Afghan Energy, Chemical & Mining Industries
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Cement

 

Lime is a powder made of calcium oxide (CaO) produced by baking the carbonate out of calcite in limestone. 

Silica is a generic term for SiO2 - quartz.  Alumina is aluminum oxide Al2O3, common in clay and many silicate minerals.

 

Calcine means to heat a rock up to a very high temperature - hot enough to burn off any water and carbon, but not hot enough to actually melt the rock.  We produce lime by calcining limestone.

(Limestone / Calcium Carbonate) CaCO + heat   ®     (Lime/Calcium Oxide)  CaO  +  CO2(g)

Calcite                         Lime

Concrete is the name we use to describe the aggregate-cement mixture. 

Aggregate is the sand and gravel, added to cement to increase bulk and strength. 

Portland Cement is the most common type of cement used today due to its vastly greater strength compared to other cements.  The ingredients are essentially the same as Natural Cement (lime, silica, alumina, and iron oxides[1]), but the calcining process is done at a much higher temperature[2].  The precise proportion of ingredients is important.  In most cases, this requires mixing of several rocks together, although the limestone in select regions naturally contains the appropriate elements in the appropriate proportions.

The powdered product is primarily a mixture of tricalcium silicate (3CaO · SiO2), tricalcium aluminate (3CaO · Al2O3), and dicalcium silicate (2CaO · SiO2), in varying proportions, along with lesser amounts of magnesium and iron compounds and gypsum.  Gypsum is often added to slow the hardening process.  Dolomite (CaMg(CO3)2) and alkalis are harmful impurities.

The compounds in active cement are unstable together.  When water is added, they rearrange their structure and interact chemically to form interlocking crystals of calcium-aluminum-silicates that bind together the particles of sand or stone.

The Kiln, a giant cylindrical pipe about 15 feet or so in diameter that slowly turns.  The kiln is on a gentle incline, so that raw materials dropped in one side slowly mix and make their way down to the lower end.  Pulverized coal is blown up into the kiln from the lower end as fuel to heat the mixture and calcine the limestone. Monitoring the temperature of the mix, composition of exiting gasses, etc. constantly is essential. Using a petrographic microscope, too, which is used to analyze the shapes and proportions of the crystals of tricalcium silicate, tricalcium aluminate, and dicalcium silicate when the cement is cured is also a strong factor in determining the strength of the cement.

The the ball mills are where the clinker is grinded  into a powder.  Ball mills are big, spinning cylinders that contain cannon balls made of hard steel.  When they put clinker in the cylinders, the spinning tube causes the balls to bounce around and break up the clinker into a powder.  The powder is mixed with gypsum (which slows the hardening process and allows for better control during pouring at the construction site)

When producing cement, the chemical composition of cement is controlled by the content of silica (SiO2), lime (CaO), alumina (Al2O3) and iron (Fe2O3). These oxides become characteristic clinker minerals which during the addition of gypsum will be ground to cement. Portland cement is made up of four main clinker compounds: Tricalcium silicate (allite) (C3S), Dicalcium silicate (belite)(C2S), Tricalcium aluminate (C3A) and Tetra-calcium aluminoferrite (C4AF). Where C stands for calcium oxide, S for silica, A for alumina, and F for iron oxide. Small amounts of uncombined lime, magnesia, alkalis and minor amounts of other elements (titanium, manganese etc.) are also present. The composition of Portland cements falls within the range of 60 to 67 percent lime, 17 to 25 percent silica, 2 to 8 percent alumina, and 0 to 6 percent iron oxide together with 1 to 7 percent sulphur trioxide, derived mainly from the added gypsum, 0.1 to 5 percent magnesia, and 0.1 to 1.5 percent alkalis.
The chemical composition of cement influences the characteristics of cement. The clinker mineral C3S is the head clinker component in cement. The strength developed by Portland cement depends on its composition of C3S, C2S, C3A, C4AF and the fineness (Blaine) to which it is ground. The strength is measured in either MPa / (N/mm2) (ASTM and BS) or psi (ASTM). The compressive strength is determined in different ways. For ASTM the strength is determined in accordance with ASTM 109 and for BS and other European countries the strength is determined in accordance with EN 196-1.  Five types of Portland cement are standardized in the U.S. (Standard Specification for Portland Cement C 150 - 97): ordinary (Type I), modified (Type II), high-early-strength (Type III), low-heat (Type IV), and sulfate-resisting (Type V). In other countries Type II is omitted, and Type III is called rapid-hardening. Type V is known in some European countries as Ferrari cement. Five types of Portland cement are also standardized in the U.K. (Specification for Portland Cement BS 12 : 1996): strength class 32.5N, 32.5R, 42.5N, 42.5R, 52.5N and 62.5N. The different Standard Specifications for Portland Cement contain different requirements of chemical and physical properties: MgO, SO3, alkalis as Na2O, loss in ignition, insoluble residue, Bogue composition, fineness (Blaine), soundness, autoclave expansion, compressive strength and initial and final setting. Besides these requirements a standard specification can contain more specific requirements.

 

C3S
  • The head clinker component in cement, typical more than 50 %
  • Quick development of strength - C3S reacts more quickly than C2S
  • High contribution to the final strength
  • Resistant to sulphur attack
  • 25 weight % water bind under hydration of C3S
  • Heat development: 500 kJ/kg
  • Hydration of C3S (Ca3SiO5 + (y+z)H2O = zCA(OH)2 + Ca(3-z)SiO(5-z)yH2O) are to some extent dependent on the presence of C3A and gypsum. Both C3A and gypsum stimulate the hydration of C3S. Also Alkalis have some influence at the hydration.  
  • C3S = 4.071*CaO-(7.600*SiO2+6.718*Al2O3+1.430*Fe2O3+2.852*SO3) according to Bogue's methods

 

C2S
  • Second clinker component in cement, between 10 - 60 %
  • Slow development of strength - C2S reacts more slow than C3S
  • High contribution to the final strength
  • Resistant to sulphur attack
  • 20 weight % water bind under hydration of C2S
  • Heat development: 250 kJ/kg
  • On hydration,  C2S shows similar behaviour to C3S, but is slower to react. It does however continue to hydrate late in the setting period, and may then contribute to the strength of the cement.  
  • C2S = 2.87*SiO2-0.754*(Ca3SiO5) according to Bogue's methods

 

C3A
  • Range in the cement between 3  - 10 %
  • High contribution to the early strength
  • Low contribution to the final strength
  • Not resistant to sulphur attack
  • 40 - 210 weight % water bind under hydration of C3A
  • Fast and high heat development: 900 kJ/kg
  • Compared with C3S, C3A reacts very rapidly with water, giving two hydrated products: 2C3A + 21H = C4AH13 + C2AH8 These forms platelets within the cement, and convert to C3AH6, which forms very quickly, and is responsible for the initial formation of a crystalline network. In the presence of free lime in the cement, the formation of C4AH13 is favoured. This slows the formation of C3AH6, but even so the formation of C4AH13 can causing the cement to set too quickly. To avoid speed setting is gypsum added the cement and the mineral ettringite is formed on hydration:
    C3A + 3CaSO42H2O + 25-26H2O = Ca6Al2O6(SO4)31-32H2O.  
  • C3A = 2.65*Al2O3-1.69*Fe2O3 according to Bogue's methods

 

C4AF
  • Range in the cement between 5 - 10 %
  • Small contribution to the development of strength
  • 37 - 70 weight % water bind under hydration of C4AF
  • Moderate to low heat development: 300 kJ/kg
  • Hydration of C4AF + 13H =  C4AFH13
  • C4AF = 3.04*Fe2O3 according to Bogue's methods

 

Alkalis
(Na2O + K2O)
  • Cements with a low alkali content may be required for use in the manufacture of concrete in which the use of aggregate introduces silica. Alkalis may enhance reactions with amorphous silica. The content of alkalis contributed to the acceleration of the early strength and lowering of the final strength. The content of alkalis is dependent on the raw materials but also the manufacturing process decide the content of alkalis. Cement manufactured by the wet processing will compared to the dry processing contain less alkalis.  
  • Alkalis as Na2O = Na2O + 0.658 * K2O

 

MgO
  • Cements with a high magnesia content may after setting hydrates and expand.

 

Free CaO
  • Cements with a high free lime content may react with water and result in expansion.

 

 

Geology

The working process of mapping and surveying raw materials Preparing for the future is very important in the cement manufacturing business. It is important when founding new cement plants, that the surrounding areas contain enough raw materials to maintain a cement production for several years. It is also important to make sure that varying qualities of raw materials are used consistently and continuously. 

The assessment of quality and quantity of geological materials is a complex process which involves choice of method and strategy. In the beginning of the working process of mapping and surveying raw materials, it is important to define clearly the task and the kind of information the mineral reserve evaluation should obtain. For most areas geological mapping can be found to various extents. This information can be used in the preparation of the preliminary geological model.

3D

Now method and strategy may be chosen on basis of task and geological model definitions. The choice of  method depends on what kind of information the mineral reserve evaluation will obtain. Geophysical methods are used to assess the quantity of minerals. Today there are several geophysical methods available. The most common and most frequently used is Direct Current Geoelectrical methods, but also Alternating Current Geoelectrical methods, Georadar, AMT, VLF, Seismic and Drillholes Logging can be used. Although these methods are most useful for the assessment of quantity, they may also be used as a tool for an overall geological interpretation of the area.

In order to assess the availability and quality of raw materials for cement manufacturing, the samples must be obtained as core samples from drillholes and analyzed using XRF. The results from the XRD analysis are usefull tools in mapmaking. By using the results from the XRF analysis (drillholes) related to the position (X-coordinate and Y-coordinate) of the basemap of the area, valuable maps can be made to optimize the coefficient of utilization of raw materials and maintain a uniform quality of raw materials intended for the production. The same procedure is of utmost importance when founding new cement plants.

The first task is to collect the input data. The data falls into four categories:

  • Drilling data (which includes the surface elevation of the drillhole, depth at which the horizon of the materials was encountered, thickness of the formation, a lithology description, the X-Y coordinates and core samples).
  • Laboratory analysis data (qualities of samples from drillholes, based on XRF analysis).
  • Surface topography data (which includes thickness of overburden).
  • Property and planning maps (which include mining property boundary, barriers to operations such as highways, railways and property ownership).

The process of manufacturing contour maps based on X-Y-Z data set involves:

  • Data checking and validation
  • Selection of gridding method

Contour

Sr.# Name of Machine Description Manufacturer Mfg.
Year
1 Kiln (Vertical shaft) #2 50 TPD -
2 Reciprocating Feeder - -
3 Hammer Mill - -
4 Roller Mill #2 - -
5 Ball Mill - -
6 Jaw Crusher - -
7 Conveyor (Screw) - -
8 Conveyor (Belt) #5 - -
9 Conveyor (Bucket) - -
10 Rotary Feeder #4 - -
11 Raw Mill #2 - -
12 Cement Mill - -
13 Nodulizer #2 - - -
14 Table Feeder #2 - - -
15 Crusher - - -
16 Blower - - -
17 Bucket Elevators #7 - - -
18 Silo (Blending) - - -
19 Silo (Storage) - - -
20 Packing Plant - - -
21 Feed hopper - - -
22 Transformer 1500 KVA - -
23 D G Set 15 KVA - -
24 Motors #lot - - -
25 Quarry Equipments #lot - - -
26 Workshop Equipments #lot - - -
27 Laboratory Equipments #lot - - -
28 Misc. Electrical Tools #lot -

 

Cement Kiln Flue Gas recovery Scubber http://www.lanl.gov/projects/cctc/factsheets/pass/cemkilndemo.html

Perhaps the most important thing to understand about concrete is the role of water. First, it provides plasticity so the concrete can be poured in a form. Its real importance, however, is in the hardening process. Wet concrete doesn’t harden by drying. Instead, the water is a chemical component in a curing process. The compounds that react with the water are in the portland cement. Isle Of Portland While cement in one form or another has been around for centuries, the type we use was invented in 1824 in Britain. It was named portland cement because it looked like the stone quarried on the Isle of Portland. Portland cement is produced by mixing ground limestone, clay or shale, sand and iron ore. This mixture is heated in a rotary kiln to temperatures as high as 1600 degrees Celsius. The heating process causes the materials to break down and recombine into new compounds that can react with water in a crystallization process called hydration. Concrete cures in several stages—a factor that allows it to be trucked to the job site and still be ready to pour. With the concrete in the form, the cement begins a slow cure and the mix hardens. After about 36 hours, most of the hydration process is complete, but the cement will continue to cure as long as water and unhydrated compounds are present. While the process can take years, strength tests are typically done after 28 days. It’s important to use the right amount of water. Too much makes for weaker concrete. However, too little makes the mix hard to pour. The best mix is a compromise between strength and workability.


  
Concrete is a combination of hydrated portland cement and aggregate. Common aggregates are sand and gravel. Air-entrained concrete has tiny bubbles to help prevent cracking. Concrete starts with the production of portland cement in the rotary kiln and grinder. From there, it’s packaged in bags for on-site use, mixed for truck delivery or used to produce concrete products such as blocks and pavers.

From Cement To Concrete While cement and water are the active components, it’s not economical to use them alone. Instead, aggregates are added to increase the volume and tailor the concrete to its final use. Typically, 60 to 80 percent of the concrete is aggregate. In most cases, the aggregates are sand and gravel. When sand is used alone, the result is mortar. When both are present, the result is concrete. However other aggregates might be used depending on the required characteristics of the cured mix. For example, vermiculite or perlite aggregates produce a lightweight concrete that has good insulating properties and can be easily sawn.

Improving Performance Concrete suppliers often use additives, called admixtures, to alter or improve the qualities of the mix for a specific application. When it’s important to have a workable concrete that pours easily without adding extra water, a mineral additive such as fly ash is added. Alternatively, superplasticizers are used to improve workability while increasing strength because less water is required. Retarding and accelerating admixtures are used to alter curing time as necessitated by climatic conditions.

One problem with concrete is a tendency for freeze/thaw cycles to cause cracks. To help remedy this, air-entraining agents are added. These admixtures create a dispersion of very fine air bubbles that cushion the concrete against the effects of freezing water.

Buying Concrete The form in which you buy concrete depends on the size and nature of your job. Concrete is normally measured in cubic yards. To determine how much you’ll need, figure the volume inside your forms in cubic feet and divide by 27 (the number of cubic feet in a cubic yard). For example, a 4-in.-thick slab that covers 90 sq. ft. takes up 30 cu. ft., or just over 1 cubic yard. Projects using up to about a cubic yard can be handled with a portable cement mixer that you can rent. The proportions of cement, sand, gravel and water can vary depending on the use of the concrete. For example, thin work—between 2 in. and 4 in. thick—will require more cement, whereas a higher-mass pour can afford to use more aggregate. An average 1:2:3 mix contains one part cement, two parts sand and three parts gravel. To make 1 cubic yard of concrete, you’d need seven 94-pound bags of cement, about 1/2 cubic yard of sand and just over 3/4 cubic yard of gravel. The amount of water you use depends on how wet the sand is. If it’s already moist, you’ll need about 4-1/2 gal. per bag of cement.

For smaller projects, you can buy premixed bags that contain cement and aggregate—you just add water. For bigger jobs, the best route to take is ready-mix concrete. In addition to the obvious advantage of having the concrete delivered, your supplier can also tailor the mix and admixtures for your job. Ready-mix prices vary based on the distance of the delivery, the type of mix and the size of the order, so it’s best to call a local dealer for a price. If your site is inaccessible to the truck, you may be able to have the concrete pumped through a hose. Or, you can simply carry the concrete from the truck with wheelbarrows.

Finally, you may be able to avoid the pouring entirely by using finished concrete products. Concrete block is available in a variety of sizes, structural qualities and surface styles for building walls that might otherwise be poured. Traditional block walls are built with mortar, but blocks designed to be laid up dry are also available. In addition, concrete pavers, bricks and small slabs are available for landscaping and walkway projects.

 

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