Synthesis and Characterization of Ca and Na Zeolites (Non-Pozzolanic Materials)

Materials Performance andCharacterization

Nevin Koshy,1 Bhagwanjee Jha,2 Srinivas Kadali,3 and D. N. Singh4

DOI: 10.1520/MPC20140053

Synthesis andCharacterization of Caand Na Zeolites (Non-Pozzolanic Materials)Obtained From FlyAsh–Ca(OH)2 Interaction

VOL. 4 / NO. 1 / 2015

Nevin Koshy,1 Bhagwanjee Jha,2 Srinivas Kadali,3 and D. N. Singh4

Synthesis and Characterization of Caand Na Zeolites (Non-PozzolanicMaterials) Obtained From FlyAsh–Ca(OH)2 Interaction

Reference

Koshy, Nevin, Jha, Bhagwanjee, Kadali, Srinivas, and Singh, D. N., “Synthesis and

Characterization of Ca and Na Zeolites (Non-Pozzolanic Materials) Obtained From Fly

Ash–Ca(OH)2 Interaction,” Materials Performance and Characterization, Vol. 4, No. 1, 2015,

pp. 1–16, doi:10.1520/MPC20140053. ISSN 2165-3992

ABSTRACT

The conventional method for alkali activation of fly ash utilizes Ca(OH)2 and

NaOH for the formation of pozzolanic material and fly ash zeolites,

respectively. Sodium-based fly ash zeolites (say, Na-zeolite) mostly employ

NaOH (the high-grade mineralizer and alkali) for activation of fly ash before its

application as an absorbent. However, the Na-zeolites as absorbents (in agro-

and aqua-culture) result in sodicity (i.e., excess of Na, present as impurity in

the zeolite), which in turn reduces their holding capacity of moisture, nutrient

[i.e., nitrogen, phosphorous, and potassium (NPK fertilizers)], microorganisms

(viz., microbial spores), or heavy metals and negatively affect the growth of

plant and aquatic life. To resolve such problems, the present study is focused

on synthesis of agro-grade blend (dominated by Ca-zeolite) of zeolites by

using Ca(OH)2 as major alkali and two well-established mineralizers, NaOH and

NaCl, used in the trace quantity. To monitor activation of the fly ash in two

different conditions, the synthesis of zeolites could be carried out by

employing (1) conventional (the open hydrothermal system), and (2)

autoclaving (the closed hydrothermal system) methods. The main attributes

that control the entire study include temperature and reaction times for both

methods. In addition, the present study demonstrates (1) effectiveness of

Ca(OH)2 in creation of blend of zeolites with considerable cation exchange

Manuscript received August 8,

2014; accepted for publication

February 16, 2015; published

online March 4, 2015.

1

Research Scholar, Dept. of Civil

Engineering, Indian Institute of

Technology Bombay, Powai,

Mumbai-400076, India,

e-mail: [email protected]

2

Head, Dept. of Civil Engineering,

Dr. B. B. A. Government

Polytechnic, Karad (DP), Dadra

and Nagar Haveli-396240, India,


3

Asst. General Manager, Powerdeal

Energy Systems Pvt. Ltd., Nashik,

Maharashtra-422010, India, e-mail:

srinivas.kadali@powerdealenergy.

com

4

Professor, Dept. of Civil

Engineering, Indian Institute of

Technology Bombay, Powai,

Mumbai-400076, India

(Corresponding author),


Copyright VC 2015 by ASTM International, 100 Barr Harbor Drive, P.O. Box C700, West Conshohocken, PA 19428-2959 1

Materials Performance and Characterization

doi:10.1520/MPC20140053 / Vol. 4 / No. 1 / 2015 / available online at www.astm.org

capacity when an optimum chemical composition (comparable to a pure agro-

and aqua-grade zeolite 4A), and (2) suitability of the conventional

hydrothermal method over autoclave method in synthesizing the blend of

Ca- and Na-zeolites possessing a cation exchange capacity up to 394 meq/

100 g. The formation of needle/star/spherule/small cube-shaped crystals (i.e.,

Na-P1, the Na-zeolite) and prismatic/cuboidal shaped crystals (i.e., heulandite,

the Ca-zeolite), confirms suitability of the end product as a good sorbent and

manure.

Keywords

fly ash, hydrothermal system, zeolites, sodium zeolite, calcium zeolite, industrial

applications

Introduction

Over the years, fly ash from coal-based industries has been established as a potential

industrial by-product for waste valorization, especially because of the enormous

quantity being produced annually across the globe and the environmental pollution

associated with it. In this context, there has been a wide interest in using it for vari-

ous purposes such as pozzolanic cement manufacture using fly ash with Ca(OH)2[1–3], land reclamation [4], brick making [5], and soft soil stabilization [6]. Of late,

raw fly ash (RFA) has been used as a source material for synthesis of the polycrystal-

line meso- to micro-porous aluminosilicate minerals known as zeolites [7,8]. Inter-

estingly, natural form of such zeolites (i.e., the natural zeolites) has been used in

agriculture industry as a controlled release fertilizer [9,10] and for wastewater treat-

ment [11]. Zeolites (the commonly produced being sodium-based zeolites,

Nomenclature

AS ¼ autoclave systemBET ¼Brunauer-Emmett-TellerCEC ¼ cation exchange capacityEDX ¼ energy dispersive X-ray spectrometryFTIR ¼Fourier transform infrared spectroscopy

G ¼ specific gravityICP-AES ¼ inductively coupled atomic emission spectroscopy

RFA ¼ raw fly ashSEM ¼ scanning electron microscopySSA ¼ specific surface area

T ¼ interaction timeWS ¼water bath system

XRD ¼X-ray diffractionXRF ¼X-ray fluorescence

h ¼ temperature

KOSHY ET AL. ON SYNTHESIS AND CHARACTERIZATION 2


designated as Na-zeolites) synthesized from the fly ash have potential as an alterna-

tive to the natural zeolites, as they contain agro-friendly and fertilizer-specific ele-

ments (viz., soil nutrients like K, S, and P and micro-nutrients, such as Mg and Fe),

accommodated in the zeolitic pores after dissolution of the fly ash ingredients in the

alkali [12–14]. However, the impurities present in the Na-zeolites in the form of

unbounded/free sodium have been identified as an undesirable constituent in the

zeolites [15]. If used in large quantities for agriculture, this situation would cause

undesirable sodicity and salinity in the soil, which is detrimental to plant growth

because of soil-pore locking and flocculation, respectively [16,17]. In such circum-

stances, the most challenging task would be to synthesize Na-zeolite with no

free/unbounded Naþ using NaOH. Considering the incomplete interaction between

fly ash and NaOH, as reported by the previous researchers [7,8], the solution would

be to use an adequate quantity of NaOH for only maintaining pH. With this in view,

an agro-friendly alkali, i.e., Ca(OH)2, would prove to be a panacea for synthesis of

Ca-zeolites [18]. The presence of NaOH in the process along with Ca(OH)2 hints at

formation of a blend of zeolites, which may be dominated by Ca-zeolites inter-

mingled with some Na-zeolites as well, in the end products.

Also, other pertinent issues include cost of production of Ca-zeolites, require-

ment of their crystallinity, and cation exchange capacity (CEC) to be at par with

Na-zeolites. Based on a critical synthesis of literature, it has been shown that pure

phase of many zeolites (viz., epistilbite, gismondine, heulandite, levynite, phillipsite,

scolecite, thomsonite, and wairakite) are Ca exchanged zeolites, which may be syn-

thesized by using rhyolitic glass, basaltic glass (conforming to glass in the fly ash), ol-

igoclase, and nepheline under an alkaline environment of Ca(OH)2 and other

mineralizing mixtures of chemicals (viz., CaCl2 and NaOH) by hydrothermal treat-

ment under autogenous pressures at temperatures varying from 100�C to 250�C

[19]. Moreover, perlite, an amorphous, rhyolitic (volcanic) glass, has also been used

as a raw material for synthesis of Ca-zeolites (viz., epistilbite, heulandite, and gis-

mondine) as reported by Khodabandeh and Davis [20]. Interestingly, out of the

common types of Ca-zeolites mentioned above, wairakite (CaAl2Si4O12 � 2H2O) has

been reported as the most popular among various lime hydration products, obtained

from various syntheses procedures [19,21,22]. In addition, based on crystal structure

and comparable CEC, wairakite has been ascertained as the calcium analog of the

popular Na-zeolite, analcime (Na2Al2Si4O12 � 2H2O), which has usually been synthe-

sized by the previous researchers from alumina-silica gel under high pH conditions

[23]. The Ca-zeolites thus synthesized are expected to perform effectively as agro-

grade zeolites, as fertilizer, which may turn out to be quite economical over natural

zeolites. To meet large fertilizer demand, large-scale synthesis of Ca-zeolites would

be highly warranted at pilot plant scale. Furthermore, innovative efforts like using

seawater (i.e., water containing NaCl) for the synthesis of zeolites from the

RFA have been attempted in the recent past [24], which ascertains significant crys-

tallization of the end products and conversion of the RFA to Na-zeolites (viz.,

hydroxysodalite and Na-X) up to 88 %.

With this in view, to ensure large-scale production of the agro- and aqua-grade

zeolites and economic scaling up of the pilot synthesis unit in the future, the present

study is focused to employ (1) NaCl as a minor reagent, (2) high pH condition, by

dosing reaction mixture with traces of NaOH, and (3) Ca(OH)2, hydraulic lime, as



major activating reagent. In addition, to establish a suitable process for low-cost pro-

duction of Ca- and Na-zeolites (the agro-grade zeolites), the present study is targeted

to undergo a comparative evaluation of two different types of synthesis methods: (1)

conventional hydrothermal activation under atmospheric pressure, and (2) autoclav-

ing the reaction mixture at autogenously set pressure and temperature under con-

trolled conditions. Furthermore, to monitor the micro-level transitions, which are

physico-chemical, mineralogical, structural, and morphological in nature, of the

RFA into the end product, various characterization tools (viz., Fourier transform

infrared spectroscopy, FTIR; X-ray diffraction spectroscopy, XRD; and scanning

electron microscopy-energy dispersive X-ray spectroscopy, SEM-EDX) have been

employed, and details of the methodology adopted are presented in this paper. It is

believed that this study would establish fly ash and its minor elements (viz., K, S, P,

Fe, and Mg conforming to micro-nutrients in a conventional fertilizer and manure)

as an agro-friendly resource material, as novel grade of fly ash zeolites [12–14]), and

in all, the study would be quite useful for scaling up the process to a commercial

(large-scale) production of fly ash zeolites for agriculture and water treatment. In all,

diversification of fly ash–Ca(OH)2 interaction from well-established formation of

pozzolanic cementitious materials to zeolites makes this study very interesting. To

grade the end products as an absorbent/adsorbent material, CEC has been identified

as a reference property of the end products [8].

Experimental Details

The RFA used for this study was obtained from the hoppers of the electrostatic pre-

cipitators of a coal thermal power plant in central Maharashtra, India. It is com-

prised of particle sizes ranging from 0.375l to 200 l. To remove major content of

unburned carbon particles, the raw fly ash was sieved through sieve No. 230 (aper-

ture size �63 l) as prescribed by ASTM E11-13 [25], and the quantity of the finer

ash passing through the sieve (designated as RFA) was employed. The activating

reagents, Ca(OH)2, NaOH, and NaCl, were procured from Merck Millipore (Mum-

bai, India). Before starting the synthesis, the initial pH of the reaction mixture was

maintained between 13 and 13.5, by adding traces of NaOH and homogeneously

mixing it with the help of a pitched blade turbine impeller (at 450 rpm). The

mixtures of different initial compositions (see Table 1) were treated in solution phase

(L/S¼ 2.54) by resorting to two types of hydrothermal treatments: (1) a Teflon-lined

5-l autoclave reactor, at controlled temperature (�100 to 175�C) and autogenously

set pressure (i.e., termed as an autoclave system, designated as AS), and (2) a water

bath (i.e., termed as water bath system, designated as WS) at controlled temperature

(�98 �C) and atmospheric pressure. Murayama et al. [26] have said that the higher

reaction rate occurs either at higher temperature or longer reaction times, and auto-

claving was carried out for short durations (1 to 6 h), whereas a water bath was set

for longer reaction times ranging from 4 to 24 h (see Table 1). After completion of

the targeted treatment, both the autoclave and the water bath were allowed to cool.

Subsequently, the end product (solid phase) was separated by vacuum filtration,

which was followed by oven drying of the end products at 60�C for 24 h. As shown

in Table 1, based on the variation in temperature, h, and the activation time, T, for

the two types of hydrothermal treatments, the end products have been designated as



http://www.astm.org/Standards/E11

WS1 to WS5 and AS1 to AS7. The terms WS and AS correspond to water bath

(open treatment) and autoclaving (closed system), respectively. Out of all the end

products, WS3 and AS3 have been identified as the two superior products, which

shows relatively higher cation exchange capacity (CEC¼ 394 and 327 meq/100 g,

respectively), and, hence, these samples were selected for further characterization.

Characterization of Raw Materials and

End Products

The CEC of raw fly ash and the various end products was determined by following

the ammonium acetate method [27], and the supernatant obtained after washing,

and centrifugation was analyzed using ICP-AES (SPECTRO ARCOS, M/s. Spectro,

Germany). The amount of Naþ and Caþ ions exchanged with the ammonium ion

by the end products was used for evaluating the CEC, and the results are shown in

Table 2.

The mineralogical composition of the RFA and its end products was determined

by employing an XRD spectrometer (PANalytical X’Pert PRO, The Netherlands),

which utilizes a graphite monochromator and Cu-Ka radiation. The sample was

scanned from 2h ranging from 5� to 120� and based on the d-spacing, qualitative

search for mineral matching prominent peaks in the XRD patterns was carried out

by using HighScore Plus with the ICDD PDF-4þ database [28]. Furthermore, the

quantification of mineral phases was done using Rietveld refinement, and the results

obtained for the mineralogy are shown in Figs. 1 and 2 corresponding to different

end products.

For observing the surface morphology, samples were initially coated with

platinum, placed on carbon tape strips, and analyzed under a scanning electron

microscope (FEI ESEM Quanta 200) coupled with EDX in high vacuum

mode [8,27,29,30]. The micrographs of the RFA and the end products are shown in

TABLE 1

Sample designation and conditions adopted for hydrothermal treatment.

Raw Mixture Ingredients (% by Weight)

Hydrothermal Treatment RFA Ca(OH)2 NaCl h ( �C) T (h) End Product

Open system (using water bath) 72.73 18.18 9.09 98 4 WS1

98 6 WS2

98 10 WS3

98 12 WS4

98 24 WS5

Closed system (using autoclave) 72.73 18.18 9.09 100 6 AS1

125 1 AS2

150 1 AS3

175 2 AS4

70 10 20 100 6 AS5

125 1 AS6

150 1 AS7



Figs. 3 and 4. The EDX results for the RFA and some superior samples (viz., WS3,

WS4, WS5, AS1, AS2, AS3, AS4) with considerable CEC are shown in Table 3.

FT-IR studies were carried out for establishing the alterations occurring in the

structural characteristics of the RFA and its end products, because of a certain

hydrothermal treatment. Approximately, 15mg of the sample was thoroughly mixed

and ground with 1 g of KBr in an agar mortar and pestle for 5min to obtain a fine

and smooth powder consisting of uniform-sized particles. To prepare pellets of the

FIG. 1

X-ray diffractograms of the

RFA and various end products

of the water bath system (with

amount of zeolitic phase in

parentheses).

TABLE 2

CEC of the raw fly ash and the end products.

End Product CEC (meq/100 g) Grade

RFA 15

WS2 272

WS3 394 Superior

WS4 354 Superior

WS5 288 Superior

AS1 189

AS2 254 Superior

AS3 327 Superior

AS4 323 Superior

AS5 118

AS6 117

AS7 137



mixture, one quarter of the mixture (�0.25 g) was pressed in a steel die by using a

hydraulically operated mini press, at a pressure of 10 t.cm�2 [27]. Finally, the infra-

red transmittance spectrum of the sample was recorded (Figs. 5 and 6) by employing

an FT-IR spectrometer (Bruker Hyperion 3000 with Vertex 80 FTIR System, Ger-

many), in the range from 4000 to 400 cm�1.

The particle size distribution was determined by employing a laser diffraction

particle size analyzer (Beckman Coulter LS 13 320), and the results are plotted in

Fig. 7. The specific gravity, G, of the materials was found as per ASTM D D5550-14

[31] using an Ultra Pycnometer (Quantachrome 1200e, Boynton Beach, FL), which

employs helium gas as a displacing fluid. The specific surface area (SSA) of the pow-

der samples was measured using the Brunauer-Emmett-Teller (BET) technique

(Smart Sorb 92/93, Smart Instruments) [27,30].

Chemical composition of the samples, in terms of oxides (% by weight), was

determined using X-ray fluorescence spectroscopy (PANalytical PW 2404, The

Netherlands). The specimen was prepared by thoroughly mixing 4 g of the finely

grounded sample and 1 g methylcellulose with isopropyl alcohol. The mixture was

then kept beneath an infrared lamp for slow drying. A small aluminum dish (inner

diameter 33mm and height 12mm) was taken, and two-thirds of the dish was filled

with methylcellulose and the remaining with the dried sample. The sample thus

obtained was subsequently compressed by applying a 15-ton load, with the help of a

hydraulic jack, to form a pallet. Afterward, the specimen was mounted on the mono-

chromatic sample holder of the XRF setup for determining its chemical composition,

and the results are shown in Table 4.

FIG. 2

X-ray diffractograms of the

RFA and various end products

of the autoclave system (with

amount of zeolitic phase in

parentheses).



http://www.astm.org/Standards/D5550

FIG. 3 Micrographs of the products of the water bath system (a) RFA (S1 and S2), (b) WS3 (S3), (c) WS4 (S4 and S5), (d) WS4,

(e) WS5, and (f) WS5 (S6), where designations S1 to S6 denote EDX spectrums (the square boxes).



Results and Discussion

As can be seen from the data presented in Table 2, the end products (WS3 to WS5)

of the open system yield higher CEC (288 to 394 meq/100 g) compared to the other

end products, which can be attributed to significant zeolitization of the RFA at 98�C

and activation time of 12 to 24 h. On the other hand, the end products (AS2 to AS4)

of the closed autoclaved system gains in CEC from 254 to 327 meq/100 g at 150�C

to 175�C and autoclaving time of 1 to 2 h. Incidentally, the CEC (¼394 meq/100 g)

of WS3 makes this product the most “outstanding product,” and further mineralogi-

cal and morphological characterization is carried out for the superior products for

their zeolitic content. In brief, based on CEC, as discussed above, the AS products

can be graded as inferior to the WS3 product from the water bath system. Also,

from a comparison of the CEC to the AS products (see Tables 1 and 2), it can be con-

cluded that the higher the NaCl, the lower the zeolitization of the fly ash. In this con-

text, zeolitic products synthesized from the water bath system (WS1 to WS5) are

FIG. 4 Micrographs of the products of the autoclave system (a) AS1 (S7), (b) AS2 (S8), (c) AS3 (S9), and (d) AS4 (S10), where

designations S7 to S10 denote EDX spectrums (the square boxes).



noticeable in their X-ray diffractograms (see Fig. 1). Out of all the products, WS3

(having 28 % zeolitic phase) exhibits new and small intensity peaks conforming to

Na-zeolite, Na-P1 (designated as Z for zeolites P, i.e., Na-P1), which considerably

resembles the higher CEC of this product, as discussed earlier.

Apart from pozzolanic materials [1,2], a novel finding from the present study is

the formation of Ca-zeolites, heulandites, and wairakite (denoted by H and W,

respectively, in the diffractogram) in WS3, which display their noticeable small

intensity peaks (see Fig. 1) close to 2h¼ 35.59� and 26.4�, respectively. This mineral-

ogical growth as a result of the RFA-Ca(OH)2 interaction results in higher CEC of

the WS3 product, and produces a different finding as compared to formation of

TABLE 3

Elemental heterogeneity (by atomic %) on the crystal surface present in the SEM/EDX spectrum.

Major Elements Present in Zeolites Agro-Friendly Elements

Sample Spectrum O Si Al Ca Na Total K S P Total I Si/Al (SiþAl)/O (SiþAl)/I Mineral

RFA S1 66.45 22.04 10.10 0.73 — 99.32 0.27 — — 0.27 0.41 2.18 0.48 78.39 M

S2 67.65 17.74 11.61 1.19 — 98.19 0.31 — — 0.31 1.08 1.52 0.43 27.17 M

WS3 S3 74.27 11.90 5.18 3.24 3.81 98.40 0.08 0.30 0.44 0.82 0.78 2.29 0.23 10.67 Z, W

WS4 S4 72.18 12.98 6.55 5.58 0.96 98.25 0.19 0.10 00 0.20 1.55 1.98 0.27 11.16 H

S5 74.51 5.65 2.44 1.22 0.76 83.77 0.11 0.22 00 0.33 15.09 2.31 0.10 0.52 P

WS5 S6 73.33 12.59 6.69 4.59 1.27 98.47 0.16 0.17 00 0.33 1.20 1.88 0.26 12.60 Z, W

AS1 S7 74.40 12.70 5.57 5.18 0.74 98.59 0.28 0.09 00 0.37 1.04 2.28 0.24 12.95 Z, W

AS2 S8 73.34 12.75 5.79 6.22 0.31 98.41 0.22 0.08 0.09 0.39 1.20 2.20 0.25 11.66 Z, W

AS3 S19 59.20 12.58 9.54 16.89 1.18 99.39 — — — — 0.61 1.31 0.37 36.26 C, H, W

AS4 S10 54.34 9.34 5.44 20.32 — 89.44 — — — — 10.56 1.71 0.27 1.39 H, W

Note: I, Impurities (¼FeþMg); -, below detection limit.

FIG. 5

The FTIR spectra of the RFA

and various end products of

the water bath system.



pozzolanic materials in similar interactions reported by previous researchers [1,2].

Superiority of WS3 over WS4 and WS5 establishes that 10 h is more effective than

12 and 24 h for activation of the fly ash with Ca(OH)2. This is an improvement in

alkali–fly ash interaction as compared to previous reports on fly ash–NaOH interac-

tion, which ascertained longer interaction for obtaining higher CEC of the products

of an open system [8,15,26,27,29,30]. On the other hand, out of all the X-ray diffrac-

tograms of the products of the autoclaved system (see Fig. 2), AS1 to AS4 show

appreciable CEC (189 to 327 meq/100 g). Furthermore, prominent calcite peaks are

observed in AS1 and AS2, which can be attributed to their low CEC. Incidentally,

FIG. 6

The FTIR spectra of the RFA

and various end products of

the autoclave system.

FIG. 7

Particle size distribution of the

RFA and the most superior end

product, WS3.



AS3 and AS4 are comprised of zeolite P and wairakite mainly, and have relatively

less peak intensity of calcite. These could be attributing factors for their high CEC.

Interestingly, both AS3 and AS4 could be graded as comparable products, as both

have nearly similar CEC (323 to 327 meq/100 g) and mineralogy. Incidentally, the

blend comprising wairakite, heulandite, zeolite P, and phillipsite, could be responsi-

ble for the higher CEC of the AS3 and AS4. The variation in CEC (see Table 2) and

phase transition in the XRD diffractogram (Fig. 2) are indicative of negligible effect

of change in temperature and time from 150�C to 175 �C and 1 to 2 h on the end

products, in the closed autoclaved system. To be more precise, it can be seen that the

amount of zeolitic formation (out of the total mineral phases) is highest for WS3

(¼28 %) and AS4 (¼36 %). Higher zeolitic content in AS4 could be because of sev-

eral crystalline Ca-zeolites (viz., heulandite and wairakite) peaks in the XRD diffrac-

tograms. However, its lower CEC can be attributed to lesser presence of Na-zeolites

(viz., zeolites P and phillipsite), which are more porous and are responsible for

imparting higher CEC, as in the case of WS3. The above-mentioned findings,

derived from the XRD diffractograms, are further substantiated from the SEM

micrographs of the various end products (see Figs. 3 and 4) and semi-quantitative

elemental heterogeneity on the crystal surface clarified by the EDX results (see

Table 3). It can be seen that the RFA (see Fig. 3(a)) consists of the majority of par-

ticles, which are <2lm, whereas fewer particles are �8lm in size, which are shaped

as spherules. These particles get converted to new morphology as bigger spheres of

wairakite and spherules of zeolite P (see Fig. 3(c)–3(e)), deposited on the end prod-

uct WS3, which are comprised of enhanced concentration of Na and Ca, as com-

pared to RFA, noted in Table 3. The noticeable peak of wairakite occurs at

2h¼ 26.299� (ICDD PDF No. 00-042-1451) in the XRD diffractogram, which indi-

cates the transformation of the mullite peak (2h¼ 26.287�, ICDD PDF No. 01-079-

1457) into zeolites (see Fig. 2).

The morphology of the superior products is clearly visible from the SEM micro-

graphs shown in Figs. 3 and 4. Figure 3(c) ascertains formation of fewer Ca-zeolite,

TABLE 4

Chemical composition (% by weight) of the raw fly ash and end products.

Oxide RFA WS3 AS1

SiO2 58.011 47.546 49.951

Al2O3 29.059 23.530 21.559

CaO 0.721 16.710 18.843

Fe2O3 6.091 4.311 2.952

TiO2 3.761 1.489 0.968

K2O 1.087 0.117 0.527

MgO 0.455 1.431 4.274

Na2O 0.183 3.138 0.741

P2O5 0.118 0.001 0.047

MnO2 0.050 0.035 0.034

SO3 0.038 0.001 0.047

SrO 0.034 0.027 0.031



heulandite, as rhombohedral pellets of mineral phase, enriched in Ca (see Table 3), a

portion of which could be in the calcite phase as well as confirmed by the micro-

graph and the XRD diffractogram (see Fig. 1) of WS4. Incidentally, Fig. 3(c) also

exhibits large numbers of thin prismatic crystals, identified as Ca-zeolite, heulandite

(see EDX spectrum S4 in Table 3), and fewer crystals of phillipsite (a Na-zeolite),

present in the product, WS4. In view of the above observations, it can be seen that

the blend of zeolites identified in WS3, and WS5 (i.e., zeolites P and wairakite) are

remarkably different than WS4 (i.e., heulandite, phillipsite, and zeolite P). Moreover,

higher CEC of WS3 could result from higher Na-zeolites (zeolite P), present in this

product, than WS4. On the other hand, micrographs of AS1 (see Fig. 4(a)) and AS2

(see Fig. 4(b)) products show similar composition of the blend (i.e., zeolite P and

wairakite), as confirmed by the EDX results (see Table 3). Moreover, AS3 (see Fig.

4(c)) and AS4 (see Fig. 4(d)) exhibit many distinct crystals of heulandite and few big-

ger spherical wairakite crystals, also confirmed by their EDX results (see Table 3). A

comparison of micrographs of all the WS and AS superior products indicates that

wairakite grows more remarkably under higher temperature and pressure conditions

in the autoclaved (closed) system, whereas longer interaction time is effective in an

open system. In addition, AS3 reveals presence of calcite crystals, which could be a

deterrent for higher CEC of the product.

The transition in particle size and the crystallization of new minerals gets elabo-

rated from the FTIR spectra (see Figs. 5 and 6), which indicate formation of the new

bands at 3454 cm�1. These bands are broader and deeper in the WS products, in

general, and much more distinct in case of WS3 than the AS products. This can be

attributed to (1) asymmetric stretching of Si-OH (the silanol) bonds, and (2) higher

rate of zeolitization reactions between hydroxyl ion of the Ca(OH)2-NaOH (the

reagents) and the Si and Al (see Tables 3 and 4), present in the RFA, for the water

bath system. On the contrary, corresponding shallow bands in the AS products

could be a result of breakage of some of the OH� ions under pressurized and high

temperatures, which may lead to oxidation of the trace elements (viz., Fe, Mg, Mn,

S, P, Ti, etc., as listed in Table 4) of the RFA, and less existence of hydroxyl ion in

the AS. Furthermore, bands at 1663 cm�1 and 1485 cm�1 also establish formation of

improved products as a result of WS treatment, which gets reflected in the bands

obtained for WS3, which yields very high CEC (see Table 2) and consists of fewer

impurities (viz., carbonates and nitrates), respectively [29,30]. Incidentally, bands at

1015 cm�1 are revelations of more crystallinity of the AS products than the WS,

which is comprised of broader but shallower bands. The broader bands in the WS

product at 1015 cm�1 might indicate its soft crystal structure and Si-O-Si bonds

[8,27,29], as compared to the AS products. The soft structures of the crystals raise a

fresh issue regarding stability of the zeolites, present in WS3. The oxidation of the

trace elements as discussed above gets verified again because of the presence of

sharper and deeper bands at 875.6 cm�1 (corresponds to calcium phosphates),

609 cm�1 (corresponds to Fe bonded compounds), and 460 cm�1 (corresponds to

nitrates) in the AS products. Based on the above FTIR spectra, effectiveness of the

fly ash-Ca(OH)2 interaction gets more established for the most superior product

WS3 than its counterpart, AS4, which is comprised of maximum zeolitic content up

to 36 % in the end product. With all the above findings on WS3, efforts have been

made to determine its specific gravity and particle size. The specific gravity, G, of



WS3 was found to be 2.637, a higher value as compared to that of RFA (¼2.224),which is in agreement with the findings of Kolay et al. [7]. This increase in G could

be because of the release of entrapped air in the cenospheres of the initial fly ash par-

ticles as a result of the surface-etching effects of the alkali reagents and their entry

into the core of the fly ash particles. Also, SSA of this product is seen to increase

from 0.5 m2/g (for RFA) to 75.4 m2/g. An increase of SSA by 150 times is indicative

of (1) the transitions in surface features of the RFA, and/or (2) formation of new

finer particles (see Fig. 7).

Moreover, the particle size distribution (see Fig. 7) for the most superior prod-

uct, WS3 shows that fly ash–Ca(OH)2 interaction results in an increase in particle

sizes of the product, as compared to those in the RFA, greater than 10lm. This

can be attributed to the crystallization on the surface of the fly ash particles after

interaction with the alkali. Subsequently, because of etching of the surface of the

particles present in the RFA (<10lm), their size in the most superior product,

WS3, gradually decreases [29]. The particles in this range could also contain zeo-

litic products, synthesized by the RFA-alkali interaction. Incidentally, the average

particle size of the conventional fertilizer falls in the range 1 to 5mm. Thus, WS3,

with <1mm size could be a better choice for application as fertilizer in

agriculture.

Based on above findings, the synthesized blend of zeolites is seen to have signifi-

cant presence of Ca with low Na concentration, which can be a better option for ag-

ricultural application and waste water decontamination. The major contribution of

this study lies in the agro-friendly ingredients present in the synthesized zeolites. In

this context, Table 3 highlights traces of K, S, and P in most of the superior products.

Incidentally, increased concentration of these elements in the most superior product,

WS3, makes it suitable to be used as agro-grade zeolites, i.e., as fertilizers in agricul-

ture [12–14]. Thus, WS3 can be shown to have double benefits like higher CEC and

agro-friendly elements in its pores, as revealed by the FTIR bands in the range of

600 to 400 cm�1 [27].

Conclusions

Based on the findings of the study, the following can be concluded:

1. The fly ash-Ca(OH)2 interaction, in an open system, is very effective for for-mation of blends of zeolite P and wairakite in a relatively less activation timeof 10 h at 98�C.

2. Higher temperature (150�C to 175�C) and less activation time (1 to 2 h) issuitable for the growth of wairakite zeolites in the products of an autoclaved(read “a closed”) system.

3. From the comparison of the CECs of autoclaved system products, it can beconcluded that the higher the NaCl, the lower the zeolitization of the fly ash.

4. Less CEC and FTIR bands corresponding to hydroxyl group in the products,from the autoclaved system, make them inferior with respect to their counter-parts from the water bath system.

5. The presence of S, K, and P in the blend of zeolites (viz., heulandites, waira-kite, zeolite P, and phillipsite) makes them suitable as agro-grade zeolites,which can be used as fertilizers.



ACKNOWLEDGMENTS

The authors acknowledge the facilities availed by them at the Sophisticated Analyti-

cal Instrument Facility (SAIF) and the XRD Laboratory, Department of Earth Scien-

ces of IIT Bombay, during the course of this study.

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