Why Alumina Is Used In Cement?

Characteristics of Asahi Alumina Cement – Rapid hardening Compared to high-early-strength or ordinary Portland cement, Asahi alumina cement harden rapidly. It strengthens enough even at low temperatures. When mixed with Portland cement or other additives, they cause quick setting according to their formulation.

  • Therefore, they are used for urgent construction of heavy traffic roads, railways, bridges, etc.
  • Refractoriness Asahi Alumina Cement is often used as a bonding material for refractory castables because it form ceramic bonds at high temperature and maintain its strength even after cooling.
  • With the accurate selection of aggregates, it is possible to produce refractory castables capable of resisting a maximum temperature of 1600°C.

Resistance to chemical attack Alumina cement is an inorganic material developed over a century ago in France as a cement having durability in sulfate. It resists sulfuric acid corrosion better than Portland cement, and it demonstrates high resistance to chemical corrosion.

What is the role of alumina in cement?

Cement Ingredients and Functions: – Below is the details given about all the ingredients present in Cement. Go through the functions and make notes for SSC JE CE exam.

  1. Lime: The function of lime is that it is the main constituent for manufacturing of cement which imparts cementing property to cement. Lime imparts strength and soundness to the cement. If it is present in excess quantity, it makes the cement unsound (causes it to expand and disintegrate). Deficiency of lime in cement reduces it strength and causes it to set quickly.
  2. Silica: Function of Silica in cement is that it imparts strength to the cement. It undergoes chemical reaction with calcium to form dicalcium silicate (C 2 S) and tricalcium silicate (C 3 S). Silica if Present in excess form adds strength to the cement but simultaneously reduces the strength to cement.
  3. Alumina: It imparts quick setting property to the cement. It acts as a flux and lowers the clinkering temperature. Alumina in excess quantity reduces the setting time of cement but simultaneously reduces the strength of cement.
  4. Calcium Sulphate: It is present in the form of Gypsum in the cement (CaSo 4,2H 2 O). Gypsum retards the setting of cement i.e it increases the setting time of cement.
  5. Iron Oxide: It imparts colour to the cement. It also gives hardness and strength to the cement.
  6. Magnesia: It also gives colour to the cement. Magnesia in excess makes the cement unsound.
  7. Sulphur: A very small quantity of Sulphur in the cement makes the cement sound.
  8. Alkalies : The most of the alkalies present in raw materials are carried away by the flue gases during heating and the cement contains only a small amount of alkalies. If they are in excess in cement, they cause a number of troubles such as alkali-aggregate reaction, efflorescence and staining when used in concrete, brickwork or masonry mortar.

Note: Magnesium oxide (MgO 2 ) and Alkali Oxide (K 2 O) are the harmful constituents of cement.

  • If the amount of alkali oxides exceeds 1 per cent, it leads to the failure of concrete made from that cement.
  • If the content of magnesium oxide exceeds 5 per cent, it causes cracks after mortar or concrete hardens.

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What are the advantages of high alumina cement?

Benefits of Refractory High Alumina Cement – High alumina refractory cement concrete has many advantages due to its large-scale properties. Furthermore, its heat resistance is useful for industries dealing with high temperatures even direct fire. Moreover, it has high compressive strength and is very reactive.

Does cement have alumina?

Alumina in cements – The cement industry, like the glass and ceramic industries, have long known about the use of minor quantities of compounds that decrease the melting point of the raw materials – known as fluxes – and that increase the rate of the reactions at both the solid and liquid phases – known as mineralizers.

Alumina is one of the main fluxes/mineralizers that is found (or added) in the cement’s raw materials, along with iron oxide (Fe2O3), alkalis (Na2O and K2O), and magnesia (MgO). Alumina being the most effective flux, while iron oxide performing better as a mineralizer. Together however, they increase the productivity of the cement plant, reduce the energy consumption, and increase the range of allowable raw materials for production.

For Portland cement (PC), the raw materials are mainly calcareous and argillaceous rock, with the former bringing the required alumina into the mix. Some cement plants further supplement the argillaceous clays and shale with small amounts of bauxite to increase the alumina content.

  1. The alumina in the PC can be found in the range of 3 – 8% and is bound in the Tricalcium aluminate (C3A) and Tetracalcium aluminoferrite (C4AF) phases which in turn typically make up to 20 % of the mass of the cement.
  2. The C3A phase requires gypsum (also added to the cement) to contribute to the early strength and setting of the concrete.

The C3A reacts with the sulfate ions provided by the dissolution of the gypsum to form Ettringite, known chemically as 6-calcium aluminate trisulfate-32 hydrate, and AFt in cement chemist notation. The AFt however is only stable for as long as there are sufficient sulfate ions in solution. On depletion of the gypsum and hence the supply of sulfate ions, any remaining C3A reacts with the AFt to form monosulfoaluminate, or AFm. If gypsum is not added to the cement the C3A would then react directly with the water to form a different product very quickly (termed as flash setting) and with a great evolution of heat. However this aluminate hydrate has little strength and would be of no benefit to the concrete.

  • The C4AF follows similar reactions to the C3A, although much slower, with less heat evolution, and is generally less reactive with the sulfate ions than the C3A.
  • This is important because the C3A could cause problems after the cement has fully hydrated and sulfate ions are re-introduced.
  • The new supply of sulfate ions would bring the AFm phase back to AFt, expanding in the process and cracking the hydrated cement.
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This ‘sulfate attack’ could be controlled by reducing the ratio of the alumina to ferric oxide – known as the Alumina Modulus – to in turn reduce the C3A phase in the cement in favor of an increased C4AF phase. The Alumina Modulus typically ranges between 0.64 and 2.5, with an optimum of 1.38 reported to yield the most melt at the lowest temperature.

Besides PC, other cements such as high alumina cements (also known as calcium aluminate cements) and calcium sulfoaluminate cements have considerable amounts of alumina. In fact these cements depend mainly on calcium aluminate phases as final product instead of the calcium silicate phases in PC. The major phases in the calcium aluminate and calcium sulfoaluminate cement clinker are monocalcium aluminate (CA) and ye’elimite (C4A3Ŝ), respectively.

Other common mineral phases are dicalcium silicate (C2S), monocalcium dialuminate (CA2), anhydrate (CaSO4), and mayenite (C12A7). The higher alumina percentage in these cements make them more suitable for usage in refractory cases and other special cases where high early strength or chemical resistance is required.

  1. The alumina forming the aluminate phases of the cement also provides an added advantage of reacting with and binding chlorides in the form of chloro aluminate hydrates, also known as Friedel’s salt, which could be thought of as a form of AFm with the sulfates replaced by chlorides.
  2. High alumina cements are hence frequently used in areas where corrosion risk due to chlorides is high such as tidal areas or where a greater amount of chlorides is present in the mixing water.

The alumina not only binds chlorides initially but also maintains their immobility due to an increased buffering capacity against acidity, especially at a lower pH (10.5 – 11.5). A high level of alumina in solution during the mixing of the concrete may also react with ferrous ions on the steel surface creating a sacrificial coating as an extra measure of corrosion protection.

Why bauxite is used in cement?

Firstly, because the iron and aluminium components of the bauxite residue are valuable additions in the production of both Portland Cement and ‘special’ cement clinkers ; and secondly it has been shown at a laboratory scale by several research groups that bauxite residue may successfully replace clinker in blended

Why is alumina a good catalyst?

Alumina is widely used as basic material of catalytic support because of its high chemical inertness, strength and hardness. ƴ- Alumina possesses excellent surface area owing to the small particle size, which results in high activity of the surface for a catalyst support.

What is the percentage of alumina in cement?

The cement contains 35 to 40 percent lime, 40 to 50 percent alumina, up to 15 percent iron oxides, and preferably not more than about 6 percent silica.

Why is alumina used in brick?

Alumina – Alumina is the main constituent of clay. It acts as a cementing material in raw brick. Brick clay is plastic due to the presence of alumina. This plasticity ensures that bricks can be molded. An excess amount of alumina in clay may cause the bricks to shrink, warp or crack on drying and burning as any other cementing material. Figure: Clay for Brick formation

Why iron oxide is used in cement?

Functions of Cement Ingredients – The main features of these cement ingredients along with their functions and usefulness or harmfulness are given below:

  1. Lime : Lime is calcium oxide or calcium hydroxide.
    • The presence of lime in a sufficient quantity is required to form silicates and aluminates of calcium.
    • Deficiency in lime reduces the strength of the property to the cement,
    • Deficiency in lime causes the cement to set quickly.
    • Excess lime makes cement unsound.
    • The excessive presence of lime causes the cement to expand and disintegrate.
  2. Silica : Silicon dioxide is known as 2 “>silica, chemical formula SiO 2,
    • A sufficient quantity of silica should be present in cement to dicalcium and tricalcium silicate.
    • Silica imparts strength to cement.
    • Silica usually presents to the extent of about 30 percent cement,
  3. Alumina : Alumina is Aluminium oxide, The chemical formula is Al 2 O 3,
    • Alumina imparts quick setting property to the cement,
    • Clinkering temperature is lowered by the presence of the requisite quantity of alumina.
    • Excess alumina weakens the cement.
  4. Magnesia : Magnesium Oxide. The chemical formula is MgO.
    • Magnesia should not be present more than 2% in cement.
    • Excess magnesia will reduce the strength of the cement.
  5. Iron oxide : Chemical formula is Fe 2 O 3,
    • Iron oxide imparts color to cement.
    • It acts as a flux.
    • At a very high temperature, it imparts into the chemical reaction with calcium and aluminum to form tricalcium alumino-ferrite.
    • Tricalcium alumino-ferrite imparts hardness and strength to cement.
  6. Calcium Sulfate : Chemical formula is CaSO 4
    • This is present in cement in the form of gypsum(CaSO 4,2H 2 O)
    • It slows down or retards the setting action of cement.
  7. Sulfur Trioxide : Chemical formula is SO 3
    • It should not be present for more than 2%.
    • Excess Sulfur Trioxide causes the cement to unsound.
  8. Alkaline :
    • It should not be present more than 1%.
    • Excess Alkaline matter causes efflorescence.

What are the properties of alumina?

Alumina is the most well-known fine ceramic material for chemical and physical stability. –

Thermal properties: High heat resistance and high thermal conductivity. Mechanical properties: High strength and high hardness. Other properties: High electrical insulation, high corrosion resistance and biocompatibility.

Is alumina an insulator or conductor?

Characteristics of Alumina and Aluminium Oxide – Alumina is the more common name of Aluminium Oxide (Al 2 O 3 ) and is a hard wearing material used for many applications. Once fired and sintered, it can only be machined using diamond-grinding methods.

What is alumina made of?

Alumina is made from bauxite, a naturally occurring ore containing variable amounts of hydrous (water-containing) aluminum oxides.

Why is it called bauxite?

12.2 ALUMINA FROM BAUXITE: THE BAYER PROCESS – Bauxite, the principal ore used for aluminum smelting, is named after Les Baux, Provence, the village where the first deposits were discovered. Bauxite contains hydrated alumina equivalent to as much as 40–60% Al 2 O 3, and is free of the other siliceous materials leached out over time.

  • However it still contains 10–30% iron oxide, and some silica and other impurities, making it unsuitable for direct electrolysis.
  • The first commercial-scale recovery of alumina from bauxite was devised by Henri Deville, but by 1900 this was largely replaced by the more economical process based on caustic extraction developed by Bayer in Austria.
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Alumina recovery from bauxite by extraction with sodium hydroxide, now referred to as the Bayer process, relies on the amphoterism of aluminum for its success ( Fig.12.1 ). Details of the alumina extraction procedure required depend on the form of hydrated alumina, which occurs in the bauxite being processed. Figure 12.1, Production of alumina from bauxite via the Bayer process. Initially the coarse ore is mechanically reduced to a finely divided form and stirred with the requisite concentration of aqueous base prior to pressure leaching. The alumina from trihydrate bauxite (Al 2 O 3 ·3H 2 O; natural bayerite or gibbsite, Al(OH) 3 ) is relatively easy to dissolve using 15–20% aqueous sodium hydroxide, under pressure, at temperatures of 120–140°C ( Eq.12.2 ).12.2 Al ( OH ) 3 + NaOH → AlO ⋅ ONa soluble + 2 H 2 O If the content of monohydrate alumina (Al 2 O 2 · H 2 O; e.g., diaspore, AlO(OH)) in the ore is significant, more concentrated sodium hydroxide (20–30%) and temperatures of 200–250°C and up to 35 atm are required to effectively leach out the alumina content ( Eq.12.3 ).12.3 AlO ⋅ OH + NaOH → AlO ⋅ ONa + H 2 O The more severe conditions will also extract trihydrate alumina from bauxite so the deciding factor for the digestion conditions required is the presence of a significant concentration of monohydrate alumina content. Fortunately, trihydrate alumina is the dominant aluminum species present in the major world deposits of Africa, Australia, the Caribbean, Central and South America, and the U.S.A. European bauxites are mainly the monohydrate. Iron oxide, clays, and most other impurities do not dissolve under alumina digestion conditions and are quite finely divided (1–10 μm). By adding wash water to decrease the viscosity, it is possible to decant the still hot sodium aluminate solution from the slowly precipitating red muds. These muds that are red from the high iron content are then washed with water to minimize losses of alumina and sodium hydroxide on disposal of the muds. By using this wash water for dilution of the next digester output, alumina and base are not lost from the Bayer circuit. Newer facilities use pressure filtration for both removal and washing of red muds. Aluminum hydroxide crystals are obtained from the supernatant sodium aluminate solution after a final filtration by ensuring a Na 2 O:Al 2 O 3 ratio of 1.5–1.8:1, by controlling the solution temperature to about 60°C, and by seeding with crystals from an earlier crystallization. Even with seeding the crystallization process requires 2–3 days. However, sufficiently large crystals are obtained by the reverse of the solution process ( Eq.12.4 ) to enable product recovery by filtration.12.4 AlO ⋅ ONa + 2 H 2 O → Al ( OH ) 3 + NaOH The aluminum hydroxide product is washed with water and then dried while still on the filters. Evaporation of most of the water from the filtrates allows return of the sodium hydroxide solution to be recycled to the initial grinding and pressure leaching circuits for reuse at appropriate concentrations. Calcination of the aluminum hydroxide at about 1200°C in either fluidized bed or rotary calciners finally yields alumina of 99.5% purity ( Eq.12.5 ).12.5 2 Al ( OH ) 3 → Al 2 O 3 + 3 H 2 O The chief contaminant is 0.3–0.5% sodium oxide, which fortunately does not affect electrolysis, with < 0.05% calcium oxide, < 0.025% of silica or iron oxide, and < 0.02% of any other metallic oxide, Apart from metal production, some of this high temperature alumina is used for the manufacture of synthetic abrasives and refractory materials. Activated alumina destined for adsorptive uses is produced in the same way, except that more moderate calcining temperatures of about 500°C are employed, which produces a highly porous product with excellent surface activity. The volume of alumina from the world's major producers is listed in Table 12.3, Australia has been the largest producer for many years ( Table 12.3 ). Table 12.3, Major World Producers of Alumina (Production in Thousands of Metric Tonnes)

1972 1980 1985 1990 2000
Australia 3,068 7,246 8,792 11,200 15,037
Canada 1,149 1,202 1,020 1,090 1,023
France 1,274 1,173 734 606 200
Jamaica 2,087 2,456 1,513 2,870 3,600
Japan 1,644 1,936 978 481 369
Surinam 1,378 1,316 1,000 b 1,530
U.S.A. 6,114 6,810 3,456 5,230 4,790
U.S.S.R. 2,300 2,700 3,500 5,900 2,850 c
W. Germany 916 1,608 1,657 922 700 d
Yugoslavia 135 1,635 1,138 1,090 370
Others 3,535 3,709 7,820 11,681 20,361
World total 23,600 33,426 31,617 42,600 49,300

a Includes all those countries producing more than a million tonnes of alumina annually by 1980. Compiled from data in Minerals Yearbooks World Mineral Statistics, b Estimate. c Russian Federation. d Germany. Gallium is a trace contaminant of chemical and commercial interest, which is present to the extent of 60–200 μg/g in Bayer process liquors.

Situated in the same group as aluminum in the periodic table, gallium’s very similar chemical properties cause it to be carried through the alumina purification and electrolytic steps along with the aluminum. With a melting point of 30°C and boiling point of 2403°C it is the element with the longest liquid interval.

Production is on the scale of 50,000 kg/year, mostly in Australia, Germany, and Russia, It is used in electrooptical devices, such as LEDs, in the GaAs used for the microchannel plates that guide and amplify photons in night vision goggles and sights, and for the integrated circuits in one of the fastest digital signal processors made.

  • Improvement of recovery methods has been examined,
  • It is also possible to obtain alumina from clays or bauxite via leaching with sulfuric acid, a technique, which is especially useful with high silica or high iron oxide bauxites ( Eq.12.6 ).12.6 calys + H 2 SO 4 → Al 2 ( SO 4 ) 3 soluble + water insoluble residue Crystals of pure aluminum sulfate hydrate (Al 2 (SO 4 ) 3 · 18H 2 O) are obtained from the leach solution, which may then be calcined to yield alumina ( Eq.12.7 ).12.7 Al 2 ( SO 4 ) 3 ⋅ 8 H 2 O → Al 2 O 3 + 3 SO 3 + 18 H 2 O Cryolite (AlF 3 3NaF) is the electrolyte used, which is the major component of the electrolytic bath.

This is a rare natural mineral originally found only in Gothaab, Greenland. Most present aluminum smelters use synthetic cryolite prepared from alumina and hydrogen fluoride in the presence of sodium hydroxide ( Eqs.12.8 and 12.9 ).12.8 CaF 2 fluorspar + H 2 SO 4 → 2 HF + CaSO 4 12.9 6 HF + Al ( OH ) 3 as briquets + 3 NaOH → Na 3 AlF 6 + 6 H 2 O This produces a cell electrolyte with characteristics identical to the natural mineral.

Why is Naoh added to bauxite?

Sodium hydroxide acts as a dissolving agent and dissolves Bauxite to form Sodium meta-aluminate in the Baeyer’s Process.

Why bauxite is red?

Mining and Refining – Bauxite Residue Management It is primarily composed of the insoluble fraction of the bauxite ore that remains after extraction of the aluminium-containing components. Iron oxides (10 – 30%), titanium dioxide (2 – 15%), silicon oxide (5 – 20%) and undissolved alumina (0 – 20%) make up the residue, together with a wide range of other oxides which will vary according to the initial bauxite source.

  1. The high concentration of iron compounds in the bauxite gives the by-product its characteristic red colour, and hence its common name “Red Mud”.
  2. Initially, the residue is washed, to extract as much valuable caustic soda and dissolved alumina as possible.
  3. The caustic soda is recycled back into the digestion process, reducing production costs and in turn lowering the alkalinity of the residue.

The pH level of the residue is generally up to 13 or higher in some cases, due to the presence of alkaline sodium compounds, such as sodium carbonate and sodium hydroxide. Like most ores and soils, bauxite can contain trace quantities of metals such as arsenic, beryllium, cadmium, chromium, lead, manganese, mercury, nickel and naturally-occurring radioactive materials, such as thorium and uranium.

Why is it added to the alumina?

Cryolite is added to alumina as to lower the fusion temperature and make the mass good conductor of electricity.

What is the function of activated alumina?

Catalysts – Activated alumina is also widely used as a catalyst, with roles as the catalyst itself, as well as an inert carrier, or substrate for other catalysts. As a catalyst, activated alumina is most well known for its role as a Claus catalyst; activated alumina is the most commonly used Claus catalyst in sulfur recovery endeavors at oil and gas refineries.

What is the disadvantage of alumina? Mechanical Properties – Alumina shares several characteristics with other polycrystalline ceramic materials, such as moderate tensile and bending resistance and the brittle fracture behavior, which is the main disadvantage of the mechanical properties of alumina.

  1. Alumina is a ionic-covalent solid that does not yield under load as metals and alloys do.
  2. The strong chemical bonds in alumina are the roots of several of its characteristics such as the low electric and thermal conductivity, the high melting point that makes it practically impossible to shape alumina by casting, and the high hardness that characterizes this material and makes its machining complex and costly.

The brittleness of alumina is the main concern of engineers while designing alumina components. In metals, crack energy is dissipated by yielding at the crack tip, while alumina components may fail without any previous plastic deformation at the location of high tensile stresses, such as surface defects, notches, internal flaws, or on the occurrence of thermal shocks.

Moreover, as polycrystalline ceramics contain a number of flaws characterized by a large scatter in size and by a random location within the solid body, the relationships between stress distribution, failure probability, and strength in ceramics need to be discussed on statistical basis.32 As a biomaterial, alumina ceramic has undergone great improvements in its mechanical properties during the 40 years of clinical use as described in Section 1.105.1,

The remarkable increase in bending strength (from less than 400 MPa to more than 630 MPa in pure alumina components) is due to the improvements in the selection of raw materials used as precursors and in the sintering process that resulted in marked reduction in grain size and in the increase in density, which is close to the theoretical one.29 However, it was the introduction of the alumina composite in clinics that made it possible to overcome the limits of alumina in terms of mechanical properties, for example, the toughness and bending strength of BIOLOX®delta are more than twice that of the former BIOLOX®forte ( Table 4 ).

Property Units Alumina (1970s) BIOLOX® (since 1974) BIOLOX®forte (since 1995) BIOLOX®delta ATZ ISO 6474:80 ISO 6474:94
Al 2 O 3 content Vol % 99.1–99.6 99.7 >99.8 80 20 ≥99.5 ≥ 99.5
Density g cm −3 3.90–3.95 3.95 3.97 4.37 n.s. ≥3.90 ≥ 3.94
Av. grain size μm ≤4.5 4 1.75 0.54 <0.5 ≤7 ≤ 4.5
Flexural strength MPa >300 400 630 1390 1090 >380 > 400
Young modulus GPa 380 410 407 n.s. n.s. 380
Hardness HV 1800 1900 2000 1760 n.s.

Data from Kuntz et al,29 and Begand.27 Read full chapter URL: https://www.sciencedirect.com/science/article/pii/B9780080552941000167

Is alumina acidic or basic?

Alumina or aluminium oxide is an amphoteric substance that can react as both acid and base.

What are two uses of alumina?

Broad industrial applications – Alumina can be used in a wide variety of industrial applications, from piping components such as elbows, tees, and straight pipes, to hydro cyclones, reducers, nozzles, and valves. It is also a very useful material for machining tools, cutting tools, thermocouple sheaths, and wear-resistant pump impellers. Why Alumina Is Used In Cement

Is alumina a catalyst?

Abstract – Aluminas are widely used as catalytic supports in chemical reactions. Reforming reactions to obtain synthesis gas requires good mechanical strength and low sintering behaviour. In this work, the influence of bentonite, aluminium phosphate and alumina gel as binder agents of a calcined α -Al 2 O 3 are analyzed with respect to support and catalytic properties.

The α -Al 2 O 3 supports, calcined at 1300 °C, are then impregnated with solutions of Ni and Al inorganic salts to obtain the catalysts and are finally tested in the reforming reaction of methane to synthesis gas at 500–900 °C. Supports and catalysts are characterized by XRD, SEM, N 2 adsorption, mechanical strength test and other techniques.

Mechanical strength depends on the type and quantity of binder material used during support preparation. The influence of the support on the performance of the resulting catalyst is evidenced by means of catalytic tests.

What is the main function of alumina in brick?

Alumina – Alumina is the main constituent of clay. It acts as a cementing material in raw brick. Brick clay is plastic due to the presence of alumina. This plasticity ensures that bricks can be molded. An excess amount of alumina in clay may cause the bricks to shrink, warp or crack on drying and burning as any other cementing material. Figure: Clay for Brick formation

What is the function of activated alumina?

Catalysts – Activated alumina is also widely used as a catalyst, with roles as the catalyst itself, as well as an inert carrier, or substrate for other catalysts. As a catalyst, activated alumina is most well known for its role as a Claus catalyst; activated alumina is the most commonly used Claus catalyst in sulfur recovery endeavors at oil and gas refineries.

What is alumina ceramic used for?

Bio Medical: – Since alumina ceramics are inert, they are insoluble in chemical reagents, have wear resistance, and can have a high polished finish, which makes alumina ceramics useful as biomaterial. Alumina ceramics are used for artificial joints, bone spacers, cochlear implants, and teeth implants. Tubes and scientific products are also made from alumina ceramics. Why Alumina Is Used In Cement