ALTERING THE MINERALOGY OF FLY ASH TO MAXIMISE ITS UTILISATION

ABSTRACT:

The Mineral phase composition of fly ashes from coal fired Thermal Plants is a function of nature of the coal, coal fineness, coal comminution system, boiler type & efficiency of collection of fly ash. The Source & Nature of coal and temperature of generation of the fly ash primarily determines the chemico - mineralogical composition and resultantly to a large extent influences the pozzolanic potential and reactivity of the generated fly ashes, it would also determine the type of crystallites formed, morphology and the chemical composition and percentages of the amorphous / glass contents of the fly ash.

It’s a of known fact that the pozzolanic reactivity of fly ash depends on glassy/ amorphous silicate phase of fly ash and specific surface area available for reaction with lime. The investigations carried out on synthetic coal ashes with sodium, sulphur and silica at different temperatures (900ºto 1500ºC) and varying residence times 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. 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.

The synthetic fly ashes produced at laboratory scale by Joshi et al concluded that prolonged heat treatment promotes sintering and devitrification which in turn influences other properties. Iron in an otherwise pure alumino-siliceous ash reduces pozzolanic strength, whereas calcium in an alumino-siliceous fly ash results in a very reactive ash. The influence of CaO addition on granulated cinders was studied by Haobo H. 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 fly ash scenario thus necessitates better understanding the influence of different minerals present in coal on the resultant fly ash properties, the fly ash Mineralogy can be altered by compositional as well as process parameters which alter the properties of fly ash, 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 to maximize its utilisation.

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 such as fly ash to synthetic slag, Mullite, Zeloite and Clinker as well as attempts on altering the fly ash Mineralogy at generation stage in a Coal Fired Power Plant.

1.0 Introduction: It is well understood that the 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, boiler over board temperature, Mineral matter composition & fly ash collection ESP fields, loading at which the thermal plant operates etc, as a result even from the same source the fly ash characteristics could vary substantially.

In India the chemico - mineralogical characteristics of Indian fly ash produced, has been observed to vary considerably. The mineralogy of fly ash available in the country shows 15-30% Mullite, 15-45 % Quartz, 1- 5% Magnetite, 1-5% Hematite and around 25-35% of amorphous glassy alumino - silicate phase [1]. 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 of 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. 1.1 Pozzolanic Reactivity of Indian Fly ashes: The pozzolanic reactivity of the Low Lime Class – F fly ashes (similar to those available in India) has been observed to be more a function of glassy amorphous content and higher spherical morphology particles (Cenospheres); in such fly ash it is relatively independent of the minor compositional variations. On an average the amorphous content of Indian fly ash is in the range of 25 - 45%,with relatively similar composition (Class – F) , Some of the fly ashes of similar chemical composition have been observed to have much 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 Class F , fly ashes [2,3] , 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 the 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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3-days 7-days 28-days

% H

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Fly ash -U

Fly ash - A

Fly ash - D

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

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

1.2 Effect of amorphous phase composition of the fly ash:

It has been observed that as the composition of the amorphous phase changes, i.e. 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 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.3 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. 4 depicts the compressive strength characteristics of PPC made with different amorphous phase composition. 2.0 Avenues for Increased Fly ash Utilization by altering the Mineralogy of Fly ash: 2.1 Improving the Pozzolanacity of fly ash during its generation

The pozzolanic reactivity of fly ash depends on glassy/ amorphous silicate phase of fly ash and specific surface area available for reaction with hydrated lime in the hydrating Cement [4].

The investigations carried out on synthetic coal ashes [5] with sodium, sulphur and silica at 900º, 1100º, 1300º, and 1500º C and residence times of 0.1, 0.5, 1.5, and 2.4 s indicated that formation of sodium silicates is favored by higher temperatures and longer residence times, the sodium sulfate 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 Cenospheres formation. The synthetic fly ashes [6,7] produced at laboratory scale, the authors R C Joshi et al concluded that prolonged heat treatment promotes sintering and devitrification which in turn influences other properties. Iron in an otherwise pure alumino-siliceous ash reduces pozzolanic strength, whereas calcium in an alumino-siliceous fly ash results in a very reactive ash.

The work by diamond [8] concluded that fly ashes with CaO content up to 20% showed maximum indicative of siliceous glass whereas Haobo [9] studied the influence of CaO addition on

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1 Day 3 Days 7 days 28 daysAge

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AP CAP Fig.4: Compressive Strength of PPC with Fly ash AP & CAP

Fig.3: XRD of fly ashes with different S/A showing difference in amorphous humps

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, 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 , at 900°C (LT) to simulate the temperature conditions of AFBC / CFBC boilers & at 1350°C (HT) simulating the overboard Boiler conditions in Pulverized fuel fired thermal Plants

2.1.1 Experimental:

Coals used for the study was having ash content of ~55% The proximate analysis along with calorific value of coals is given in Table-1. Synthetic coal ashes were produced with coal blended with different minerals, the coals with combusted at conditions essentially, identical to those found in suspension fired furnaces. The change in the coal ash mineral matter was brought about by addition of different additives such as Na2O, K2O, Al2O3, P2O5 & CaO. Emphasis was given on chemical composition, fineness or particle size and crystallinity. The effect of chemical composition was studied by varying the proportions of natural minerals namely Lime, ATH, phosphate compound and alkali compounds, from which these ashes were prepared, The laboratory experimental set up for manufacture of synthetic coal ashes is shown in Fig.5.The minerals were ground to pass a 200 mesh (74 microns) and were blended with fine coal (15% Residue on 90 microns), nodulised with water followed by drying at 110°C and subjected directly into furnace with use of platinum crucible connected to a platinum wire at the temperatures of 900° & 1350°C, the particles of the coal were in contact with that temperature for fixed time interval. The coal ashes were produced in the lab by firing the coal at 900 °C in the furnace. The composition of the coal ash is given in Table -2 and the Mineralogy of the coal ashes is given in Fig. 6

Table- 1: Proximate analysis and Calorific values of Coals

Coal % Moisture 10.1 % VM 14.3 % Ash 54.7 % Fixed Carbon 21.0 CV (Cal /gm) (Gross on Dry basis)

3072

%S 1.1

Platinum Wire

Platinum Crucible

Furnace Cavity Furnace

Fig.5: Experimental set up for manufacture of synthetic coal ashes

Table -2: Composition of Coal ash %Oxides SiO2 Al2O3 Fe2O3 CaO MgO LOI Na2O K2O SO3 TiO2 IR

Coal ash from Coal 64.2 22.0 4.2 1.8 0.8 1.3 0.33 1.27 1.8 1.1 78.3 2.1.2 Effect Alkalis, Lime, Alumina & Phosphates on fly ash mineralogy:

The effect of other additives such as Na2O, K2O, CaO, Al2O3 &P2O5 on the phase formation of simulated fly ash (coal ashes) was studied with Coal, wherein coal was blended with 1% Sodium carbonate,1% Potassium carbonate, 3% hydrated lime , 2.5% phosphate compound & 5% ATH.

This coal mix was fired at the temperatures of 900°C (LT) and 1350°C (HT). The phases formed were identified by XRD and quality of cement was studied with use of these coal ashes, for comparison, Coal Ash without any additive was considered. The nomenclature used for the coal ash samples is given in Table -3 Table -3: nomenclature used for coal ash samples Denotation Fired at 900°C Denotation Fired at 1350°C

CA- LT Coal ash without additive at 900°C CA-HT Coal ash without additive at 1350°C CA- LT -CaO Coal ash with Lime at 900°C CA-HT-CaO Coal ash with Lime at 1350°C CA– LT -NC Coal ash with Sodium carbonate at

900°C CA- HT -NC Coal ash with Sodium carbonate at

1350°C CA- LT -KC Coal ash with Potassium carbonate at

900°C CA- HT -KC Coal ash with Potassium carbonate at

1350°C CA- LT -P Coal ash with Phosphate compound at

900°C CA- HT - P Coal ash with Phosphate compound at

1350°C CA- LT -AL Coal ash with ATH at 900°C CA- HT -AL Coal ash with ATH at 1350°C

The chemical composition of the coal ashes is given in Table -4 and the different phases formed at 900°C and 1350°C is given below in Table - 5. The changes in the amorphous phases with addition of different minerals is given in Fig. 7 & 8 and the formation of Aluminium phosphate in coal ash –with addition of phosphate compound at 900& 1350°C phase in coal ashes is shown in Fig.9, whereas anorthite formation with addition of CaO is shown in Fig.10 and the formation of mullite phase in different coal ashes is shown in Fig.11.

The PPC’s were prepared by intergrinding clinker, 25% simulated Fly ashes (coal ash) without and with different additives and gypsum in a lab ball mill targeting a sp.surface area of ~380 M2/Kg and SO3 content of 2.5% in resultant cement, these PPC’s were tested for their physical properties and the strength of PPC with different coal ashes (fired at 900°C & 1350°C) is graphically shown in Fig.12 & 13.

Fig.6: Mineralogy of simulated Fly ash without additive

Table -4: Composition of the Coal ashes

%Oxides SiO2 Al2O3 Fe2O3 CaO MgO LOI Na2O K2O SO3 P2O5 IR Coal Ash -A 64.2 22.0 4.2 1.8 0.8 1.3 0.33 1.27 1.0 0.20 78.3 Coal ash –C 59.2 20.0 4.3 5.8 0.8 3.2 0.33 1.20 1.1 0.17 57.1

Coal ash –NC 64.1 22.1 4.2 1.6 0.8 1.4 1.08 1.27 1.0 0.18 72.9 Coal ash –KC 64.1 22.0 3.9 1.7 0.8 1.5 0.37 2.52 1.1 0.22 78.0 Coal ash –P 62.2 24.1 4.0 1.8 0.8 1.5 0.32 1.33 0.9 1.63 70.7

Table-5 : Mineralogical Phases formed in different coal ashes Phases identified at 900°C Phases identified at 1350°C CA-LT Quartz (SiO2), Anhydrite(CaSO4) ,

Hematite(Fe2O3), amorphous phase CA-HT Quartz (SiO2), Anhydrite(CaSO4) ,

Hematite(Fe2O3), amorphous phase CA- LT - CaO Quartz (SiO2), Anhydrite(CaSO4) ,

Hematite(Fe2O3), amorphous phase, Anorthite ( CAS2)

CA- HT -CaO Quartz (SiO2), Anhydrite(CaSO4) , Hematite(Fe2O3), amorphous phase, Anorthite ( CAS2)

CA – LT -NC Quartz (SiO2), Anhydrite(CaSO4) , Hematite(Fe2O3), amorphous phase, anorthoclase (Na,K) (Si3Al)O8

CA – HT -NC Quartz (SiO2), Anhydrite(CaSO4) , Hematite(Fe2O3), amorphous phase, anorthoclase (Na,K) (Si3Al)O8

CA- LT -KC Quartz (SiO2), Anhydrite(CaSO4) , Hematite(Fe2O3), amorphous phase, anorthoclase (Na,K) (Si3Al)O8

CA- HT -KC Quartz (SiO2), Anhydrite(CaSO4) , Hematite(Fe2O3), amorphous phase, anorthoclase (Na,K) (Si3Al)O8

CA – LT -P Quartz (SiO2), Anhydrite(CaSO4) , Hematite(Fe2O3), amorphous phase, Aluminum phosphate

CA – HT - P Quartz (SiO2), Anhydrite(CaSO4) , Hematite(Fe2O3), amorphous phase, Aluminum phosphate

CA – V-AL Quartz (SiO2), Anhydrite(CaSO4) , Hematite(Fe2O3), amorphous phase

CA – HT -AL Quartz (SiO2), Anhydrite(CaSO4) , Hematite(Fe2O3), amorphous phase

Fig.7: Changes in the amorphous phase after addition of different Additives at 900°C

Fig.8: Changes in the amorphous phase after addition of different Additives at 1350°C

CA- HT

CA-HT- KC

CA-HT- NC CA- HT-CaO

CA- HT-AL CA-HT - P

CA- LT CA-LT -KC

CA-LT NC

CA-LT -CaO

CA- LT -AL

CA-LT P

Fig.12: Compressive Strength of PPC With 25% Coal ashes at 900° C

Fig.9: Aluminium phosphate formation in coal ashes at 900° & 1350°C

Fig.10: Anorthite formation in coal ashes at 900° & 1350°C

900°C

1350°C

900°C

1350°C

CA- B-NC

CA-B- KC

CA-B-AL

CA- B-P

CA- A

CA- B-AL

Fig.11: Mullite formation in coal ashes with use of different Additives at1350°C

The effect of additives at two temperatures could be clearly observed, that at 9000C, most of the additives added are a part of the amorphous phase, the temperature is lower to form any of the corresponding crystalline phases, however in case of addition of CaO and phosphate compound, the initiation of crystalline anorthite and aluminum phosphate is clearly seen, however ,the change in the amorphous hump in all the coal ashes having additives is higher than the coal ash, without any additive (Fig.7) and a shift of amorphous hump at ~270 is observed with use of CaO.

At the temperature of 13500C, higher amorphous phase is seen to form with additions of alkalis (Na,K) and decreases with addition of Alumina and phosphate mainly due to formation of crystalline phases of mullite and aluminum phosphates , whereas in case of addition of CaO a crystalline phase (anorthite)is formed.

The formation of Mullite (Fig.11) amongst all additives is higher with addition of Alumina followed by Na when compared with Coal ash-B, whereas formation of mullite is lower in case of addition of K and Phosphate compound. The compressive strength of the PPC’s with coal ashes(25%) having additives is observed to be higher as compared with coal ash without any additives irrespective of the temperatures, however within the temperatures the strengths of PPC with coal ashes at 13500C are higher than the PPC’s with coal ashes at 9000C

2.2 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 [10] developed a synthetic slag from different industrial wastes such as municipal incinerator bottom ash, granulated sewage sludge and automotive shredder (A.S.W). The X-ray amorphous slag was ground and used for as a binder along with 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.

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 quaternary Phase diagram, the Molten mass at 1300°C was aqueous quenched. The composition of synthetic Slag thus produced is given in Table – 6 and the mineralogical phases identified by XRD are shown in Fig. 14, The Glass content in Synthetic slag quantified by microscopy as observed under Optical Microscope is shown in Fig. 15 and the normal GBS slag is shown in Fig. 16. The Quality of slag Cement produced with 50% synthetic slag, gypsum and clinker is given in Table -7, for comparison a PSC was also prepared with GBS slag.

Fig.13: Compressive Strength of PPC With 25% Coal ashes at 1350° C

Table -6 : Chemical Composition of Synthetic Slag

2.3 Conversion of Fly ash to Mullite:

A lot of efforts have been made to synthesize Mullite ceramics using various starting materials [11 - 14]

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. 17 & 18 and was compared with as such fly ash.

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

% 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. 14: Comparative XRD Scan of Synthetic Slag at 1300°C

Fig.16 : Granulated Slag from Steel plant under Optical Microscope showing Glassy phase % Glass content : 94.5%

Fig.15: Synthetic Slag at 1300°C under Optical Microscope showing Glassy phase with some crystallites % Glass content : 97.0%

Fig. 17 : XRD Scan of as such fly ash, Fly ash + 30% α-Alumina & Fly ash + 40% α-Alumina

Fig. 18 : 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

Table - 7 : 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

2.4 Conversion of Fly ash to Zeolite : The possibility of converting fly ash to Zeolite has been reported by various researchers [15 - 17], A typical SEM photomicrograph of A- type Zeolite produced from Fly ash is shown in Fig.19. 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. 20

2.5 Conversion of Fly ash to Clinker:

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 [18]. 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 [19-23] 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. 21 and the microstructure of the Reactive Belite Clinker is shown in Fig.22a & 22b.

Conclusions:

Fig. 21: 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.22a : Photomicrograph of Clinker showing well developed C2S

Fig.22b : Photomicrograph of Clinker showing well developed C3S

Fig.19 : A typical SEM image of A-type Zeolite produced from Fly ash

Fig.20 : SEM photomicrograph of Na-P type Zeolite produced from Fly ash at Author’s Lab

To maximize 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 (simulated Coal 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

References: 1. Chatterjee A.K., “Coal ash utilisation in the Indian cement and concrete industry–Present status

and future opportunities”(private communication) 2. Karandikar M.V., Khadilkar S.A..and.Cursetji R.M., "Comparative Assessment of Pozzolanicity of Low

Lime Class -F Fly ashes of Varying Amorphous Content " 5th Int. Sym. On Cement & Concrete, Oct. 28 to Nov.1, 2002, Shanghai, China.

3. Khadilkar S.A., Karandikar M.V., Tiwari A.K., Cursetji R.M., "Assessment of Pozzolanic Reactivity-An Attempt on Formulating a Hydraulic Index"11th International Conference on Chemistry of Cement, 11-16 May 2003, Durban, South Africa, Vol. 2, pp.868-877.

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5. Erickson, T.A.; Ludlow, D.K. and Benson, S.A “Interaction of Sodium, Sulfur, and Silica during Coal Combustion”, Energy & Fuels, 5:539-547, 1991.

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7. Joshi R.C and Rosauer E.A., “Pozzolanic Activity in Synthetic Fly ashes: II, Pozzolanic Behavior”, Ceramic Bulletin,Vol.52, No.5,1973,pp.459-463.

8. Diamond S.,”On the Glass Present in Low Calcium and High Calcium fly ashes”, Cement and Concrete Research, Vo.13,1983, pp.459-464.

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10. Pera J.,Ambroise J and Chabannet M.,” Transformation of Wastes into Complementary Cementing Materials”, 7th CANMET/ACI Int. Conf. on Fly ash, Silica Fume, Slag and Natural Pozzolanas in Concrete, Sp-199,Vol. II, 2001, Chennai,India,pp.459-475.

11. Suriyanarayanan N., Nithin Kannan K.V. & Bernardo E., “ Mullite Glass cERmamics Production from Coal ash and Alumina by High Temperature Plasma”, Journal of Non- Oxide Glasses., Vol.1, No.4, December,2009,pp.247-260

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16. Chang H.L. and Shih W.H., “A General Method for the Conversion of Fly ash into Zeolites as Ion Exchangers for Cesium”, Ind.Eng.Chem. Res., 1998, 37, pp,71-78.

17. Querol X.,Moreno N.,Umana J.C.,Alastuey A.,Hernandez E., Soler A.L.,Plana F., “Synthesis of Zeolites from Coal Fly ash :an overview”,Int.Journal of Coal Geology,50,2002,pp.413-423

18. Roy. D.M. and Jiang. A. W., “Hydrothermal processing of new fly ash cement”, Ceram.Bull. Vol.71 No.4, 1992, pp.642-647.

19. Khadilkar S.A., Karandikar M.V., Ghosh D., Chatterjee A.K., “A Novel clinkerisation Process for Fly ash Utilisation”, 6th Inter. Conf. on Fly ash, Silica Fume & Natural Pozzolanas in Concrete, Thailand, Vol.I, June 1998.SP-178-13.

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23. Karandikar M.V., Khadilkar S.A. and Cursetji R.M., “Manufacture of Fly ash Belite Cement Through Use of Alkali Activated Fly ash”, 7th CANMET-ACI inter. Conf.on Fly ash, Silica Fume, Slag and Natural Pozzolans in Concrete, Madras, India.,July 2001,Supplementary .