A cement is a binder, a substance used for construction that makes, solidifies and attaches to other materials, binds them together. Cement rarely used alone, but to bind sand and gravel (aggregate) together. Cement is used with fine aggregate to produce mortar for rock pairs, or with sand and gravel aggregates to produce concrete.
The cement used in construction is usually inorganic, often lime or calcium silicate based, and may be characterized as either non-hydraulic hydraulic or , depending on the ability of cement to be arranged in the presence of water see hydraulic and non-hydraulic lime plaster).
Non-hydraulic cement will not be regulated under wet or underwater conditions; instead, it dries as it dries and reacts with carbon dioxide in the air. It is resistant to attack by chemicals after setting.
Hydraulic cement (eg, Portland cement) is adjusted and adhesives due to chemical reactions between dry matter and water. Chemical reactions produce mineral hydrates that are not very soluble in water and are extremely durable in water and safe from chemical attacks. This allows the arrangement in wet or underwater conditions and further protects the hardened material from chemical attack. The chemical process for hydraulic cement discovered by the ancient Romans used volcanic ash (pozzolana) in addition to lime (calcium oxide).
The word "cement" can be traced back to the Roman term opus caementicium , used to describe rocks resembling modern concrete made of crushed stone with burnt lime as a binder. Volcanic ash and powdered brick powder are added to the burned lime, to obtain a hydraulic binder, then referred to as cementum , cimentum , cÃÆ'¤ment , and cement . In modern times, organic polymers are sometimes used as cement in concrete.
Video Cement
Chemistry
Non-hydraulic cement , such as dead lime (calcium oxide mixed with water), hardened by carbonation in the presence of carbon dioxide that is naturally present in the air. The first calcium oxide (lime) is produced from calcium carbonate (lime or lime) with calcination at temperatures above 825 ° C (1,517 ° F) for about 10 hours at atmospheric pressure:
- CaCO 3 -> CaO CO 2
Calcium oxide then is spent (dipipis) mix it with water to make dead lime (calcium hydroxide):
- CaO H 2 O -> Ca (OH) 2
Once the excess water actually evaporates (this process is technically called setting ), the carbonation begins:
- Ca (OH) 2 CO 2 -> CaCO 3 H 2 O
This reaction takes a lot of time because the partial pressure of carbon dioxide in the air is low. The carbonation reaction requires dry cement to be exposed to air, and for this reason dead lime is a non-hydraulic cement and can not be used under water. The whole process is called lime cycle .
In contrast, hydraulic cement solidifies with hydration when water is added. Hydraulic cement (such as Portland cement) is made from a mixture of silicates and oxides, the four main components are:
- Belite (2CaO Â · SiO 2 );
- Alite (3CaOÃ, Â · SiO 2 );
- Tricalcium aluminate (3CaOÃ, Â · Al 2 O 3 ) (historically, and occasionally, called 'celite');
- Brownmillerite (4CaOÃ, Â · Al 2 O 3 Ã, Â · Fe 2 O 3 ).
Silicates responsible for the mechanical properties of cement, tricalcium aluminate and brownmillerite are essential for allowing the formation of a liquid phase during a sintering kiln (burning). The chemical reactions listed above are not entirely clear and are still the object of research.
Maps Cement
History
Perhaps the earliest known cement events are from twelve million years ago. Cement deposits are formed after the occurrence of oil shale located next to the limestone bed burned by natural causes. This ancient sediment was investigated in the 1960s and 1970s.
Alternative to cement used in ancient times
Cement, chemically, is a product that includes lime as the main preservative, but is far from the first ingredient used for cement . Babylonians and Assyrians use asphalt to bind bricks or burnt alabaster sheets. In Egypt, stone blocks cemented together with sand and rough-burned gypsum mortars (CaSO 4 Ã, Â · 2H 2 O), which often contain calcium carbonate ( CaCO 3 ).
Macedonia and Rome
Lime (calcium oxide) is used in Crete and by the ancient Greeks. There is evidence that Minoan from Crete used the crushed lump as an artificial pozzolan for hydraulic cement. It is uncertain where it was first discovered that the combination of hydrated non-hydraulic lime and pozzolan produced a hydraulic mixture (see also: Pozzolan reaction), but the concrete made from the mixture was used by Ancient Macedonians and three centuries later on a large scale by Roman engineers.
There is... a kind of powder that from natural causes produces amazing results. It is found in the neighborhood of Baiae and in the country that belongs to the cities around Mt. Vesuvius. This substance when mixed with lime and debris not only gives strength to other types of buildings, but even when the dock is built in the sea, they harden under the water.
The Greeks used volcanic tuffs from the island of Thera as their pozzolan and the Romans used volcanic ash that had been mashed (aluminum active silicate) with lime. This mixture can be set under water to increase its durability. The ingredients are called pozzolana from the town of Pozzuoli, west of Naples where volcanic ash is extracted. In the absence of pozzolanic ash, the Romans used brick powder or pottery instead and they may have used crushed tiles for this purpose before finding natural resources near Rome. The great dome of the Pantheon in Rome and the great Caracalla Baths are examples of these ancient structures made of concrete, many of which are still standing. The extensive Roman aqueduct system also uses extensive hydraulic cement.
Medieval
Although any preservation of this knowledge in medieval literary sources is unknown, medieval stonemasons and some military engineers maintain an active tradition of using hydraulic cement in structures such as canals, forts, ports and shipbuilding facilities.
16th century
Tabby, building materials using chalk oysters, sand, and oyster shells intact to form concrete, was introduced to America by the Spaniards in the sixteenth century.
18th century
The technical knowledge to make hydraulic cement was formalized by French and English engineers in the 18th century.
John Smeaton made an important contribution to the development of cement while planning the construction of the third Eddystone Lighthouse (1755-59) in the English Channel now known as the Smeaton Tower. He needs a hydraulic mortar that will regulate and develop some power in a twelve hour period between successive rising tides. He conducted experiments with different combinations of limestone and additives including trass and pozzolanas and conducted in-depth market research on available hydraulic limes, visited their production sites, and noted that "limestone hydraulicity" is directly related to limestone clay content. from where it was made. Smeaton is a civil engineer with a profession, and takes that idea no farther.
In the South Atlantic coastal area of ​​the United States, cats that rely on oyster shells from the early Native American population were used in house construction from the 1730s to the 1860s.
In the UK in particular, good quality building stones become more expensive during periods of rapid growth, and it becomes common practice to build prestige buildings of new industrial bricks, and to finish them with stucco to imitate stones. Hydraulic flora is favored for this, but the need for fast time encourages the development of new cement. The most famous is the "Roman cement" Parker. It was developed by James Parker in the 1780s, and eventually patented in 1796. In fact, unlike the material used by Romans, it is a "natural cement" made by burning septaria - nodules found in certain clay deposits. , and which contain clay minerals and calcium carbonate. The burned nodules were ground into fine powder. This product, made into mortar with sand, is set in 5-15 minutes. The success of "Roman cement" led to other producers developing rival products by burning artificial hydraulic cement cement from clay and lime. Roman cement quickly became popular but was largely replaced by Portland cement in the 1850s.
19th century
Apparently unaware of Smeaton's work, the same principle was identified by the Frenchman Louis Vicat in the first decade of the nineteenth century. Vicat went on to design a method of combining chalk and clay into an intimate mixture, and, burning this, yielding "artificial cement" in 1817 was considered a "major pioneer" of Portland cement and "... Edgar Dobbs of Southwark patented this cement on year 1811. "
In Russia, Egor Cheliev makes a new binder by mixing chalk and clay. The results were published in 1822 in his book A Treatise on the Art for Preparing the Good Mortar published in St. Petersburg. A few years later in 1825, he published another book, which described the various methods of making cement and concrete, as well as the benefits of cement in building construction and embankments.
Portland Cement, the most commonly used cement type in the world as a base material of concrete, mortar, cement, and non-specialty groats, was developed in England in the mid-19th century, and is usually derived from limestone. James Frost produced what he called "English cement" in the same way around the same time, but did not get a patent until 1822. In 1824, Joseph Aspdin patented the same material, which he called Portland Cement, because the rendering made from it is a color similar to the prestigious Portland stone unearthed on the Isle of Portland, Dorset, England. However, Aspdins cement is not like modern Portland cement but is the first step in its development, called the proto-Portland cement. Joseph Aspdins's son, William Aspdin, had abandoned his father's company and in his cement factory accidentally produced calcium silicate in the 1840s, a central step in the development of Portland cement. William Aspdin's innovation goes against the producers of "artificial cement", because they need more lime in the mix (a problem for his father), much higher kiln temperatures (and therefore more fuel), and clinker produced very hard and fast. lowered the millstone, which was the only milling technology available at the time. Therefore, manufacturing costs are much higher, but the product sets slowly and develops the power quickly, thus opening up the market for use in concrete. The use of concrete in construction grew rapidly from 1850 onwards, and soon became the dominant use for cement. So Portland cement started its leading role. Isaac Charles Johnson improved the production of meso-Portland cement (middle stage of development) and claimed to be the original father of Portland cement.
Timing and "initial strength" are important cement characteristics. Hydraulic limes, "natural" cement, and "artificial" cement all depend on their belite content for strength development. Belite develops the power slowly. Since they are burned at temperatures below 1,250 ° C (2,280 ° F), they contain no alite, which is responsible for initial strength in modern cement. The first cement consistently containing alit was made by William Aspdin in the early 1840s: This is what we call now the "modern" Portland cement. Because of the mystery air surrounded by William Aspin's products, others ( for example, Vicat and Johnson) have claimed to be prioritized in this discovery, but recent analyzes of both raw concrete and cement have shown that William Aspdin Products is made in Northfleet, Kent is a true alter-based cement. However, the Aspdin method is a "rule-of-thumb": Vicat is responsible for building the chemical base of this cement, and Johnson establishes the importance of sintering the mixture in a kiln.
In the US the first large-scale use of cement was Rosendale cement, a natural cement that was mined from the massive deposit of large dolostone stone deposits found in the early 19th century near Rosendale, New York. Rosendale Cement is very popular for foundation buildings ( for example. , Statue of Liberty, Capitol Building, Brooklyn Bridge) and marched water pipes.
Cement Sorel was patented in 1867 by French Stanislas Sorel and was stronger than Portland cement but poor water resistance and corrosive qualities restricted its use in building construction. Subsequent developments with Portland cement manufacturing are the introduction of rotary kilns which allow for stronger, more homogeneous mixtures and sustainable manufacturing processes.
20th century
The calcium aluminate cement was patented in 1908 in France by Jules Bied for better resistance to sulfate.
In the US, a long preservation time of at least one month for Rosendale cement made it unpopular after the First World War in highway and bridge construction and many countries and construction companies switched to the use of Portland cement. Due to the switch to Portland cement, in the late 1920s from 15 Rosendale cement companies, only one survived. But in the early 1930s it was discovered that, while Portland cement had a shorter regulatory time it was not durable, especially for highways, to the extent that some countries stopped building roads and roads with cement. Bertrain H. Wait, an engineer whose company has worked on the construction of Catskill Aqueduct in New York City, is impressed with the durability of Rosendale cement, and comes with a mixture of Rosendale and synthetic cement that has both the attributes of both. : it is very durable and has a faster setting time. Wait to convince the New York Commissioner of the Highway to build an experimental section of a highway near New Paltz, New York, using a sack of Rosendale to six sacks of synthetic cement. It proved successful and for decades the synthetic-Rosendale cement mix became commonly used in the construction of highways and bridges.
Modern cement
Modern hydraulic cement has been developed since the start of the Industrial Revolution (around 1800), driven by three main needs:
- Making hydraulic cement (plastering) to finish building brick in wet climate.
- Hydraulic mortar for construction of rock pairs at ports, etc., in contact with sea water.
- Strong concrete development.
Modern cement is often Portland cement or Portland cement mix, but other cements are used in industry.
Portland cement
Portland cement is by far the most common type of cement used worldwide. This cement is made by heating limestone (calcium carbonate) with other materials (such as clay) up to 1450 Â ° C in kilns, in a process known as calcination, in which carbon dioxide molecules are liberated from calcium carbonate to form calcium. oxide, or chalk, which then chemically combine with other ingredients that have been incorporated in the mixture to form calcium silicates and other cement compounds. The resulting hard material, called 'clinker', is then ground with a small amount of gypsum to powder to make 'ordinary Portland cement', the most commonly used type of cement (often referred to as OPC). Portland Cement is a base material of concrete, cement and non-special grout. The most common use for Portland cement is in the production of concrete. Concrete is a composite material consisting of aggregates (gravel and sand), cement, and water. As a construction material, concrete can be cast in almost any desired shape, and after hardening, it can be a structural element (load bearing). Portland cement may be gray or white.
Portland cement together
Portland cement mixtures are often available as an inter-soil mixture of cement producers, but similar formulations are often also mixed from soil components in concrete mixing plants.
Portland slag-cement slag furnace, or Blast furnace cement (containing ASTM C595 and EN 197-1 nomenclature respectively), containing up to 95% blast furnace powder, with the remaining Portland clinker and a bit of gypsum. All compositions produce ultimate high strength, but because the slag content increases, the initial strength decreases, while the sulfate resistance increases and the evolution of heat decreases. Used as an economic alternative to Portland sulfate-resistant cement and low heat.
Portland-fly ash cement contains up to 40% fly ash below ASTM standard (ASTM C595), or 35% below EN standard (EN 197-1). Fly ash is pozzolanic, so its main strength is maintained. Because the addition of fly ash allows for lower concrete water content, initial strength can also be maintained. Where good quality fly ash is available, this could be an economic alternative to ordinary Portland cement.
Portland pozzolan cement includes fly ash cement, because fly ash is pozzolan, but also includes cement made from natural or other artificial pozzolans. In countries where volcanic ash is available (eg Italy, Chile, Mexico, Philippines) this cement is often the most commonly used form. Maximum replacement ratio is generally defined as for Portland fly-ash cement.
Portland silica smoke cement . The addition of silica fume can produce very high strength, and cement containing 5-20% silica fume is sometimes produced, with 10% being the maximum allowable allowance under EN 197-1. However, silica fume is usually added to Portland cement in a concrete mixer.
Mason Cement is used to prepare the cement and cement mixture, and should not be used in concrete. They are usually complex ownership formulations containing Portland clinker and a number of other materials that may include limestone, hydrated lime, air entrainers, retarders, waterproofer and dyes. They are formulated to produce a workable mortar that allows fast and consistent brick work. Subtle variations of Masonry cement in the US are Plastic Cement and Cement Cement. It's designed to produce a controlled bond with a brick block.
Expansive Peninsula contains, in addition to the Portland clinker, expansive clinker (usually sulfoaluminate clinker), and is designed to offset the drying effects of shrinking that are usually encountered with hydraulic cement. This allows large floor sheets (up to 60 sq. M.) To be prepared without contractionary connections.
White cement can be made using a white clinker (containing little or no iron) and white additives such as metaccharides with high purity.
Colored cement is used for decorative purposes. In some standards, the addition of pigments to produce "Portland-colored cement" is permissible. In other standards (eg ASTM), pigments are not allowed Portland cement constituents, and colored cement is sold as "mixed hydraulic cement".
A very fine cement is made from a mixture of cement with sand or with slag or other very finely ground polished minerals. Such cement may have the same physical characteristics as normal cement but with 50% less cement primarily due to increased surface area for chemical reactions. Even with intensive milling they can use up to 50% less energy to fabricate than ordinary Portland cement.
Other cements
Pozzolan-lime Cement. The mixture of pozzolan and lime soil is a cement used by the Romans, and is present in the existing Roman structures (eg the Pantheon in Rome). They develop strength slowly, but their main strength can be very high. The hydration product that produces strength is essentially the same as that produced by Portland cement.
Slag-lime cements. Ground-grained ground blast-furnace powder is not hydraulic by itself, but is "activated" by the addition of alkali, the most economical use of lime. They are similar to cement pozzolan on their property. Only granular slag (ie water slag, glass container) is effective as a cement component.
Supersulfated cement contains about 80% ground furnace stirred powder furnace, 15% gypsum or anhydrite and a bit of clinker or Portland chalk as an activator. They produce strength with ettringite formation, with a power growth similar to that of slow Portland cement. They exhibit good resistance to aggressive agents, including sulfates. Calcium aluminate cement is a hydraulic cement made primarily of limestone and bauxite. The active ingredient is monocalcium aluminate CaAl 2 O 4 (CaO Â · Al 2 O 3 or CA in Cement notation chemistry, CCN) and Ca masenite 12 Al 14 O 33 (12 CaO Ã, Â · 7 Al 2 O 3 , or C 12 A 7 in CCN). Strength is formed by hydration to the hydrate of calcium aluminate. They adapt well for use in fireproof concrete (high temperature resistant), for example for furnace layers.
Calcium sulfoaluminate cement is made from clinker which includes ye'elimite (Ca 4 4 SO 4 or C 4 A 3 S in the chemical notation of Cement) as the main phase. They are used in expansive cement, in very high initial cement, and in "low energy" cement. Hydration produces ettringite, and special physical properties (such as expansion or rapid reaction) are obtained by adjusting the availability of calcium and sulfate ions. Their use as a low energy alternative to Portland cement has been pioneered in China, where several million tons per year are produced. Lower energy requirements due to lower kiln temperatures are required for the reaction, and lower amounts of limestone (which must be terminated endothermically) in the mixture. In addition, lower limestone content and lower fuel consumption result in approximately half the CO 2 emissions associated with the Portland clinker. However, SO 2 emissions are usually much higher.
"Natural" cement corresponds to a particular cement from the pre-Portland era, which is produced by burning argillaceous limestones at moderate temperatures. The level of clay component in limestone (about 30-35%) is such that a large amount of belite (low initial strength, high-end mineral strength in Portland cement) is formed without the formation of excessive amounts of free lime. As with any other natural material, such cement has a highly variable nature.
Geopolymer cement is made from a mixture of water-soluble alkali metals and aluminosilicate mineral powder such as fly ash and metakaolin.
Polymer cement is made from polymerized organic chemicals. Often thermoset materials are used. While they are often significantly more expensive, they can provide water proof materials that have useful tensile strength.
Settings and curing
Semen begins to form when mixed with water causing a series of chemical hydration reactions. The constituents slowly hydrate and the mineral hydrates solidify; interlocking hydrate gives its strength. Contrary to popular perception, hydraulic cement is not regulated by drying; Proper curing requires proper moisture maintenance during the curing process. If the hydraulic cement dries during the drying process, the resulting product may weaken significantly. However, a minimum temperature of 5 ° C is recommended.
Security issues
Bags from cement routinely have health and safety warnings printed on them for not only very alkaline cement, but the exothermic regulating process. As a result, wet cement is very caustic (pH = 13.5) and can easily cause severe skin burns if not immediately cleaned with water. Similarly, dry cement powder in contact with mucous membranes can cause severe eye irritation or respiration. Some trace elements, such as chromium, from impurities naturally present in the raw materials used to produce cement can cause allergic dermatitis. Reductors such as ferrous sulfate (FeSO 4 ) are often added to cement to convert carcinogenic hexavalent chromosomes (CrO 4 2 - ) to trivalent chromium (Cr < soup> 3 ), less toxic chemical species. Users of cement should also wear proper protective gloves and clothing.
The world's cement industry
In 2010, the world's hydraulic cement production reached 3,300 million tons (3.2 ÃÆ' - 10 9 ton length; 3.6 ÃÆ' - 10 9 short ton). The top three manufacturers are China with 1,800, India with 220, and the United States with 63.5 million tonnes for a combined total of more than half of the world by the world's three most populous countries.
For the world's capacity to produce cement in 2010, the situation is similar to the top three states (China, India and the US) accounting for less than half of the world's total capacity.
During 2011 and 2012, global consumption continued to increase, rising to 3,585 Mt in 2011 and 3736 Mt in 2012, while annual growth rates declined respectively to 8.3% and 4.2%.
China, which represents an increasing share of world cement consumption, continues to be the main engine of global growth. In 2012, China's demand was recorded at 2,196 Mt, representing 58% of world consumption. The annual growth rate, which reached 16% in 2010, seems to have softened, slowing to 5-6% during 2011 and 2012, as the Chinese economy is targeting a more sustainable growth rate.
Outside of China, world consumption rose 4.4% to 1462 Mt in 2010, 5% to 1535 Mt in 2011, and eventually 2.7% to 1576 Mt in 2012.
Iran is now the world's third largest cement producer and has increased its production by more than 10% from 2008 to 2011. Due to rising energy costs in Pakistan and other major cement producing countries, Iran is in a unique position as a trading partner, utilizing its advantages own oil for clinker power plant. Now as a major producer in the Middle East, Iran is increasingly increasing its dominant position in local and overseas markets.
Performance in North America and Europe during the period 2010-12 is very different from China, as the global financial crisis has evolved into a sovereign debt crisis for many countries in the region and recession. The level of cement consumption for this region fell 1.9% in 2010 to 445 Mt, recovering 4.9% in 2011, then down another 1.1% in 2012.
Worldwide performance, which covers many developing countries in Asia, Africa and Latin America and represents about 1,020 Mt of cement demand in 2010, is positive and more than offsets the decline in North America and Europe. The annual consumption growth was recorded at 7.4% in 2010, respectively to 5.1% and 4.3% in 2011 and 2012.
By the end of 2012, the global cement industry comprises 5,773 cement production facilities, including integrated and grinding, of which 3900 are located in China and 1773 worldwide.
Total cement capacity worldwide is 5245 Mt in 2012, with 2,950 Mt located in China and 2,295 Mt worldwide.
China
"Over the past 18 years, China has consistently produced more cement than any other country in the world. [...] (However,) China's cement exports peaked in 1994 with 11 million tons being shipped out and steadily declining since Just 5.18 million tons were exported out of China in 2002. Offered at a price of $ 34 per ton, China cement set its own price because Thailand asked $ 20 for the same quality. "
In 2006, China is estimated to produce 1,235 billion tons of cement, accounting for 44% of the world's total cement production. "Demand for cement in China is forecast to rise 5.4% annually and exceed 1 billion tonnes in 2008, driven by a slowdown but healthy growth in construction spending The cement consumed in China will account for 44% of global demand, and China will remain in the world's largest national consumer cement with a large margin. "
In 2010, 3.3 billion tons of cement were consumed globally. Of this amount, China reached 1.8 billion tons.
Environmental impact
Cement making causes environmental impact at all stages of the process. These include air pollution emissions in the form of dust, gas, noise and vibration when operating machinery and during blasting in mining, and rural damage from excavation. Equipment to reduce dust emissions during excavation and cement making is widely used, and equipment for trapping and separate exhaust gases are being used. Environmental protection also includes re-integration of mining into rural areas after they are closed by returning them to nature or cultivating them.
CO 2 emissions
Carbon concentration in the cement range from? 5% in the structure of cement to be? 8% in case of road in cement. Cement production releases CO 2 in the atmosphere both directly when calcium carbonate is heated, resulting in chalk and carbon dioxide, and also indirectly through energy use if its production involves CO 2 emissions. The cement industry generates about 10% of global CO2 emissions 2 , of which 60% comes from chemical processes, and 40% of fuel combustion.
Nearly 900 kg of CO 2 is emitted for every 1000 kg of Portland cement produced. In the EU, specific energy consumption for cement clinker production has been reduced by about 30% since the 1970s. This reduction in primary energy demand is equivalent to about 11 million tonnes of coal per year with the corresponding benefits in reducing CO 2 emissions. It accounts for about 5% of anthropogenic CO 2 .
Most of the carbon dioxide emissions in Portland cement making (about 60%) are produced from limestone chemical decomposition to lime, an ingredient in Portland cement clinker. This emission can be reduced by decreasing the content of cement clinker.
To reduce the transport of heavier raw materials and to minimize related costs, it would be more economical if the cement plant is closer to a limestone quarry than to a consumer center.
In certain applications, lime mortar recovers some CO 2 as released in its manufacture, and has lower energy requirements in production than main cement (citation). Newly developed cement types from Novacem and Eco-cement can absorb carbon dioxide from the surrounding air during hardening. The use of the Kalina cycle during production can also improve energy efficiency.
Heavy metal emissions in air
In some circumstances, depending mainly on the origin and composition of the raw materials used, the high-temperature calcination process of limestone and clay minerals releases atmospheric gases and dust rich in volatile heavy metals, ao, thallium, cadmium and mercury are the most toxic. Heavy metals (Tl, Cd, Hg,...) and selenium are often found as trace elements in common metal sulphides (pyrite (ZnS), galena (PbS), pyrite (FeS 2 ),...) are present as secondary minerals in most raw materials. Environmental regulations exist in many countries to limit these emissions. In 2011 in the United States, cement kilns "are legally allowed to pump more toxins into the air than incinerators of hazardous waste."
Heavy metal is present in clinker
The presence of heavy metals in clinker appears both from natural raw materials and from the use of recyclable byproducts or alternative fuels. The high pH applicable in porewater cement (12.5 & lt; pH & lt; 13.5) limits the mobility of many heavy metals by decreasing their solubility and increasing their absorption to the cement mineral phase. Nickel, zinc and tin are found in cement in non-negligible concentrations. Chromium may also appear immediately as a natural impurity of the raw material or as a secondary contamination of the abrasion of the hard chromium alloys used in the ball mill when the clinker is milled. As chromates (CrO 4 2 - ) are toxic and may cause severe skin allergies at trace concentrations, sometimes reduced to Cr (III) trivalent with iron sulphate addition (FeSO 4 ).
Use of alternative fuels and by-products
The cement plant consumes 3 to 6 GJ of fuel per ton of clinker produced, depending on the raw materials and processes used. Most cement kilns currently use coal and petroleum coke as the main fuel, and to a lesser extent natural gas and fuel oil. Garbage and by-products selected with recoverable calorific values ​​can be used as fuel in cement kilns (referred to as co-processing), replacing some conventional fossil fuels, such as coal, if they meet stringent specifications. Wastes and by-products containing useful minerals such as calcium, silica, alumina, and iron can be used as raw materials in kilns, replacing raw materials such as clay, shale, and limestone. Because some materials have beneficial mineral content and recoverable caloric values, the difference between alternative fuels and raw materials is not always clear. For example, waste sludge has a low but significant calorific value, and burns to provide mineral-ash useful in clinker matrix.
The normal operation of cement kilns provides more than enough combustion conditions for even the most difficult destruction to destroy organic matter. This is mainly due to the very high temperatures of kiln gases (2000 Â ° C in the combustion gases of the main burner and 1100 Â ° C in the gas from the burners in the precalciner). The residence time of the gas at high temperatures in the rotary kiln is about 5-10 seconds and in precalciner over 3 seconds.
Because of bovine spongiform encephalopathy (BSE) in the European beef industry, the use of animal-derived products to feed livestock is now severely restricted. Large amounts of animal meat and bone meal (MBM), also known as animal flour, must be disposed of or transformed safely. The production of cement kilns, along with combustion, is the date of one of the two main ways to treat this food industry's solid waste.
Green cement
Green cement is a cement material that satisfies or exceeds the functional performance capabilities of regular Portland cement by combining and optimizing recycled materials, thereby reducing the consumption of raw materials, water, and energy, resulting in more sustainable construction materials.
The new manufacturing process for producing green cement is being investigated with the aim of reducing, or even eliminating, the production and release of harmful pollutants and greenhouse gases, especially CO 2 .
Growing environmental concerns and rising fossil fuel costs have led many countries to reduce the resources needed to produce cement and waste (dust and exhaust).
Source of the article : Wikipedia
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