Arquitectura, Obras y Construcción
Cement and concrete could be synonymous regarding the union of materials but they are of different nature: the cement, a powder ultra-fine ties rock and sand inside a concrete mass. In conclusion, the cement is the main ingredient of the concrete.
The annual global production of concrete is approximately of 5 trillion yards you cube, some 1,25 trillion tons.
EARLY HISTORY AND DEVELOPMENT OF THE PORTLAND CEMENT
Since the first civilization began to build, the man has looked for a material that tied rocks transforming them into solids forming a mass.
The Assyrians and Babylonians used clay for this purpose and the Egyptians discovered the slimes and the mortars of plaster like an agent of union to build grandiose structures like they are the pyramids.
The Greeks created further improvements and the Romans finally developed a cement that produced a great durability to the buildings. Most of the Roman forums are based in kind of a concrete tossed in occasions to a depth 12 feet. The Roman big bathroom was built approximately B.C. in the 27, the Coliseum and Constantino's Basilica are examples of Roman architecture in which a cement mortar is used. The Roman secret in the production of the cement, was to mix out lime with pozzolana a volcanic ash of the Vesuvius, this process produced a cement able to become hard under the water. During the Middle Ages this art was lost and it was not until our revived scientific spirit of question induced us to rediscover it.
The repeated failures in the construction of the lighthouse of Eddystone, in the outskirts of Cornwall, England, they took to John Smeaton, a British engineer to direct experiments with mortars in salted water. In 1756 these experiments took him to discover that the cement created with limestone, containing a clay proportion, it would harden under the salted water.
With this discovery I build the lighthouse of Eddystone in 1 759. Before he/she was discovered the cement portland and during some years after their discovery, the natural cement was used that was produced burning a mixture of lime and clay. Their properties vary as thoroughly as the natural resources of those that I toss you.
In 1824 Joseph Aspdin, bricklayer of Leeds, took out the patent of a cement that he called portland.
The portland is consolidated today, as Aspdin it predetermined and carefully it provided the chemical combination of calcium, silicon, iron and aluminum.
The natural cement left step to the portland, that which was predictable, knowing the consistency and high quality of the product. Almost 20 years later J.D. White and children mounted one it manufactures it prospers in Kent, here it was when the industry of the cement portland saw its maximum expansion, not alone in England, but also in Belgium and Germany. Portland was used to build the system of sewers of London 1859-1867.
PROCESSES OF PRODUCTION.
The demanding nature of the production of the portland requires to make some 80 operations, use of a machinery and weighed team and big quantities of heat and energy. Each step in the production of the portland is verified by frequent physical and chemical tests in laboratory plants. The final product is analyzed and it is proven to assure that it completes all the specifications.
Two processes of production of the cement portland exist: dry and wet. The first step after taking out the stone of a quarry is the same one for both processes, the one milled of the same one. In the first milled decreases the stone until an approximate size of 6 inches, then it passes to secondary crushers or hammer mills and their size is reduced to 3 inches or less. In the wet process, the main materials, are milled with water.
The raw material warms to some Fahrenheit 2700 degrees, the ovens are of 12 feet of diameter in many cases and extremely long, they are lightly inclined, and for up the material is introduced and for below there is a blaze (burning coal, petroleum or gas) that burns the material.
Certain materials leave in form of gas, the remaining elements unite to form a new material with characteristic new. The new substance, called Clinker, is formed in similar pieces to the form of the marble. The Clinker is discharged to the red-hot from the lower end of the kiln, going by some refrigerators; the heated air of the refrigerators is returned to the oven, a process that saves fuel and the effectiveness of the burnt one increases.
THERE´S NOT ANYTHING SIMILAR TO THE CEMENT OF THE SIDEWALKS
The cement can be easily the finest construction material in the earth.
It forms our reservoirs, drainage pipes, basements and foundries, blocks and bricks, streets, etc...The secret of this mixture of sand, stone and it dilutes he/she is a fourth ingredient teacher: the cement portland. The cement portland is an extremely fine gray powder, produced of the terrestrial but common minerals. It is the paste that unites sand and it burdens inside the rock-mass that we call concrete.
THE RIGHT MIX.
PORTLAND IS NOT TO BRAND
The natural cement exists from the Roman empire.
But the production of the cement portland in compensation was not invented up to 1824.
The cement this compound essentially for minerals, containing: calcium, silicon, aluminum and iron. The limestone is the biggest source of calcium. The clay, schist, bauxite and iron mineral, produce the silicon, aluminum and iron components.
Other sources of main materials include the shells of the sea and industrial byproducts as the scum of high oven, flying ashes,...
How Cement Works
When stone, sand, water, and portland cement are mixed together in the proper proportions, the water and cement form a paste that coats every stone and grain of sand and fills the spaces between them. The water triggers a chemical reaction called hydration. This reaction forms a gel which, as it hardens, binds the stones and sand into a solid mass that becomes stronger and stronger.
The cement's rate of hydration determines the setting and hardening time of the concrete. The initial reaction must be slow enough to permit workers to place and finish the concrete in its plastic state. Once in place, however, the concrete is allowed to harden into its finished form.
The rate of hardening can be precisely controlled. The gypsum ground into cement at the plant regulates setting time. Different types of cement have different setting times, and concrete additives are available to speed or slow setting. Temperature also affects setting time. The reaction of hydration itself releases heat-a boon in winter construction.
Although most strength development occurs in the first few days, concrete can gain additional strength for years if moisture is present and cement hydration is sustained.
Strength of Concrete
The strength of concrete is very much dependent upon the hydration reaction just discussed. Water plays a critical role, particularly the amount used. The strength of concrete increases when less water is used to make concrete. The hydration reaction itself consumes a specific amount of water. Concrete is actually mixed with more water than is needed for the hydration reactions. This extra water is added to give concrete sufficient workability. Flowing concrete is desired to achieve proper filling and composition of the forms. The water not consumed in the hydration reaction will remain in the microstructure pore space. These pores make the concrete weaker due to the lack of strength-forming calcium silicate hydrate bonds. Some pores will remain no matter how well the concrete has been compacted.
The empty space (porosity) is determined by the water to cement ratio. The relationship between the water to cement ratio and strength is shown in the graph that follows.
Low water to cement ratio leads to high strength but low workability. High water to cement ratio leads to low strength, but good workability.
The physical characteristics of aggregates are shape, texture, and size. These can indirectly affect strength because they affect the workability of the concrete. If the aggregate makes the concrete unworkable, the contractor is likely to add more water which will weaken the concrete by increasing the water to cement mass ratio.
Time is also an important factor in determining concrete strength. Concrete hardens as time passes. Why? Remember the hydration reactions get slower and slower as the tricalcium silicate hydrate forms. It takes a great deal of time (even years!) for all of the bonds to form which determine concrete's strength. It is common to use a 28-day test to determine the relative strength of concrete.
Concrete's strength may also be affected by the addition of admixtures. Admixtures are substances other than the key ingredients or reinforcements which are added during the mixing process. Some admixtures add fluidity to concrete while requiring less water to be used. An example of an admixture which affects strength is superplasticizer. This makes concrete more workable or fluid without adding excess water. A list of some other admixtures and their functions is given below. Note that not all admixtures increase concrete strength. The selection and use of an admixture are based on the need of the concrete user.
Sometimes other materials are incorporated into the batch of concrete to create specific characteristics. These additives are called admixtures. Admixtures are used to: alter the fluidity (plasticity) of the cement paste; increase (accelerate) or decrease (retard) the setting time; increase strength (both bending and compression); or to extend the life of a structure. The making of concrete is a very complex process involving both chemical and physical changes. It is a material of great importance in our lives.
Some admixtures and functions
•AIR ENTRAINING improves durability, workability, reduces bleeding, reduces freezing/thawing problems (e.g. special detergents)
•SUPERPLASTICIZERS increase strength by decreasing water needed for workable concrete (e.g. special polymers)
•RETARDING delays setting time, more long term strength, offsets adverse high temp. weather (e.g. sugar )
•ACCELERATING speeds setting time, more early strength, offsets adverse low temp. weather (e.g. calcium chloride)
•MINERAL ADMIXTURES improves workability, plasticity, strength (e.g. fly ash)
•PIGMENT adds color (e.g. metal oxides)
It Begins at the Quarry
Rock blasted from the quarry face is transported to the primary crusher, where the piano-sized rocks are broken into pieces the size of baseballs. A secondary crusher reduces them to the size of gravel. Some plants now crush materials in a single stage.
The types of crushers used in a plant depend upon the types of raw materials processed. While the plant's quarry yields the primary ingredient, limestone, other raw materials are often brought in from outside sources.
In the plant laboratory, technicians analyze the raw materials and determine the correct proportions of limestone and other materials needed for the final cement product. At all manufacturing stages, the laboratory keeps a close watch on the raw materials to assure the quality and uniformity of the product.
The Daily Grind
Once analyzed, the raw materials are blended in the proper proportions and ground even finer. Some cement plants use heavy wheel-type rollers that crush the materials into powder against a rotating table. Other facilities grind the raw materials in ball or tube mills-horizontal steel cylinders filled with thousands of steel balls. As the mill turns, the balls tumble onto the materials and crush it into powder.
In the type of cement-making called the dry process, the raw materials are now ready for the kiln. In an older system known as the wet process, water is added to the raw feed during grinding, producing a creamy mixture called slurry, to simplify mixing and proportioning. After grinding, the slurry is stored in large, open tanks under agitation.
As the raw materials move down the progressively hotter kiln, they undergo complex chemical and physical changes requiring intense heat. Expressed at its simplest, the series of chemical reactions in cement-making converts calcium and silicon oxides into calcium silicates, cement's principal constituents.
While kiln systems vary, there are three major zones:
1. Drying and preheating zone, 70oF to 1650oF (20oC to 900oC)
Water is evaporated and calcination-driving off carbon dioxide from limestone-begins.
2. Calcining zone, 1100oF to 1650oF (600oC to 900oC)
Calcination is complete, removing carbon dioxide from calcium carbonate to produce the lime (calcium oxide) needed for subsequent reactions.
3. Sintering or burning zone, 2200oF to 2700oF (1200oC to 1480oC)
Calcium oxide reacts with silica to form dicalcium silicates and alumina- and iron-bearing materials to form tricalcium aluminate and tetracalcium aluminoferrite. These two compounds, in liquid phase, meld solids together into the pellets called clinker. Remaining calcium oxide reacts with dicalcium silicate to form tricalcium silicate.
Conversion by Fire
Whether in dry powder or slurry form, the raw meal is ready for the huge rotating furnace called a kiln. It's the heart of the cement-making process -- a horizontally sloped steel cylinder, lined with firebrick, turning from about one to three revolutions per minute. The kiln is the world's largest piece of moving industrial equipment.
1. To save energy, modern cement plants preheat the materials before they enter the kiln. The preheater tower dominates the landscape, rising more than 200 feet. The tower supports a series of vertical cyclone chambers through which the raw meal passes one its way to the kiln. Hot exit gases rising from the kiln heat the material as it swirls through the cyclones.
2. Some preheaters contain a furnace or precalciner at the bottom of the preheater tower just before the kiln. Material from the last stage cyclone enters the precalciner along with hot combustion air and fuel. As much as 95% of calcination-the removal of carbon dioxide from raw materials-takes place here.
3. From the preheater, the material now enters the kiln at the upper or feed end. It slides and tumbles down the kiln through progressively hotter zones toward the flame. Remaining carbon dioxide in the raw materials is driven off, and the intense heat triggers other chemical reactions.
4. At the lower end of the kiln, powdered coal, natural gas, oil or waste-derived fuels feed a white-hot flame that reaches 3400oF (187oC) -- one-third of the temperature of the Sun's surface. Here in the hottest zone, the materials reach nearly 2700oF (1480oC) and become partially molten. They emerge from the lower end of the kiln as a new substance: red-hot particles called clinker.
5. Pollution control devices, such as electrostatic precipitators or fabric filters called baghouses, remove particulates from exit gases before they enter the atmosphere. This strict control of emissions enables cement plants to meet high air-quality standards. Many plants return all or a portion of the collected particles, called cement kiln dust, to the kiln as part of the raw feed. Cement kiln dust not returned to the kiln is responsibly managed or sold for uses such as liming or stabilizing agents.
The Final Grind
As the clinker leaves the kiln, it tumbles onto a reciprocating grate through which fans force cool air. The heat recovered as the clinker cools is returned to the kiln or preheater to save energy. Once cooled, the clinker is ready to be ground into the familiar gray powder we know as cement. During final grinding, a small amount of gypsum is added to the clinker to control the setting time of the cement.
In ball mills, the clinker is ground to a super-fine powder composed of micron-sized particles as small as 1/25,000 of an inch. It can now be considered portland cement. The cement is so fine it will easily pass through a sieve that is fine enough to hold water.
Bagged or Bulk
From the grinding mills, the cement is conveyed to silos where it awaits shipment. Most cement is shipped in bulk by transport trucks, railroad cars, or barges. They're gravity-loaded at the storage silos by overhead equipment that can fill a large tanker in only a few minutes. A small percentage of the finished product is bagged for customers who need only small amounts of cement.
Although they may use different raw materials, fuels, equipment, and methods, cement plants produce portland cement of consistently high quality no matter where and how it is made. This is because cement is manufactured to rigid specifications that are carefully adhered to by all manufacturers. Cement produced in the United States conforms to the American Society for Testing and Materials Specification for Portland Cement (ASTM C 150).
From the plant, most cement is shipped to ready mixed concrete producers. It's combined with water, sand, and gravel to make the concrete delivered to construction sites in the familiar trucks with revolving drums. Cement is also used for an array of precast concrete products: everything from pipe and concrete block to parking lot bumpers, and median barriers. It's also used in mortar for brick and block.
A polyvalent material
Construction demands different types of cements for specific conditions and purposes. Most portland cement made today is a general-purpose cement known as Type I. Other specialty cements are produced from the same basic raw materials, but vary in chemical composition and physical performance.
One of the great advances in concrete technology was the development of air-entrained concrete in the 1930's. Scientists discovered that incorporating tiny bubbles of air in the concrete mix dramatically improves hardened concrete's resistance to deterioration from freezing and thawing. As moisture in the concrete freezes and expands, these bubbles act as expansion chambers, relieving stresses that could cause cracking.
The Case for Waste
Cement-making is ideal for recycling wastes by recovering their energy value. Many wastes, such as used motor oils, solvents, chemical by products -- even scrap tires -- have high energy content. As supplemental fuels, they replace significant quantities of traditional fuels such as coal and natural gas. Besides conserving scarce fossil fuels, burning waste in cement kilns safely rids society of undesirable and hazardous materials.
With its 3400oF degree flame, the rotary cement kiln provides the high temperatures and long burning time needed to completely destroy hazardous wastes. The U.S. Environmental Protection Agency requires incinerators and other waste processors to achieve 99.99% destruction efficiency. Cement kilns easily reach this destruction level and are often 100 times more efficient. And rather than simply destroying wastes, the energy is recovered to make portland cement.
Waste fuel burning does not affect the quality of the cement. In fact, some waste products can be processed as raw materials if they contain essential elements for cement. The rubber in scrap tires, for example, is completely consumed as fuel, and the steel belts provide iron, an essential ingredient of cement.
Cement plants must meet strict emission limits regardless of the fuel used. Using waste as supplemental fuel does not materially change air emissions. The tight controls over cement production processes assure that wastes are effectively managed and destroyed.
And the final product -- cement -- does not contain any toxic organic compounds from waste fuel. Cement plants are thus contributing to a cleaner environment by safely destroying unwanted wastes while using the energy created to make a useful and essential product -- portland cement.
But cement plants can't just burn any waste -- the materials must have fuel value and be compatible with the cement-making process. Any wastes not meeting these standards are rejected. Cement plants in the U.S. do not accept PCB's, dioxins, pesticides, or radioactive wastes.
Most wastes recycled in a cement-making are generated in the manufacture or use of everyday goods and services we all take for granted.
Used motor oil
Solvents and inks used to print newspapers and other publications
Solvents used to recycle paper
Paint thinners and paint residues
Sludge from the petroleum industry
Agricultural wastes such as almond shells
Organic compounds are burned as fuel, essentially destroyed. Inorganics, such as metals, become locked into cement's crystalline structure or are incorporated into cement kiln dust, a by product that is responsibly managed by waste-fuel users.
The Shape of Things to Come
A steady stream of advances continue to build on concrete's traditional strengths and expand on its already considerable versatility.
A whole range of admixtures can customize concrete to flow like water, reduce setting time from days to hours, or dramatically increase strength. The last decade has seen breakthroughs that have pushed concrete's strength to nearly 20,000 pounds per square inch (140 MPa)-about 6 times that of a sidewalk or driveway. And in the laboratory, researchers have achieved strengths of 100,000 psi (690 (MPa). For construction, this means taller and more economical buildings, longer and more durable bridges.
Other research is transforming Joseph Aspdin's "artificial stone" into wholly new products with properties that rival plastic, aluminum, and ceramics.
"Concrete is where metals were in 1960, when their strength shot up five or ten fold, and where ceramics were in 1970, when they did the same thing," says Francis Young, professor of civil engineering at the University of Illinois. "Concrete will go through an explosion of understanding in the 1990's. It's a material for the twenty-first century."
Yet for all it sophistication, modern concrete retains the stone-like properties that have made it the foundation of all we build -- durable, fire -- resistant, and immune to rot and rust. And it all begins with the fine gray powder called portland cement.
Accelerators: Admixtures that decrease the setting time by increasing the rate of hydration.
Admixture: A material other than water, aggregates, or cement that is used as an ingredient of concrete or mortar to control setting and early hardening, workability, or to provide additional cementing properties.
Aggregate: Inert solid bodies such as crushed rock, sand, gravel.
Binder: Hardened cement paste.
Bleed: To have water seep to the surface of the cement paste due to settling.
Calcination: Decomposition due to the loss of bound water and carbon dioxide.
Cement: Finely powdered mixtures of inorganic compounds which when combined with water hardens with hydration.
Cement paste: Cement plus water. When the mass has reacted with water and developed strength it is called hardened cement paste.
Clay: Type of soil consisting of very fine particles.
Clinker: The material that emerges from the cement kiln after burning. It is in the form of dark, porous nodules which are ground with a small amount of gypsum to give cement.
Compression: Forces acting inwardly on a body.
Concrete: A hard compact building material formed when a mixture of cement, sand, gravel, and water undergoes hydration.
Cure: To keep concrete moist during initial hardening.
Deformation: The process of changing the dimensions of a structure by applying a force.
Dormancy period: Time period that concrete retains it workability.
Elasticity: The ability of a material to return to its original shape after being stretched.
Forms: Holders in which concrete is placed to harden.
Gypsum: Calcium sulfate dihydrate, CaSO4.2H2O added to cement to regulate setting.
Hydration: The reaction of cement with water to form a chemical compound.
Kiln: High temperature oven.
Limestone: Mineral rock of calcium carbonate.
Mortar: Cement paste mixed with sand.
Pozzolan cement: Volcanic rock powdered and used in making hydraulic cement.
Porosity: The amount of empty space in concrete.
Portland cement: A cement consisting predominantly of calcium silicates which reacts with water to form a hard mass.
Retardants: Admixtures that increase the setting time by slowing down hydration.
Set: Transformation of cement paste or concrete from a fluid-like consistency to a stiff mass.
Slump test: Test used to determine workability.
Tension: The stress resulting from elongation.
Workability: How easily fresh concrete can be placed and consolidated in forms.
|Enviado por:||Javier Jimenez|