MINERALOGY OF FLY ASH & ITS INCREASED USAGE · MINERALOGY OF FLY ASH & ITS INCREASED USAGE ....

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Transcript of MINERALOGY OF FLY ASH & ITS INCREASED USAGE · MINERALOGY OF FLY ASH & ITS INCREASED USAGE ....

Page 1: MINERALOGY OF FLY ASH & ITS INCREASED USAGE · MINERALOGY OF FLY ASH & ITS INCREASED USAGE . ABSTRACT: The Chemical and Mineralogical haracteristics of fly ash produced in the coal
Page 2: MINERALOGY OF FLY ASH & ITS INCREASED USAGE · MINERALOGY OF FLY ASH & ITS INCREASED USAGE . ABSTRACT: The Chemical and Mineralogical haracteristics of fly ash produced in the coal

MINERALOGY OF FLY ASH & ITS INCREASED USAGE

ABSTRACT:

The Chemical and Mineralogical Characteristics of fly ash produced in the coal fired thermal plant is a function of nature of the coal, coal comminution system, boiler type & efficiency, fly ash collection ESP fields, as well as on the loading at which the thermal plant operates etc, as a result from the same thermal Plant source the fly ash characteristics could vary substantially in its Mineralogical and Morphological properties .

The Mineralogical make up of fly ash is of course governed by the Chemical composition of the Mineral matter of coal as well as the mineral matter Contents . The mineralogy of fly ash determines the reactivity of fly ash in its various applications, for e.g the pozzolanic reactivity of fly ash would depends on glassy/ amorphous silicate phase of fly ash and specific surface area available for reaction with lime [1].

Studies carried out by Joshi R.C and Rosauer E.A on synthetic fly ashes [2,3] produced at laboratory scale, indicated that prolonged heat treatment promotes sintering and de-vitrification which in turn influences other properties . Iron in an otherwise pure alumino-siliceous ash reduces pozzolanic strength, whereas calcium in an aluminosiliceous fly ash results in a very reactive ash.

The work by diamond [4] concluded that fly ashes with CaO content upto 20% showed maximum indicative of siliceous glass whereas Haobo[5]studied the influence of CaO addition on granulated cinders concluded that with increase in CaO content, the vitreous network of granulated cinders becomes disordered and is destroyed, the polymerization of network formed reduces and the hydraulic activity of granulated cinder improves.

Thus fly ash Mineralogy can be altered by Compositional as well as process parameters which alter the properties of fly ash, however this aspect necessitates better understanding of the influence of different minerals and minor constituents present in coal and processing conditions on the resultant fly ash properties, so as to Tailor make a fly ash with enhanced reactivity in its applications.

Altering the Mineralogy of an available fly ash is another avenue available on hand to alter the Mineralogy of the fly ash for its desired applications and enhanced usage.

This Paper refers some of the reported work on Mineralogical alteration with the Fly ashes available and altering the Mineralogy of fly ashes during generation. The Paper also discusses some of the avenues studied at the author’s lab to alter Mineralogy of available fly ash as well as attempts on altering the fly ash Mineralogy at generation stage in a Coal Fired Power Plant.

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Introduction:

The fly ashes available from the coal fired thermal plants in country are fairly uniform in their chemico-mineralogical characteristics. Compositionally they could be classified as Low Lime Class – F fly ash, mineralogically composed of 15 - 30% mullite 15-45 % quatz,1-5% Magnetite, 1-5% Hematite and around 25 - 45% of amorphous glassy aluminao silicate phase [6] . The mineralogical composition being governed by coal source, Coal pulverising system used, boiler type & efficiency, fly ash collection system etc. The paper initially discusses briefly the quality characteristics of the low lime Class – F fly ashes available in the country and different avenues of altering fly ash mineralogy during generation as well as processing fly ash to Tailor make the Mineralogy of available fly ash for increased usage in its applications , exemplified through some of the studies carried out at authors lab .

Pozzolanic Reactivity of Indian Fly ashes: The shift in the emphasis of construction industry from high strength to high performance concrete, has resulted in increased acceptability of fly ash based blended cements in India. Irrespective of the arguments on the necessity of strength gradation for blended cements, it is the requirements of the present market scenario to have the product at comparative levels of quality to OPC, with maximized levels of the pozzolanic component in order to provide the product with added merits of lower heat of hydration, lower permeability, improved durability, through higher sulphate resistance, corrosion resistance and resistance to ASR expansion thus resulting in an improved performance of the resultant concrete . The USP’s of these fly ash blended cements can be primarily attributed to decreased availability of calcium hydroxide in hydrated cement paste due to secondary pozzolanic reactions with fly ash and the low lime class – F fly ashes available in the country are compositionally most suited in terms of durability of the resultant concrete . The pozzolanic reactivity of the Low Lime Class – F fly ashes available in the country has been observed to be more a function of glassy amorphous content and the particle characteristics; it is independent of the minor variations in the chemical compositions. On an average the amorphous content of Indian fly ash is in the range of 25 - 45%, where as comparatively compositionally similar Class – F, European fly ashes have been observed to have relatively higher amorphous phase contents (40 -70%). At the authors laboratory comparative hydration studies carried out to assess the relation between the amorphous glassy phase contents and the reactivity of low lime fly ashes[7] indicates that the percentage reaction of the fly ash of different amorphous phase contents at different ages of hydration appear to be nearly similar except for the difference in the initial rate of hydration and the type of the different types of hydrates formed during the hydration of fly ash based blended cements made with fly ashes of low (Fly Ash-In) , medium (Fly ash – U) and higher ( Fly ash –A) amorphous glassy phase contents. The Fig. 1 indicates the free calcium hydroxide content of the hydrated cement pastes (PPCs), at different ages of hydration and Fig:2 illustrates the % reaction of the fly ash component in the different PPCs at the tested ages of hydration

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Fly ash - A

Fly ash - D

Fig: 2 : % Reaction of the fly ash in different PPC

Fig.1: Free Calcium Hydroxide in hydrated Cements at different age of hydration

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The studies indicate that the particle characteristics of the fly ash plays a dominant role in influencing the properties of the resultant PPC and PPC - Concrete as this property would govern the particulate plasticising effect, as also the packing effect.

Effect of Mineralogical composition of the fly ash:

It would be immensely important to understand that the characteristics of fly ashes are assemblages of particles produced by combustion and melting of individual small particles of ground coal. Each particle is heated and undergoes changes independently of other particles, while passing through the burning zone of the power plant boiler. Its composition reflects that of the inorganic portion of the particular coal fragment, with whatever changes have occurred due to selective vaporization of components and perhaps subsequent surface deposition. In any of these events, the composition of each particle is necessarily different from its neighboring particles and overall chemical analysis is only an average description of the assemblage. Another feature of fly ash is those individual fly ash particles vary in content of crystalline component like quartz, Mullite, Iron oxide, calcium bearing compounds (in Class C fly ashes) and amorphous glassy phases.

As discussed above a considerable distinction exists between low lime class F fly ash from bituminous coal and high lime class C fly ash produced from lignitic or sub-bituminous coal. Depending on composition of the clay mineral constituents, the boiler temperatures, coal fineness used in the boiler type as well as the efficiency of the heat recuperation systems the fly ashes would show a difference in the glassy amorphous phase contents and the nature and extent of minerals present. Which would determine the pozzolanic potential of the fly ash and its resultant effect on the performance characteristics of the cements /concrete?

The Fig.3 depicts comparative pozzolanic reactivity of two compositionally similar low lime class –F fly ashes differing in Mineralogy and amorphous contents. The method used has been evolved at the author’s laboratory for comparing the reactivity of fly ashes [8]. The XRD showing the difference in mineralogy is shown in Fig.4.

Effect of amorphous phase composition of the fly ash:

It has been observed that as the composition of the amorphous phase changes that is as the alumino-silicate amorphous glassy phase becomes calcium rich or as the Si /Al ratio of the amorphous phase changes there is distinct shift observed in the peak maxima of the amorphous hump observed in XRD, i.e there is a shift in the maxima of the amorphous hump towards that of the hump maxima of the granulated blast furnace slag. This could be related to the changes in the composition of the amorphous glassy phase, however some more evaluations need to be done to confirm and quantify this observation. The Fig 5 depicts the amorphous hump maxima of different fly ashes of different CaO content; the figure also shows the nature of the amorphous hump of Slag for comparison. As already indicated that the lime rich amorphous alumino silicate has higher pozzolanic reactivity. Fig. 6 depicts the compressive strength characteristics of PPC made with different amorphous phase composition.

Fig.3: Comparative Pozzolanic reactivity of Class F Fly ashes differing in the Amorphous content

Fig.4: XRD fly ashes of differing mineralogy

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Avenues for Fly ash Utilization by altering the Mineralogy of Fly ash: Enhancing the pozzolanic Reactivity of Fly ash: Various options for enhancing the reactivity of fly ash were studied by researchers globally could be categorized as use of Alkali source followed by thermal treatment and use of lime source followed by thermal treatment, or lime and alkali combination, Poon et al [9] studied the activation of fly ash / cement system using calcium sulfate anhydrite (CaSO4). The authors reported the effectiveness of anhydrite in activating fly ash cement system. Ma and Brown [10] investigated the hydrothermal reactions of fly ash with Ca (OH)2 or CaSO4.2H2O. Fly ash was activated by calcium salts and activation influenced hydration rates as determined by the heat evolution. The principle underlying activation by gypsum is based on the ability of sulfate ions to react with alumina, the latter being one of the principle components of fly ash. Class F and Class C fly ashes were taken for the study. Shi [11] & Shi and Day [12] studied the effect of various chemical activators like CaCl2 and Na2SO4 on strength of lime-fly ash (low lime and high lime). Results indicated that a small amount of activators can increase the pozzolanic reactivity of fly ash, this results in significant improvement in early strength. Li et al [13] studied the hydrothermal activation of fly ash by Ca (OH)2. The changes accompanying the activation were studied employing TGA, XRD and SEM analysis. Results showed that the reaction of fly ash was very low in fly ash cement because Ca (OH)2 had less activation capability on fly ash at room temperature it is enhanced only if curing temperature is enhanced to ~500C. Sharma et al[14] studied the morphological and microstructural changes induced in fly ash due to thermo chemical activation by iron ore, soda ash and red mud and thermal activation and concluded that lime reactivity (LR) of untreated fly ash was increased from 48 to 92 kg/cm2 after treating fly ash and the order was observed to be (LR) untreated fly ash > soda ash treated>thermally treated > red mud treated > iron ore treated. Also activated fly ash showed increased reactivity with lower degree of polymerization due to breaking of the continuous polymeric nature of silica network by introduction of lower valency atom (Al or Fe). When CaO source is added to coal, at the combustion temperature the CaO combines with the ash forming Calcium alumino silicate amorphous phase and at higher CaO in Fly ash it would form partly Calcium aluminate and thus prevents formation of Mullite (crystalline alumino silicate) as alumina availability decreases. As the total calcium rich amorphous phase is increased the fly ash, its reactivity in PPC increases, which reflects in increased early strengths, as well as later age strengths of PPC. At ~ 10 % CaO it has been observed that even the 1 & 3-day strengths of PPC are enhanced, partly the enhancing effect is also due to formation of small amounts of Calcium rich glassy phases as well as at times traces of Calcium aluminates, The Change in the nature of amorphous phase is reflected in the XRD maxima of the amorphous hump it shifts from 23o towards 26 - 29 o. At the author’s lab also studies were carried out on fly ash converting it to a Reactive Fly ash (RFA) with use of Lime source insufflation in AFBC Captive Power plant (targeting a CaO content of 12%) at 900°C (the temperatures in the CPP boilers), the composition of fly ash used along with RFA is given in Table – 1 and the mineralogy of the RFA is shown in Fig. 6

Fig.6: Compressive Strength of PPC with Fly ash AP & CAP

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Table -1 : Composition of Fly ash and Limestone & RFA % Oxides Fly Ash Limestone RFA

SiO2 50.2 11.8 52.6 Al2O3 20.0 2.5 19.9 Fe2O3 5.0 2.6 4.7 CaO 1.5 41.6 13.0 MgO 0.90 3.3 2.1 LOI 18.6 36.9 5.0 Na2O 0.08 0.06 0.11 K2O 1.17 0.39 1.04 SO3 1.20 0.10 0.3 IR 75.0 0.2 52.0

The Quality of PPC with the RFA was compared with OPC and PPC produced with the Normal CPP Fly ash, the PPC were produced by blending the respective ground fly ash with OPC at 30% levels , at similar fineness levels, the results are graphically presented in Fig .7 Effect of aggressive environment on PPC Concrete prepared with PPC (30% RFA) To assess the effect of aggressive environment on the quality of PPC prepared with 30% RFA and for comparison OPC and PPC prepared with 30% Low Lime Class F fly ash from super thermal power plant were considered. The OPC and PPC samples were subjected to Aggressive environment (0.5M Na2SO4 +0.5 M NaCl) and the results of flexural strength of mortar & Concrete (M-20 grade, W/c ratio =0.57) cubes in water and in aggressive environment are tabulated in Fig. 8 Table-2 . The PPC with RFA shows improves resistance to aggressive environment. Table – 2 : Concrete Strength and effect of Suplhate environment on Concrete Density Slump

mm fc, MPa 7 days

fc, MPa 28 days

PPC with Low Lime Class F Fly ash 2493 30 13.9 22.3 PPC- with RFA 2487 25 23.6 37.7 Curing condition : 7 days normal curing + 42 days curing in 5% sodium sulphate solution Control concrete

strength (MPa) Concrete in sulphate solution Strength (MPa)

PPC with Low Lime Class F Fly ash 33.0 32.5 PPC- with RFA 37.9 40.8 Improving the Pozzolanacity of fly ash during its generation:

The investigations carried out on synthetic coal ashes [1] with sodium, sulphur and silica at 900º, 1100º, 1300º, and 1500º C and residence times of 0.1, 0.5, 1.5, and 2.4s indicated that formation of sodium silicates is favored by higher temperatures and longer residence times, the sodium sulfate particles were detected on the surface of the larger sodium silicate fly ash particles formed at lower temperatures.

The size and prevalence of the sodium sulfate particles decreases as temperature was increased. Fly ash particle formation was characterized by fragmentation followed by coalescence. Larger particles were formed at lower temperatures, indicating more complete coalescence with some cenosphere formation.

Fig. 7: Quality of PPC with RFA

Fig. 6 : XRD Scan of RFA

Fig. 8: Flexural Strength of Mortar OPC,PPC with Low Lime Class F Fly ash & PPC with RFA in Water & Aggressive Environment

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The synthetic fly ashes [2,3] produced at laboratory scale, the authors concluded that prolonged heat treatment promotes sintering and devitrification which in turn influences other properties. Iron in an otherwise pure aluminosiliceous ash reduces pozzolanic strength, whereas calcium in an aluminosiliceous fly ash results in a very reactive ash.

The work by diamond [4] concluded that fly ashes with CaO content upto 20% showed maximum indicative of siliceous glass whereas Haobo[5] studied the influence of CaO addition on granulated cinders and concluded that with increase in CaO content, the vitreous network of granulated cinders becomes disordered and is destroyed, the polymerization of network former reduces and the hydraulic activity of granulated cinder improves.

This avenue of tailor making the Fly ash Mineralogy in generation , needs better understanding of the influence of different minerals present in coal as well as effect of the Process conditions on properties of the resultant fly ash .

The changes in the composition of the mineral matter in coal could also be simulated through external additions to coal of different additives such as sodium, calcium and alumina based minerals and its impact on fly ash Mineralogy and fly ash properties could be studied at different temperatures.

At the author’s lab, simulation experiments were carried out to enhance the pozzolanic activity of fly ash during its formation. The addition of different mineral additives such as CaO, alkalis, phosphates and alumina were added to the coal . The coal was combusted at different temperatures (900°C & 1350°C ) simulating the over board Boiler conditions in CPP and Pulverized fuel fired thermal Plant

The Synthetic coal ashes with coal were produced at the two temperatures using additions such as CaO , alkalis, alumina and phosphates , were characterized for chemical composition and changes in Mineralogy especially the amorphous phase contents and Mullite content which is shown in Fig. 9,10& 11 Effect of CaO The detail studies were carried out on addition of CaO to Coal targeting CaO content of ~13% in the coal ash at different temperatures. The composition of Synthetic Coal ash is given in Table -3. The Changes in the mineralogical phases and morphology was assessed at temperatures of 900°C & 1350°C and is shown in Fig. 12a &12b & 13a & 13b. The quality of PPC prepared with coal ashes at 900°C and 1350°C is graphically shown Fig.14

CA- NC

CA- KC

CA- CC

CA- P

CA- AA

CA- A

Fig.9: Changes in the amorphous phase after addition of different Additives at 900°C

CA- AA

CA- NC CA- KC

CA- P

CA- CC

CA- A

Fig.10: Changes in the amorphous phase after addition of different Additives at 1350°C

CA- AA

CA- KC

CA- NC CA- CC

CA- A

CA- P

Fig.11: Mullite formation in Synthetic coal ash with use of different Additives at1350°C Abbreviation used: CA – A : Coal ash as such CA – NC : Coal ash with Na2CO3 CA – KC : Coal ash with K2CO3 CA – CC : Coal ash with CaCO3 CA – P : Coal ash with P2O5 CA – AA : Coal ash with Al2O3

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Table -3 : Composition of Coal ashes produced %Oxides SiO2 Al2O3 Fe2O3 CaO MgO LOI

Synthetic Coal ash 51.4 22.5 5.3 13.3 0.8 1.0

With Lime additions to coal the phase formation are different at different at the above temperatures, at 1350°C a complete formation of calcium alumino silicate phase (Anorthite) was observed, whereas at 900°C the added Lime is in the amorphous phase, since no crystalline calcium rich phases were observed. The findings were also confirmed by SEM, where a fused mass is seen under SEM at 1350°C, where as morphology of synthetic coal ash (at 900°C) is observed to be similar to coal ash. The new phase formation has a positive impact on the strength development when compared with Synthetic coal ash as shown in Fig.14 Converting fly ash to Synthetic Slag: Indian standard IS: 12089 -1987, “Specification for granulated slag for the manufacture of Portland slag cement” covers the requirements of granulated slag used in the manufacture of Portland slag cement conforming to IS: 455-1976. The definition of slag as per IS: 12089-1987 is as follows .“Slag is a non-metallic product consisting essentially of glass containing silicates and aluminates of lime and other bases, as in the case of blast furnace slag, which is developed simultaneously with iron in blast furnace or electric pig iron furnace. Granulated slag is obtained by further processing the molten slag by rapidly chilling or quenching it with water or steam and air”.

Pera et al[15] developed a synthetic slag from different industrial wastes such as municipal incinerator bottom ash, granulated sewage sludge and automotive shredder (A.S.W). These wastes were mixed with Marl or Limestone as a source of calcium and molten at 14500C, then cooled and granulated with water. The resulting vitreous product was X-ray amorphous slag was ground and used for as a binder alongwith cement and gypsum. The best results in terms of compressive strength of 22.0 MPa at 7 days and 800 MPa at 28 days was obtained by binder having 60% slag, 20% cement and 20% gypsum.

Fig.12a &12b: Changes in the Mineralogical phases with respect to temperature for Synthetic Coal ash

Fig.13a: SEM Photomicrographs of Synthetic Coal ash at 900°C

Fig.13b: SEM Photomicrographs of Synthetic Coal ash at 1350°C

Fig.14: Quality of PPC with use of Synthetic Coal ash

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Burge [16] demonstrated that burning oil shale at 8000C results in a residue having hydraulic properties which without addition of lime gives very good compressive strength. The cementitious binder was prepared by mixing burnt oil shale with Portland cement clinker and activator (having 79% calcium sulphoaluminate and 21% anhydrous calcium sulphate). The results on compressive strength indicated blends of burnt oil shale and portland cement clinker increased 1day to 28 days strength, ternary blends of burnt oil shale, Portland cement clinker and sulphoaluminate based activator lead to high 3 hrs and 28 days compressive strength.

The Feasibility of conversion of fly ash to synthetic slag composition was experimented at the author’s Lab with use of Fly ash and Lime source in ratios, as per the quarternary Phase diagram, the Molten mass at 1250 & 1300°C was aqueous quenched.

The composition of synthetic Slag thus produced is given in Table – 4 and the mineralogical phases identified by XRD are shown in Fig. 15, The Glass content in Synthetic slag quantified by microscopy as observed under Optical Microscope is shown in Fig. 16 & 17 and the normal GBS slag is shown in Fig. 18. The Quality of slag Cement produced with 50% synthetic slag, gypsum and clinker is given in Table -5, for comparison a PSC was also prepared with GBS slag. Table -4 : Chemical Composition of Synthetic Slag

Table - 5 : Quality of PSC with Synthetic Slag

PSC with Granulated Slag with Glass content of 94.5%

PSC with Synthetic Slag with Glass content of 97%

Sp. surface area (M2/kg) 382 379 % NC 30.3 30.0

Setting time (mins.) Initial 110 95 Final 170 185

Compressive Strength (MPa) 1 Day 12.4 12.7 3 Days 21.8 22.2 7 Days 30.9 31.3

28 Days 50.6 52.7

% Oxides GBS Slag Synthetic Slag BIS Requirement

SiO2 34.2 47.6 Al2O3 19.2 16.5 Fe2O3 0.5 4.1 CaO 33 26.7 MgO 8.8 2.1 17.0 Max. LOI 0.8 0.0 Na2O 0.1 0.17 K2O 0.87 1.16 SO3 0.3 0.34

Sulphide sulphur 0.48 -- 2.0 Max. Cl 0.004 0.01

MnO 0.07 0.03 5.5 Max. Hydraulic Index ** 1.03 ≥ 1.0

(CaO + MgO + 1/3 Al2O3) ** Hydraulic Index = (SiO2 + 2/3 Al2O3)

Fig. 15: Comparative XRD Scan of Synthetic Slag at 1250° & 1300°C

Fig.16: Synthetic Slag at 1250°C under Optical Microscope showing Glassy Phase with some crystalline phases % Glass content : 85%

Fig.18 : Granulated Slag from Steel plant under Optical Microscope showing Glassy phase % Glass content : 94.5%

Fig.17 : Synthetic Slag at 1300°C under Optical Microscope showing Glassy phase with some crystallites % Glass content : 97.0%

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Conversion of Fly ash to Mullite:

A lot of efforts have been made to synthesize Mullite ceramics using various starting materials such as industrial grade and chemicals. All of which follow different mullitization routes of heating. The path can be classified in two types (1) Mullite formation above 1200°C through Al2O3 precursor, Mullite is observed in the mixture of kaolinite and alumina sol mixtures and diphasic gels. (2) Direct formation of Alumina rich Mullite at about 900-1000°C [23] is observed in monophonic gels and glasses. The occurrence of differ mullitization routes is due to the different degrees of mixing of Al2O3 and SiO2 with grains and on a molecular scale. Suriyanarayanan etal [17] produce glass ceramics by high temperature plasma using coal ash. Different % of coal ash - alumina mixture was used, coal ash -100%, coal ash 75% + 25% alumina, coal ash 50% +50% alumina and coal ash 25% and 75%alumina. The samples were heated to 1600°C for 20 mins and the XRD scans of the same are given in Fig. 19 a to 19d.

Mullite formation from coal ash with use of MgO (1 % to 5%)was studied by Sultana [18,19] the % Mullite formation increased when fly ash samples were sintered at 1600°C and XRD results indicated the presence of Mullite, cristobalite and corundum phases. With increase in MgO content, the peak intensity of Mullite and corundum increased while that of cristobalite decreased Monteiro et al [20] studied the effect of 10% dolomite sintered in air at 900& 1300°C for 2hrs.. The addition of dolomite caused the formation of an increased amount of anorthite. The density, thermal expansion coefficient and the modulus of rupture of the densest fly ash based ceramics materials are identical to those exhibited by some traditional ceramics used in civil construction. The XRD scan & SEM of the sintered fly ash and as such fly ash is shown in Fig.20, 21 & 22

Typical low lime class F Indian fly ash has a potential to convert it into low cost Mullite glass ceramic material, the feasibility studies were carried out at the author’s lab to convert fly ash to Mullite by addition of α-Alumina at different proportions at high temperature (1450°C). The α-Alumina was mixed at 30% & 40% by weight and the mix was fired at 1450°C in a static furnace. The fired material was subjected to XRD for mineral phase identification. The XRD confirms the formation of Mulllite with traces of Quartz as shown in Fig. 23 & 24 and was compared with as such fly ash.

Fig. 19a 100% Coal ash Fig.19b: coal ash 75% + 25% alumina Fig.19c: coal ash 50% + 50% alumina Fig.19d: coal ash 25% + 75% alumina

Fig. 21: SEM photomicrograph of Fig. 22: SEM photomicrograph of 10 % 100 % Calcined Fly ash dolomite added sintered fly ash at 1100°C

Fig. 20: XRD Scan of 100 % Calcined Fly ash & 10 % dolomite added sintered fly ash at 1100°C

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Thus the studies indicates that a convenient way to treat fly ash transforming it into useful ceramic product (Mullite) via a simple and cost effective powder technology and sintering route Conversion of Fly ash to Zeolite :

Zeolite is the group name of materials of microporous crystalline aluminosilicate having various useful physicochemical properties like selective adsorption, hydrophilicity, ion exchange property and catalysis. These materials would possess a wide variety of possibilities to apply such as in agriculture, reclamation, cement and brick, stabilization of soil, building materials etc. while all industrial process for zeolite production have so far been batch hydrothermal processes, work carried out by Wang et al [21] have alkali activated the fly ash with 1:10mol/L of NaOH, where Zeolite P crystallized early at low alkaline concentration, which was replaced then by zeolites X and A. At high concentration, hydroxy sodalite was the only new phase. Quartz, in fly ash and NaOH solution system, gradually dissolved, and mullite, however, remained stable. It was concluded that, with Al/Si and Na/Si finally reaching equilibrium in molar ratio, composition of starting mixtures affects the crystallization of zeolite from fly ash. Chang and Shih [22] converted fly ash to zeolite by fusing with NaOH at 550°C followed by dissolution in water and hydrothermal treatment to precipitate faujasites. The ion exchange capabilities of converted faujasites with respect to Cs+ ions were measured. Querol et al [23] overviewed the literature on synthesis of zeolites from coal fly ash and remarked that all the methodologies developed are based on the dissolution of Al-Si bearing fly ash phases with alkaline solutions (mainly NaOH & KOH solutions) and the subsequent precipitation of zeolitic material depending upon the concentration of alkalis used and the composition of fly ash, various zelolites could be synthesized. A typical SEM photomicrograph of A- type Zeolite produced from Fly ash is shown in Fig.25. Studies carried out at the author’s lab on zeolite synthesis from fly ash revealed that Na-P type of zeolite could be produced with use of 5% NaOH and activating it in furnace at 1000°C to form Na-P type zeolite as shown in Fig. 26

Fig. 23 : XRD Scan of as such fly ash, Fly ash + 30% α-Alumina & Fly ash + 40% α-Alumina

Fig. 24 : XRD Scan of as such fly ash, Fly ash + 30% α-Alumina & Fly ash + 40% α-Alumina

Fly ash – as such

Fly ash + 30% α-Alumina

Fly ash + 40% α-Alumina

Fly ash – as such

Fly ash + 30% α-Alumina

Fly ash + 40% α-Alumina

Fig.25 : A typical SEM image of A-type Zeolite produced from Fly ash

Fig.26 : SEM photomicrograph of Na-P type Zeolite produced from Fly ash at Author’s Lab

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Conversion of Fly ash to Clinker: The latently hydraulic Alumino-Silicate Phase of fly ash is converted to the more reactive Calcium

Alumino-silicate Phase by thermo chemical activation. Use of such a reactive fly ash would also help to maintain the alkalinity (pH) of the hydrated cement paste Matrix which would become an issue at higher absorption levels in PPC and the resultant PPC has excellent Sulphate Resistance property. In absence of Lime, the ash matter under the combustion temperature forms Alumino silicate (amorphous phase-hydraulic)/Alumino silicate (Mullite - crystalline non-hydraulic) less quantity at CPP temperature and remaining silica as quartz and Fe2O3 as magnetite etc. One of the approaches that was investigated in detail was conversion of fly ash (low lime Class-F) to cementitious products through hydrothermal activation route of Jiang and Roy[24]. However the studies at the author’s laboratory revealed that the low lime class -F Indian fly ashes with lower amorphous phase and higher crystalline contents like mullite, quartz are not amenable to hydrothermal activation. The authors however developed a new process route [25-28] based on the principles of alkali activation of fly ash at 4500C. This alkali activated fly ash is reacted with hydrated lime in pre- determined proportions to produce the Calcium silicate /aluminate hydrates gels (hydrogel), which is dehydrated, dried and sintered at 800-13500C to produce the cement clinker. The schematic representation Hydrogel process is given in Fig. 27 and the microstructure of the Reactive Belite Clinker is shown in Fig.28a & 28b.

Conclusions:

To maximise the fly ash utilisation, Some of the technological options discussed in the paper of altering the Mineralogy of fly ash , would result in increased utilisation of fly ash in different areas of its applications, the most promising ones could be:

• Enhancing the fly ash pozzalinicity ( Reactive Fly ash & simulated Synthetic fly ash) for its use in PPC • Synthetic Slag • Fly ash to Mullite /Zeolite • Fly ash to Clinker

These would substantially enhance the utilisation levels of fly ash, with fly ash becoming a resource material through altering fly ash mineralogy in generation or through subsequent processing

Fig. 27: Schematic representation of Hydrogel Process

FLY ASH & NaOH SOLUTION

i) NODULISED ii) PRE HEAT AT 450 0C

ADD WATER AND STIR & ADD HYDRATED LIME

FILTER/DE-WATERED, OVEN DRY

HYDRO GEL

CALCINATION /SINTER AT 1100-13500C

REACTIVE BELITE/OPC CLINKER

Fig.28a : Photomicrograph of Clinker showing well developed C2S

Fig.28b : Photomicrograph of Clinker showing well developed C3S

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