11.5 to 11.8, and a non-crystalline phase with a similar composition from pH 12.5 to 12.8. Ghorab and Kishar (1986) found the pH of an ettringite solution to be 11.2, whereas a group of workers based at the University of North Dakota have variously observed pH values from 9.8 to 12, although their most recent work states that ettringite is not stable below pH 11 (McCarthy, Hassett and Bender, 1992, Kumarathasan et al., 1990, and Hassett et al., 1989).
Literature values of the solubility product for ettringite range from 10-35 to 10-45 (Day, 1992, and Deng and Tang. 1994), from which the theoretical pH of a saturated solution may be calculated as ranging between 11.0 and 11.6.
The hydraulicity of C4AF is reported to be dependent on the conditions of formation, formation at lower temperatures (1200C) resulting in a more hydraulic cement.27 The hydration of pure calcium aluminoferrite has been investigated;122 high strengths are found to result from its hydration. The cements were formed by melting at 1360C and then hydrated after cooling and grinding with 3 per cent gypsum (Table 9.18) to 300 m2/kg. High early strengths were obtained from high-iron mixes containing C4AF.122 The hydration of C4AF synthesised by solid-state reaction at 1200C, was faster than that for C4AF synthesised from the melt.124 Compressive strengths have been reported for ferrites prepared by the separate burning of pure compounds;123 the ferrites were fired at temperatures such that the free lime was reduced to zero (12301350C). It is postulated that more structural vacancies exist in material burned rapidly than in normally fired products, and this leads to higher hydration rates and increased strength. Hydration studies in the system C2F-C4AF-C4A3S showed that C4AF hydrated more rapidly than C2F and that the presence of C4A3S accelerates the hydration of the ferrite. The character of the ferrite phase was found to alter with cooling rate and Na2O content;125 the Fe2O3/Al2O3 ratio in the ferrite phase is increased by slow cooling and by the presence of Na2O. The hydration of samples of C3A and C2(A,F) (obtained by extraction from Portland cements) show that the ferrite hydrates more rapidly in the presence of calcium sulfate. The reaction rate of the ferrite phase in Portland cement is increased by the presence of 13 per cent potassium citrate or potassium carbonate, or combinations of the two. A normal Portland cement and a sulfate-resisting Portland cement clinker were examined.126 Compressive strengths of cement mortars were measured on 40 40 160 mm prisms between 4 h and 28 days; after 28 days, strengths lay between 56 and 81.5 MPa. Strength increases were very marked at ages up to 1 day in the presence of citrate.
Ferruginous bauxite plus limestone and gypsum may be used to produce (at 135050C) ferroaluminate cement clinkers in the C-S-A-F-S system.127 These materials belong to a group of special cements whose relatively low energy consumption in manufacture results mainly from the low CaO content of their main phases, i.e. belite and calcium aluminoferrite, and in the lower clinkering temperatures required. The mineral composition is C4A3S (3560 per cent), C4AF (1545 per cent) and C2S (1530 per cent). By adjusting the feed proportions, a high early strength, an expansive or a self-stressing cement could be produced. The cements gave a high resistance to sulfates (tested over 12 months). The energy consumption for the production of these cements was about 65 per cent of that required for Portland cement production. The lattice distortions present due to impurities and the low temperature of burning contribute to their hydraulic properties128. The compressive strengths of high early strength cements produced industrially attained 19.7 MPa after 1 day and 101.8 MPa after 3 days.127 The Ca(OH)2 content of the hydrated cement was low, with a pH for the set paste between 12 and 12.5; FH3 is present in the set material, both factors leading to a high sulfate resistance. Firing temperatures were low, in the region of 1250C, leading to good grindability; their properties could be adapted by varying the proportions of clinker minerals to give either high early strengths or expansive cements. The phase composition, for example, of a Chinese cement127 was stated to be 15 to 45 per cent C4AF, 3560 per cent C4A3S, and 1530 per cent C2S. The SO3 content of the clinker is up to 10 per cent.78 The designation C4AF does not imply a particular member of the ferrite solid-solution series. Increasing the Fe2O3 content in the feed resulted in a lowering of the clinkering temperature.
Four-component clinkers containing Al2O3, Fe2O3, CaO and SiO2 with additions of SO3 and MgO may be synthesised, at varying Al2O3/Fe2O3 ratios, at 1350C followed by either slow cooling or air quenching. When this ratio is high (1.75), the sulfate is first consumed to form C4A3S and a high Fe2O3 aluminoferrite phase (C6AF2), in spite of the high aluminate composition of the clinker, as well as - and -belites. When the Al2O3/Fe2O3 ratio is low (<1.0), belite and a high ferruginous ferrite, C6AF2 is formed containing significant amounts of SO, in solid solution. With increasing additions of SO3 the amounts of C4A3S and -belite increase while the amount of aluminoferrite phase decreases, and this phase becomes more ferruginous. The hydraulic character of these ferrites is greatly influenced by the speed of quenching.128 The possibility of forming C4F3S was not confirmed.128 Compressive strengths were measured on 10 mm cubes, at water/cement = 0.3 and the degree of hydration is established by quantitative X-ray diffraction. Hydration kinetic measurements showed that both the belites and the ferrites were more reactive for clinkers with lower Al2O3/Fe2O3 ratios. The belites in these clinkers contained significant amounts of SO3 (34 per cent) in solid solution, and possessed a strongly distorted structure. Maximum strengths were obtained when the ratio (C4AF or C4A3S equalled 3:1. The hydration products formed from calcium aluminoferrite cements in the presence of gypsum have been studied:129 the hydration products were essentially similar to those formed from C3A. Fe(OH)3 was found as a gel phase and Fe3+ ions were incorporated into the AFm hydration products.
The hydration of calcium sulfoferrites and the addition of sulfoferrite clinker to Portland cement results in improved properties for the cement.130 In the C-F-CS system, clinkering starts at 800C with the formation of CF initially. Within the interval 9501205C, CF and CaSO4 interacted to form calcium monosulfoferrite (C4F3S), which decomposes at 1205C to form C3FS in clinkers of high basicity. CF hydrates slowly to form C3FH6, which has little or no strength. Commercial clinkers containing sulfoferrites can form at temperatures between 1200 and 1350C, to produce cements showing high resistance to sea water, and 28-day strengths of >80 MPa.130
The aluminoferrite phase has a variable composition, even within individual clinker nodules,131 and this phase can accommodate 10 per cent by oxide mass of impurity ions.46 Heterovalent substitution in the lattice leads to greatly increased hydraulic activity and to a near-amorphous structure. The aluminoferrite phase in Portland cement clinker contains considerable quantities of MgO, Mn2O3 and TiO2.46
Iron-rich cements in the system C2S-C4A3S have been examined; Table 9.19 summarises the results of an early investigation.26 Cements fired at the lower clinkering temperatures possible (1200C for 1 h), show that C4AF is formed with increased hydraulic activity and practically no free CaO. Similar experimental cements have been prepared again more recently132 from a range of waste materials; setting times were about 45 min. The clinkers are very easy to grind, partly as a result of the very low temperature of firing. The hydraulic activity of these cements is closely related to the poor crystallinity and high impurity content, arising from the low temperature of formation. Limestone is added as setting control rather than gypsum. The hydrates forming are AFt, AFm, AH3 and C-S-H; strengths in the range 5090 MPa at 1 day are achieved, with good frost resistance.
Cements containing the series of compounds C2S, C4A3S, C4AF, CSH2 and free CaO have been prepared.20,110,127 It is observed that free lime can be accommodated in these cements without resulting in unsoundness. Hydration studies showed very high early strengths (1-day strength up to 40 MPa) and good durability. Systems that incorporated MgO were important in that high-magnesium limestones and dolomites are commonly available and not normally useful in the production of Portland cement. Systems were found to be able to accommodate up to 10 per cent MgO without expansion; high strengths were obtained on rather coarsely ground cements (250300 m2/kg) (37 and 49 MPa at 1 and 28 days, respectively).133 Natural raw minerals combining limestone, dolomite, bauxite, laterite and gypsum are ground and blended to contain 512 per cent MgO, and 3549 per cent CaO. They were burned in an electric oven at 1350C for up to 35 min. The phases -C2S, C4A3S, C4AF, C4A3S, C3MS2 and C2AS were present. On hydration, ettringite was the main product.
The conventional tunneling method is a cyclical process of tunnel construction that involves excavation by drilling and blasting or by mechanical excavators (except the full-face tunnel boring machine (TBM)). This is followed by application of an appropriate primary support.
Mainly used in rock formations, several variants exist including the mining method, drill-and-blast method, new Austrian tunneling method, etc. Conventional tunneling has the following steps (Fig. 4.21):
Before blasting it is necessary to drill holes in the rock to insert explosives. These holes are drilled in a pattern (Fig. 4.22) by drill rigs. The three types are:1.Parallel cuts. They are usually used in hard and intact rocks. The direction should be precisely controlled.2.V cuts. They are applicable to all kinds of rock and no large-diameter drilling machine is needed. The drilling angle is hard to control.3.Fan cuts. They are not as common as the two previous cuts due to the asymmetry of blast holes and delicately designed position and depth. This cut is suitable for rocks with fractures.
Once the pattern has been drilled, the depth of its holes is verified and the presence of water for efficiency and safety checked. The holes are then charged with a detonator and a primer is lowered to the bottom of the hole. Explosives are then pumped down the hole around the detonator and primer, before the end of the holes are filled with stemming. This acts as a plug and forces the explosive energy to go into the surrounding rock rather than along and out of the hole. Once all the holes have been charged, they are connected to explode in a certain order.
Regarding the choice of explosives, several variations exist:Ammonium nitrate/fuel oil. Cheap and bulk dosed but sensitive to water.Emulsions. Pumpable, resistant to water, and thus growing in use.Explosive gelatines that are mainly used for smooth blasting.Explosive powders that are less and less commonly used.
The ignition system can include different elements.Electric detonators (Fig. 4.23) used to initiate explosives such as blasting works in noncoal mines, open pits, and demolition and other engineering projects.Figure 4.23. Electric detonators: image of electric detonators for rocks.Nonelectric detonators. They have the advantage of enabling fast and easy connection of all detonators at one ignition point (compared to the time-consuming and complicated process of connecting electric detonators).Detonating cords. They are used to reliably and cheaply chain together multiple explosive charges. Typical uses include mining, drilling, demolitions, and warfare.
Nonelectric detonators. They have the advantage of enabling fast and easy connection of all detonators at one ignition point (compared to the time-consuming and complicated process of connecting electric detonators).
Scaling is the removal of insecure blocks of rock from the back, the sidewalls, and the face. It is also regarded as preparation for shotcrete and/or rock bolts. Due to its danger, the safety of workers must be carefully considered. Procedures vary throughout the world, from manual to completely mechanized (Fig. 4.25).
Similar to the support method, the excavation method depends on the ground conditions. Conventional tunneling allows either full-face or partial excavation of the tunnel cross section. An appropriate excavation sequence must be defined on the basis of the expected geotechnical conditions, tunnel size, modeling results, structural analysis, and experience.
Full-face excavation is normally used for small-sized tunnels of rock class I to III. The excavation tool can be a jumbo (Fig. 4.28) or handheld air-leg for a small-scale excavation when access is difficult.
A short-stage method (K>550m) can improve the stress conditions of initial supports to control the deformation of surrounding rocks. Nevertheless, muck in the upper stage may influence the construction of the lower stage.
When the stage is 35m long (K=35m), it is known as an ultrashort stage method. But the interference between the upper and lower stage is significant, which means the excavation efficiency decreases.
For crown heading excavation, the bench and invert excavation is only started after the excavation of the tunnel crown (Fig. 4.30). This method is typically useful when the area of excavation is in fragmentary strata. Crown heading is normally used for classes IV or V and if the tunneling size is larger, classes III or IV.
CD excavation (midwall method, Fig. 4.31) is normally used for classes IV and V whose span is less than 18m. There is a greater chance of settlement and the middle temporary support has to be removed later.
Center cross diaphragm method (CRD) excavation (cross-midwall method, Fig. 4.32) is used for fractured rocks as it provides top and lateral supports. It has been adopted in many projects, but the construction period is long and the cost for temporary support construction and removal is high. It is normally used for large caverns for rock class III and for smaller ones, classes IV and V.
The sidewall adit heading method (Fig. 4.33) is advantageous in weak rock (class V) such as clay and silt and for tunnels with shallow depth and wide span. Its drawbacks are lengthier construction periods, higher costs, and weaker waterproofing. The Xiangan tunnel in Xiamen (Fig. 4.34) uses this construction method.
Anchors and rock bolts (Fig. 4.35) involve the same mechanical principle, but they are employed in soft soil and rock, respectively. Anchors and rock bolts are active reinforcing elements designed to anchor and stabilize the rock mass during tunnel excavations. In case of rock mass movement, the bolts will develop a restraining force that is transferred back to the rock mass as in Fig. 4.36. The driving force is countered in this way, such that the total resistance mobilized within the rock mass is equal to the applied force.
Lattice girders (Fig. 4.37) are lightweight triangular steel frames. In crown heading excavation, for example, immediate support should be applied to the excavation with lattice girders. They are used in conjunction with shotcrete and act as armor to achieve a good level of support in tunneling. To provide additional forward support, strata bolts can be inserted through the lattice girders (Tunnel Ausbau Technik, n.d.).
I beam girders (Fig. 4.38) are relative heavy steel frames. They are widely used in China tunneling projects for weak rock formation. However, the use of I beam girders will have a bad influence on shotcrete quality due to their geometric shape.
Sprayed concrete is a fast hardening material used to stabilize and support tunnels. It comes in two forms: gunite (dry mix) and shotcrete (wet mix). Shotcrete is more efficient than gunite as shotcrete has a lower rebound rate.
Dry sprayed concrete is used when small amounts and high early strength is needed (Hfler, Schlumpf, & Jahn, 2011). The material is conveyed in a dry or semidry state to the nozzle, where water is added to the mix before being applied at high velocity onto the substrate.
Shotcrete is used more when a high concrete quality is required (Hfler et al., 2011). It requires a mechanized nozzle, as the wet mixture is too heavy to be held by a worker (Fig. 4.39). This wet sprayable concrete is workable, premixed, and consists of aggregate, cement, and water. For spraying, wet-sprayed concrete is mixed with air and shotcrete accelerators before being applied. Fibers are frequently applied in shotcrete to achieve better performance in terms of tensile strength, durability, and antifire function of the lining.
Presupport measures in fractured, yet good rock mass aim to increase standup time. This limits the overbreak, ensuring safe excavation and allowing efficient initial support installation. Spilling (Fig. 4.40) and forepoling are two common presupport measures.
The former is the insertion of tensile strengthening elements in the ground such as dowels. The latter involves the installation of steel sheets or bars. In cases where the rock mass is severely fractured, self-drilling rock reinforcement pipes are used to avoid borehole collapse (U.S. Department of Transportation Federal Highway Administration (FHWA), 2009).
Blasting technologies cause serious ground vibrations that can be reduced by wave-screening methods. Waterjet precutting is one recent example technology. It involves an abrasive waterjet that performs precise precutting to prevent the propagation of elastic waves (Oh, Joo, Hong, Cho, & Ji, 2013).
cement manufacturing process - civil engineering
The raw cement ingredients needed for cement production are limestone (calcium), sand and clay (silicon, aluminum, iron), shale, fly ash, mill scale and bauxite. The ore rocks are quarried and crushed to smaller pieces of about 6 inches. Secondary crushers or hammer mills then reduce them to even smaller size of 3 inches. After that, the ingredients are prepared for pyroprocessing.
The crushed raw ingredients are made ready for the cement making process in the kiln by combining them with additives and grinding them to ensure a fine homogenous mixture. The composition of cement is proportioned here depending on the desired properties of the cement. Generally, limestone is 80% and remaining 20% is the clay. In the cement plant, the raw mix is dried (moisture content reduced to less than 1%); heavy wheel type rollers and rotating tables blend the raw mix and then the roller crushes it to a fine powder to be stored in silos and fed to the kiln.
A pre-heating chamber consists of a series of cyclones that utilizes the hot gases produced from the kiln in order to reduce energy consumption and make the cement making process more environment-friendly. The raw materials are passed through here and turned into oxides to be burned in the kiln.
The kiln phase is the principal stage of the cement production process. Here, clinker is produced from the raw mix through a series of chemical reactions between calcium and silicon dioxide compounds. Though the process is complex, the events of the clinker production can be written in the following sequence:
The kiln is angled by 3 degrees to the horizontal to allow the material to pass through it, over a period of 20 to 30 minutes. By the time the raw-mix reaches the lower part of the kiln, clinker forms and comes out of the kiln in marble-sized nodules.
After exiting the kiln, the clinker is rapidly cooled down from 2000C to 100C-200C by passing air over it. At this stage, different additives are combined with the clinker to be ground in order to produce the final product, cement. Gypsum, added to and ground with clinker, regulates the setting time and gives the most important property of cement, compressive strength. It also prevents agglomeration and coating of the powder at the surface of balls and mill wall. Some organic substances, such as Triethanolamine (used at 0.1 wt.%), are added as grinding aids to avoid powder agglomeration. Other additives sometimes used are ethylene glycol, oleic acid and dodecyl-benzene sulphonate.
The heat produced by the clinker is circulated back to the kiln to save energy. The last stage of making cement is the final grinding process. In the cement plant, there are rotating drums fitted with steel balls. Clinker, after being cooled, is transferred to these rotating drums and ground into such a fine powder that each pound of it contains 150 billion grains. This powder is the final product, cement.
Cement is conveyed from grinding mills to silos (large storage tanks) where it is packed in 20-40 kg bags. Most of the product is shipped in bulk quantities by trucks, trains or ships, and only a small amount is packed for customers who need small quantities.
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wet process of cement manufacturing - cement wet process & cement dry process
Cement is a kind of powdery material. When properly mixed with water, it will turn into slurry. The slurry will gradually harden in air and glue together the granular or fibrous materials such as sand and stone firmly. It is widely used in all aspects of our lives, such as subway construction, bridge construction, and residential building construction. It is an indispensable part of our city.
The production process of silicate cement (also known as Portland cement) is representative in cement production. It usually adopts limestone and clay as main materials. After been crushed, proportioned and ground into appropriate granularity, most of the raw materials will be fed into cement kiln for calcining clinker, and then we usually add an appropriate amount of gypsum (sometimes mixed with other materials or additives) in the cement grinding process, finally obtaining the cement products with a qualified fineness. At cement plant, according to different raw materials preparation methods, cement manufacturing can be divided into the dry process (including semi-dry process) and wet process (including semi-wet process). Next, we will discuss the wet process of cement manufacturing in details.
The wet process of cement manufacturing refers to grinding raw material into slurry after mixing with water and then feeding them into the wet process kiln for drying and calcination and finally forming clinker. The slurrys water content is usually between 32%-36%. In addition, the raw material slurry can also be dehydrated into raw material blocks and put into the kiln to calcine clinker. This method is called the semi-wet process, which still belongs to the cement wet process production.
Advantages: the wet process of cement production has the characteristics of simple operation, low dust and easy conveying. Because the slurry has fluidity so that its homogeneity is good and the quality of clinker is improved. Whats more, the energy consumption of raw material grinding in the wet process is reduced by nearly 30%.
Disadvantages: the heat consumption of the wet process is too high, usually between 5234-6490 J/kg and the consumption of ball mill vulnerable parts is also large. Compared with other processing methods, the clinker manufactured by the wet process has a low temperature when it comes out of the kiln, so this method is not suitable to produce the clinker with a high silica rate and high aluminum-oxygen rate.
The dry process of cement manufacturing means that after raw materials with different particle sizes are dried, broken and ground into powders of certain fineness, they will be sent into the dry process kiln for calcining, finally forming clinker. Besides, the raw material powder can also be made into raw material balls by adding a proper amount of water and then be directly sent to the Lepol kiln for calcining. This method is called a semi-dry process, which belongs to the cement dry process production.
Advantages: as the dry process is to directly feed raw material powder into the rotary kiln for calcination, and the moisture content of raw materials is about 1% 2%, it saves the heat consumption needed for the moisture evaporation. Therefore, this method has the advantages of energy-saving, high production efficiency and stable output, which can meet the production needs of large cement plants. At the same time, there is less sewage discharged in the dry process cement production. It is conducive to environmental protection. Nowadays, we call the production line with preheater and precalciner as the new dry process cement production line, which is the development direction of dry process cement manufacturing in the future.
The procedures of the wet process are basically the same with the dry process, which can be divided into three stages: raw materials preparation, clinker calcination, and the cement grinding. All of these stages are covered in the article What You Need to Know about Portland Cement Manufacturing Process we mentioned before.
Similar to the dry process, materials also need
to undergo quarrying, primary crushing, secondary crushing, proportioning and
grinding in the raw materials preparation stage of the wet process. The biggest
difference between the two methods is that in the wet process, water is usually
required as a process media added in the raw mix to form slurry. After mixing
and blending, the slurry will be stored in the slurry tank waiting for further
processing. While in the dry method cement production line, the raw mix doesnt
In the calcination stage, the cement kiln
used by the wet process is longer in comparison to the dry process, and there
is no preheater and precalciner in front of the kiln. The temperature in cement
kiln can reach 1400-1500, slurry in it is heated and dried and finally forming
the clinker compounds, namely Di-calcium Silicate, Tricalcium Silicate,
Tri-calcium Aluminate and Tetra Calcium Alumino-Ferrite. Clinker is a kind of
particle with a variety of size and dark green color. After cooled down in the
grate cooler, they will be sent into the grinding mill for the last processing.
In the last stage, clinker will be ground
into qualified fineness in grinding mills. During this process, we usually add
some gypsum and other materials into clinker to give the final cement product
different properties and usages. For example, we add gypsum to obtain the
ordinary Portland cement and add gypsum and fly ash to obtain the Pozzolana
Wet process cement manufacturing method can be used to produce various types of Portland cement, such as ordinary Portland cement, white Portland cement, oil well cement, etc. It can help your cement plant to achieve high quality and high output cement production.
AGICO Group is an integrative enterprise group. It is a Chinese company that specialized in manufacturing and exporting cement plants and cement equipment, providing the turnkey project from project design, equipment installation and equipment commissioning to equipment maintenance.