phD

orM

rra

M

fone

A working procedure was developed for determining the degree of reaction of y ash subjected to alkali activation (with 8 M NaOH)

data gathered to quantify the phases formed as a result of

research showed, among other things, the eectiveness ofthe various techniques used to reach the objectives pur-

for determining its SiO2 and Al2O3 content.

ume) followed by an attack with 5% Na2CO3 solution. Themethod has more recently been modied to optimize results[6], and now consists of a 1:20 HCl attack at laboratorytemperature.

Finally, the alkali activation of y ash can be brieydescribed as a physicalchemical process in which this

* Corresponding authors.E-mail address: anafj@ietcc.csic.es (A. Fernandez-Jimenez).

Fuel 85 (2006) 196the alkali activation of y ash (a powdery by-product of thecoal-red steam generation of electric power). The studydiscussed here is actually a logical extension of researchbegun some time ago [1] aiming to establish a procedurefor determining the reactive capacity of y ash when mixedwith a highly concentrated alkaline medium in the produc-tion of alkaline cements [2,3]. In a previous paper [4], theauthors described a methodology for quantifying the crys-talline and vitreous components of y ash with dierentanalytical and instrumental techniques (selective chemicalattack), X-ray power diraction (XRPD), and magic-anglespinning nuclear magnetic resonance (MAS-NMR). That

Any further research in this direction necessitated theexploration of ways to identify and quantify the reactionproducts forming during the alkali activation of y ash.And that, essentially, is the specic objective of the presentstudy. In this case also, the techniques used included selec-tive chemical attack (acid attack with HCl) and instrumen-tal analysis (XRPD combined with the Rietveld methodand MAS-NMR).

A technique similar to the HCl attack used here wasproposed earlier by Granizo et al. [5]. Their procedure con-sisted in dissolving the reaction products obtained in thealkali activation of metakaolin in HCl 1:9 (solution in vol-at mild temperatures. Since the reaction products dissolve in HCl, the residue left after this acid attack contains only the fraction of theoriginal ash that failed to react with the basic solution. This residue was analysed with Rietveld XRPD quantication and NMR and thendings were compared to the results of the analyses run on the activated ash to obtain a very precise quantication of all of the (crys-talline, vitreous and amorphous) phases present in the systems studied. 2006 Elsevier Ltd. All rights reserved.

Keywords: Fly ash; Alkali-activation; Rietveld

1. Introduction

The present paper describes and interprets experimental

sued. The study likewise provided a fuller understandingof the vitreous phase (fundamental element that controlsalkali reactivity) of y ash, as well as a very precise methodQuantitative determination ofof y ash. Part II:

A. Fernandez-Jimenez a,*, A.G. de la TM.M. Alonso a,

a Eduardo Torroja Institute (CSIC), c/Seb Department of Inorganic Chemistry, University of

Received 7 March 2006; received in revisedAvailable onli

Abstract0016-2361/$ - see front matter 2006 Elsevier Ltd. All rights reserved.doi:10.1016/j.fuel.2006.04.006ases in the alkaline activationegree of reaction

re b, A. Palomo a,*, G. Lopez-Olmo b,.A.G. Aranda b

no Galvache, n 4, 28080, Madrid, Spainalaga, Campus Teations s/n, 29071, Malaga, Spain

rm 11 April 2006; accepted 13 April 200615 May 2006

www.fuelrst.com

01969

powdery solid is mixed with a concentrated alkali solution(in a suitable proportion to produce a workable and moul-dable paste) and stored at mild temperatures (T < 100 C)for a short period of time to produce a material with goodbinding properties [7,8]. The main reaction product formedin this process is an X-ray amorphous alkaline aluminosil-icate gel characterized by somewhat limited short-rangestructural order [6]. In addition, Na-Herschelite-type zeo-lites and hydroxysodalite are formed as secondary reaction

of ash that had been converted to cement and the por-tion that had not reacted with the alkaline solutions; inshort, to determine the degree of reaction (a), since thisattack provokes the dissolution of the chief reaction prod-ucts of the alkali activation of y ash (alkaline aluminosil-icate gel and zeolites) in the acid, while the fraction of ashnot activated by the alkalis remains in the insoluble resi-due. The 1:20 HCl solution was prepared using a concen-trated reagent HCl (37%) supplied by Panreac.

c

A. Fernandez-Jimenez et al. / Fuel 85 (2006) 19601969 1961products [1,6].

2. Experimental methods

2.1. Characterization of raw materials

Two dierent y ashes type F were used in this study.The chemical compositions as well as, the amount of yash retained in dierent sieves were given in the previouspaper [4]. Table 1 is a summary of data, concluded fromthat mentioned previous paper, concerning the chief com-ponents of y ashes of the two types of ash: reactive silica,alumina and vitreous silica content, as well as the majoritycrystalline phase (quartz and mullite). The dierencesbetween the reactive SiO2 and vitreous SiO2 shown in Table1 might be due to the very dierent analytical techniquesused for the quantication (in principle reactive and vitre-ous silica should be the same): reactive SiO2 is determinedby chemical attack (quartz and mullite could be slightlyattacked during this process), while vitreous SiO2 is calcu-lated by XRPD (see Ref. [4]).

2.2. Alkali activation of y ash

The two types of ash studied were activated with an 8 MNaOH solution having alkaline solution/ash ratios of 0.4and 0.56 in mass for y ashes L andM, respectively (the dif-ferent alkaline solution/ash ratios used had the purposeof getting the same workability values in the pastes). Thepaste obtained was placed in air-tight plastic bags and keptat 85 C for 7 days. The hardened material was subse-quently examined in keeping with the objectives pursued.

2.3. Selective chemical attack: reference tests

After thermal curing (at 85 C), the alkali activated yash was attacked with 1:20 HCl to determine the amount

Table 1Main components of y ashes

Reactive SiO2a (%) Vitreous phaseb Vitreous phase

Fly Ash L 42.17 64.94 80.6Fly Ash M 45.07 54.28 65.7

a Value determined as specied in Spanish standard UNE 80-225-93.b Value determined by acid attack with 1% HF (see [1,9]).

c Value determined by XRPD Rietveld quantication (see [4]).d Al2O3 vitreous = Al2O3 content in glass phase of y ash. Value determinedThe experimental procedure followed in the acid attackconsisted in adding 1 g of activated y ash to a beaker con-taining 250 ml of (1:20) HCl. The mixture was stirred witha plastic rotor for three hours, after which it was lteredand washed with de-ionized water to a neutral pH. Theinsoluble residue was rst dried at 100 C and then calcinedat 1000 C; the degree of reaction, a, was found by deter-mining weight loss. These trials were repeated at least threetimes to guarantee reproducibility.

In addition, one of the residues obtained after attackingthe sample of activated y ash L with 1:20 HCl (but notcalcinated at 1000 C) was re-activated with 8 M NaOHfor three days at 85 C. The material so obtained had beenthen subjected to all the steps mentioned before for thechemical attack performed after the rst activation process,these are: acid attack, washing process, drying and calcina-tions process.

2.4. Techniques

The crystalline phases present in the solids studied werequantied with Rietveld XRPD analysis. NMR spectros-copy was also used to attain a fuller understanding of thesystems studied and determine the degree of reaction, a.

2.4.1. XRPD

Sample preparation [4]. Standard a-Al2O3 was synthe-sized as follows: 6 g of c-Al2O3 (99.997% from Alfa) wasground in an agate ball mill at 200 rpm for 30 min. Theresulting powder was placed in a Pt crucible and heatedat 1200 C for 4 h. The oxide was allowed to cool to150 C over 5 h and ground at room temperature in anagate mortar for 5 min. The sample was subjected to a sec-ond thermal treatment at 1300 C for 6 h and then cooledas above. This standard was then ground in an agate mor-tar for 5 min and sieved (

an agate mortar for 10 min, adding acetone to facilitateparticle dispersion, and then heated at 60 C. Around30% (wt) of corundum was added, with the exact quantityrecorded for each measurement. The samples were gentlyloaded (vertically) onto an aluminium X-ray sampleholder.

X-ray data collection [4]. Laboratory XRPD measure-ments were performed for all mixtures on a SiemensD5000 automated diractometer with Cu Ka1,2 radiation(1.5418 A) and a secondary curved graphite monochroma-tor. The readings were taken in vertical BraggBrentano

(h/2h) geometry (at reection mode) between 18 and70 (2h) at 0.03 steps, measuring 15 s per step. The sam-ples were rotated at 15 rpm during acquisition to improvepowder averaging, which is essential to have accurateintensities and hence good phase analysis. The D5000 dif-fractometer optic consisted of a system of primary Sollerfoils between the X-ray tube and the 2-mm xed apertureslit. One 2-mm scattered-radiation slit was placed down-stream of the sample, followed by a system of secondarySoller slits and a 0.2-mm detector slit. The X-ray tube oper-ated at 40 kV and 30 mA.

2-Theta, deg

he

Coun

ts

20.0 30.0 40 50.0 60.0 70.0

X10

E 3

X10

E 3

-2.

00.

02.

04.

06.

08.

0

SiO

2

Al 2O

3

Mu

Mu;

Ma

Mu;

chab

azite

Al 2O

3

Al 2O

3

Al 2O

3

Al 2O

3

Al 2O

3A

l 2O3

Al 2O

3

chab

azite

chab

azite ch

abaz

itech

abaz

itech

abaz

ite

chab

azite

;Ma

soda

lite

(a) LAc

40.0

-4.

06.

08.

0

Al 2O

3

Al 2O

3;M

u

lita u

SiO

2

Al 2O

3

Al 2O

3

Al 2O

3

Al 2O

3

Al 2O

3

SiO

2

Mu

Mu;

Ma

Al 2O

3

3acita

abac

ita;C

aCO

3

abac

ita;M

ut

(b) MAc

SiO

2

ulat

1962 A. Fernandez-Jimenez et al. / Fuel 85 (2006) 196019692-T

Coun

ts

20.0 30.0 40.0

-2.

00.

02.

0 Soda M

CaC

O

Cha

b

Ch ChMu

Fig. 1. Rietveld plots (1870/2h) showing the observed (crosses) and calc

line), for y ash activated with 8 M NaOH and cured at 85 C for 7 days: (a) sadierent phases; the main diraction peaks of each phase are highlighted (a-Ata, deg50.0 60.0 70.0

ed (solid line) powder patterns, and the dierence between them (bottom

mple LAc and (b) sample MAc. The marks denote the Bragg peaks of thel2O3 was used as standard).

X-ray data analysis. Rietveld renement was used toanalyse the powder patterns [10,11], applying GSAS soft-ware [12] with a pseudo-Voigt function [13] and includinga correction for asymmetry due to axial divergence [14].The crystal structures used to calculate the powder patternswere taken from the Inorganic Crystal Structure Database(ICSD). The collection codes for the various structureswere: corundum 73725; a-quartz 63532; mullite 66263;maghemite 87119; calcite 80869; albite 68193; chabazite

denominated LAc and MAc). Alkali activation transformsthe y ash into an X-ray amorphous alkaline aluminosili-cate that casts the characteristic halo on the respective dif-fractogram. Although this halo partially overlaps with thehalo from the original ash [4], it is shifted towards slightlyhigher 2h values. The alkaline activation of the ash alsogenerates new crystalline phases.

Such new crystalline phases formed during the y ashreaction were identied as Na-Herschelite-type zeolites(majority) and hydroxysodalite. These phases were identi-ed by comparing the signals on the diractograms withthe reported patterns of the phases in the PDF database.Since there is no structural description of Na-Herschelite(formula: NaAlSi2O6 3H2O) in the ICSD (Inorganic Crys-tal Structure Database), the structure of the Na-chabazitecrystal (a zeolite practically identical to Na-Herschelite)was used in the Rietveld analysis. The stoichiometry usedfor the sodium chabazite was that described in ICSDrecord 201584: Na0.92Al0.92Si2.081O6 3.25H2O. Similarly,ICSD record 72059, which describes a sodalite with theempirical formula Na2(Al1.5Si1.5O6) 0.5(OH) 0.5H2Owas used for analyzing the pattern of hydroxysodalite.

Table 2 gives the results obtained with XRPD quanti-cation of the dierent mineral phases present in the materi-

idu

Ch

18

3

A. Fernandez-Jimenez et al. / Fuel 85 (2006) 19601969 1963201584 and sodalite 72059. Neither the positional nor thethermal vibration parameters were rened. The parametersoptimized were: background coecients, cell parameters,zero-shift error, peak shape parameters (including aniso-tropic terms if needed), and phase fractions.

29Si MAS-NMR. 29Si MAS-NMR spectroscopic charac-terization was conducted with a Bruker apparatus, modelMSL-400. The resonance frequency used in this study was79.5 MHz and the spinning rate was 4 kHz. The measure-ments were taken at laboratory temperature with TMS asthe external standard. The error in the chemical shift valueswas estimated to be lower than 1 ppm. Magnetic materialswere removed from the samples prior to NMR spectrarecording by exposing the sample to a strong magnetic eld.

3. Results

The results of characterizing the y ash used in thisresearch have been discussed in previous publications[1,4] and are therefore not included here. Nonetheless, abrief description is provided below of some of the similar-ities and dierences between these two types of ash:namely, the ones that are most closely related to the objec-tives of the study. The two materials, denominated L andM, have similar elemental chemical characteristics (seeTable 1 of the previous part I [4]) and even similar particlesize distributions (see Table 2 of the previous part I [4]).They dier substantially, however, in terms of their respec-tive mineralogical compositions (see Table 1). Ash L, forinstance, has a mullite content of approximately 12%,and a 7% quartz content, while ash M contains 22% mulliteand 11% quartz. These dierences in mineralogical compo-sition have a sizeable impact on the reactive alumina (vitre-ous alumina) content of the ash; and this in turn is closelyassociated with the intensity of alkali activation.

Fig. 1 shows the XRPD traces for the y ash activatedwith 8 M NaOH and cured at 85 C for 7 days (material

Table 2XRPD Rietveld quantication of activated ash and 1:20 HCl-insoluble res

Sample Quartz (%) Mullite (%) Maghemite (%)

LAc 4.4 8.7 0.9LAcHCl 9.2 23.9 0.9LAc2HCl 6.4 17.7 1.8

MAc 8.9 18.2 0.4

MAcHCl 15.9 33.7 0.4

a Amorphous = vitreous phase from y ash + amorphous phase formed durals (including the amorphous fraction) by using theRietveld method.

Once characterized, cements LAc and MAc wereattacked with a 1:20 HCl solution to dissolve the reactionproducts formed in the alkali activation process (alkalinealuminosilicate gel + zeolites). The results of this attackare shown in Table 3.

es

abazite (%) Sodalite (%) CaCO3 (%) Amorphous (%)a

.2 0.7 67.1 66.0 74.1

.0 1.2 0.9 67.4

Table 3Results of 1:20 HCl cold acid attack

Name Characteristic (%) Insolublein HCl

(%) Soluble= reactiondegree a

LAca NaOH-activated LLAcHCl Attacked with HCl 35.44 64.5LAc2b NaOH-activated LAcHClLAc2HCl LAc2 attacked with HCl 85.60 (Ref. [2]) 14.4

Normalized to IR = 35.44% 30.33 69.6

MAca NaOH-activated MMAcHCl MAc attacked with HCl 61.34 38.66

a Activated with 8 M NaOH at 85 C for 7 days.b Activated with 8 M NaOH at 85 C for 3 days. 49.4

ing the activation process.

According to these results, the degree of reaction in ashL, when activated under the conditions specied above, ison the order of 64.5%, or around 25% higher than ashM. In addition, the insoluble residues obtained as a resultof the HCl attack (presumably unreacted ash: vitreousphase + quartz + mullite) were characterized using theRietveld renement technique (see Fig. 2, (a) sampleLAcHCl and (b) MAcHCl).

Finally, the HCl-insoluble residue from the LAc samplewas re-activated with the alkali to drive system reactivity to

its farthest limit (to quantify the eectiveness of the ashactivation process). This residue was mixed with an 8 MNaOH solution (alkaline solution/solid = 0.4) and theresulting paste was cured at 85 C for 3 days. This sample(LAc2) was subsequently attacked with HCl acid. Accord-ing to the results shown in Table 3, the degree of reaction inthis second attack was 14.4%, for a rise in the degree ofreaction of approximately 5% (14.4 35.44/100). Thismeans that the conditions for the rst alkali activation pro-cess (7 days at 85 C) were suciently intense for most of

2-Theta, deg

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20.0 30.0 40.0 50.0 60.0 70.0

X10

E 3

-2.

00.

02.

04.

06.

08.

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SiO

2

Ma

Al2O

3

SiO

2M

u

Mu

;Ma

Mu

Mu

Al 2O

3

Al2O

3

Al2O

3

Al2O

3

Al2O

3;M

u Al 2O

3

Al 2O

3

Mu

(a) LAcHClA

l 2O3

Al2O

3Al 2O

3

het

X10

E 4

0.5

1.0

SiO

2Al

2O3

SiO

2M

u

Mu;

Ma

Mu

u

Al 2O

3

Al2O

3

Al2O

3

Al2O

3

Al2O

3;M

u Al 2O

3

Al 2O

3

Mu

(b) MAcHCl

Cris

t

2

l 23

23

2

l

23

ulat

1964 A. Fernandez-Jimenez et al. / Fuel 85 (2006) 1960196920.0 30.0 40.02-T

Coun

ts0.

0

M

Fig. 2. Rietveld plots (1870/2h) showing the observed (crosses) and calc

line), for the HCl-insoluble residues in the activated ash: (a) sample LAcHCl anphases; the main diraction peaks of each phase are highlighted (a-Al2O3 was50.0 60.0 70.0a, deg

ed (solid line) powder patterns, and the dierence between them (bottom

d (b) sample MAcHCl. The marks denote the Bragg peaks of the dierentused as standard).

the vitreous phase of the y ash to react. After this secondattack, the total degree of reaction came to 69.6%, whilethe insoluble residue amounted to 30.33%. Given that thecrystalline phases in the original ash (quartz, mullite, andso forth, which accounted for 20% of the total material)are inert in the alkali activation process, it may be inferredthat, after the second acid attack, the degree of reaction ofthe total ash liable to alkali activation came to 86%.

As in the preceding cases, the HCl-insoluble residue fromthe LAc2 sample was characterized using Rietveld XRPDquantication. The pattern obtained is shown in Fig. 3.

Fig. 4 gives the results of 29Si MAS-NMR analysis ofthe alkali activated ashes (LAc and MAc) and their HCl-insoluble residues (LAcHCl and MAcHCl). The spectrafor the initial y ashes are likewise shown on the gurefor readier interpretation of the results. The most promi-nent feature on these 29Si NMR spectra for the initial ashesis a wide signal, indicative of the heterogeneous distribu-tion of the Si atoms in this type of matrices. The interpre-tation of such signals has been discussed in previous papers[1,4,6]. The spectra for the materials activated with NaOHfor 7 days are more distinct. The results of their deconvo-lution, conducted in accordance with the constant band

activation. Alternatively, it may be attributed to hydroxy-sodalite [16], a mineral which, despite its characteristicallyzeolitic composition, does not exhibit the properties typi-cally associated with these materials. Hydroxysodalite hasa signal at 84.8 ppm.

The spectra for the insoluble residues, however, have awider signal centred at 108 ppm associated withQ4(0Al) units which, as mentioned above, are indicativeof the presence of quartz (a phase that is essentially inertto alkali activation). The deconvolution of this signalreveals the presence of other less intense signals associatedwith the mullite [17] contained in the initial ash (signal at87/88 ppm) as well as with another phase or phases.While not wholly dened (signals at 92, 98 and103 ppm), these latter elements may correspond eitherto an unreacted fraction of the vitreous material or to asmall group of incipient mullite or quartz crystals exhibit-ing low crystallinity.

4. Discussion

The alkali activation of y ash is a physicalchemicalprocess that transforms a powdery ash into a material with

-Th

Mu

AlO

ulat

A. Fernandez-Jimenez et al. / Fuel 85 (2006) 19601969 1965width criterion, are shown in Table 4. The well-denedpeaks appearing on these spectra at 104, 99, 94 and88 ppm are associated with the presence of Q4(nAl) units[15], where n = 1, 2, 3, 4. In addition, the signals appearingat values greater than or equal to 108 ppm were associ-ated with Q4(0Al) units, which can in turn be attributedto the unreacted fraction of the ash (especially quartz).Finally, the signal appearing at 84 ppm may correspondto possible residues of the products formed during alkali

2

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20.0 30.0 40.0

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00.

02.

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Al 2O

3

Mu M

uA

l 2O3

SiO

2

Al 2O

3

SiO

2

Mu

Mu;

Ma

Mag

Mu;

Ma

g

Fig. 3. Rietveld plots (1870/2h) showing the observed (crosses) and calc

line), for the HCl-insoluble residue in the sample of ash L after alkali re-activathe main diraction peaks of each phase are highlighted (a-Al2O3 was used asgood cementitious properties [1,68,18]: high mechanicalstrength, excellent bonding to reinforcement steel [19],and so on. The interaction of the ash during this processgenerates an alkaline aluminosilicate gel as the main reac-tion product. This three-dimensional, XRPD-amorphouscompound may be regarded to be a zeolite precursor.Crystalline zeolites such as Na-Herschelite and hydroxy-sodalite are also found to appear as secondary reactionproducts.

eta, deg50.0 60.0 70.0

Al 2O

3; M

u

Al 2O

3

23

Al 2O

3

Al 2O

3

LAc2HCl

3

3

ed (solid line) powder patterns, and the dierence between them (bottom

tion, LAc2HC. The marks denote the Bragg peaks of the dierent phases;standard).

. / F1966 A. Fernandez-Jimenez et alThe subsequent attack on the hardened material (acti-vated ash) with 1:20 HCl dissolves the reaction products(alkaline aluminosilicate gel and zeolites) (see Fig. 2). Acomparison of the patterns in Figs. 1 and 2 readily showsthat the reaction products disappear after the acid attack,

Fig. 4. 29Si MAS-NMR spectra of (a) the initial ash; (b) 8 M NaOH-alkali

Table 4Results of deconvolution of 29Si MAS-NMR spectra for L and M y ash and

Sample 82 84 88 9L Pos. (ppm) 84 88.3 9

Width 7.15 7.15Integration (%) 5.32 11.0 1

LAc Pos. (ppm) 81.18 84.3 88.5 9Width 3.62 3.62 3.62Integration (%) 2.36 5.98 11.78 2

LAcHCl Pos. (ppm) 87 9Width 6.43Integration (%) 5.10

M Pos. (ppm) 89 9Width 8.52Integration (%) 14.25

MAc Pos. (ppm) 82.7 87.6 9Width 4.93 4.93Integration (%) 5.1 20.9 2

MAcHCl Pos. (ppm) 87.65 9Width 5.70Integration (%) 6.12uel 85 (2006) 19601969leaving a residue that contains only the unreacted phasesof the original y ashes.

The results of quantifying the various phases found insamples LAc, MAc, LAcHCl, LAc2HCL and MAcHClare given in Table 5. These values (the ones corresponding

activated ash cured at 85 C for 7 days; (c) the HCl-insoluble residue.

alkali activated y ash pastes

4 98 104 108 112 1183.6 98.6 103.4 108 1157.15 7.15 7.15 7.15 7.156.98 20.66 19.46 20.9 5.61

3.7 98.9 104.5 1093.62 3.62 3.62 3.624.25 31.62 19.01 5.00

3.7 99.2 103.5 108 112 1186.43 6.43 6.43 6.43 6.43 6.438.93 12.86 16.66 26.59 23.12 6.75

5.2 99.4 104 109 1168.52 8.52 8.52 8.52 8.527.76 9.15 17.58 34.17 16.84

2.5 98.11 103.6 108.54.93 4.93 4.93 4.931.1 27.0 16.2 9.7

2.63 97.60 102.7 108.3 113 1175.70 5.70 5.70 5.70 5.70 5.704.49 3.8 13.3 35.7 26.5 10.2

ite

.ee o

l. / Fto the samples attacked with HCl) were normalized to theinsoluble residue content with Eq. (1).

F R F XRPD IRHCl=100 1where FR = residual phase (HCl-insoluble phase); FXRPD =XRPD-quantied phase in % (see Table 2); IRHCl =HCl-insoluble residue (see Table 3).

In the LAc and MAc samples, what is quantied as theamorphous phase is the sum of the unreacted vitreous frac-tion of the ash plus the aluminosilicate gel formed duringalkali activation with sodium hydroxide. Subtracting thenon-crystalline fraction of the samples attacked with HCl(fraction associated only with the vitreous phase of theash) from that sum gives an estimate of the amount of alu-minosilicate gel formed. For LAc this value would be 67.123.4 = 43.7. Adding this sum to the amount of zeoliteformed (18.9%) yields the total percentage of reactionproducts (degree of reaction, a) or 62.6%, which is practi-cally the same as the value obtained with the HCl attackprocedure (LAcHCl, a = 64.5, see Table 3). For ash Mthe results are 37.1% of alkaline aluminosilicate gel and a41.3% (37.1 + 3.0 + 1.2) degree of reaction or a, which isalso quite similar to that measured directly with the HClattack method, 38.5%.

In the ash L sample subjected to double activation(LAc2) to attain the maximum degree of reaction in thesystem, only a small amount of amorphous material isobserved to react after the second alkali activation (seeTable 5). In other words, the reaction is essentially com-pleted when 20% of the non-crystalline phase found inthe original ash is still intact. Table 5 also shows that

Table 5Normalization of XRPD Rietveld results

Sample Quartz Mullite Maghem

LAc 4.4 8.7 0.9LAcHClb IR = 35.44 a = 64.5 3.26 8.47 0.32LAc2HClb IR = 30.33 a = 69.6 1.94 5.37 0.54

MAc 8.9 18.2 0.4MAcHClb IR = 61.34 a = 38.66 9.75 20.67 0.24a Amorphous = vitreous phase in y ash + amorphous zeolite precursorb Values in italics represent total ash content recalculated from the degrc CaO was also identied in this sample.

A. Fernandez-Jimenez et athe quartz and mullite content is clearly lower than in theinitial ash [4], suggesting that both minerals may undergopartial attack in the aggressive conditions prevailing in thisreaction. This nding is consistent with previous researchby the authors [20], in which a scanning electronic micro-scopic study detected alterations in the surface texture ofmullite crystals, indicative of an attack by the surroundingalkaline medium.

Another important fact deduced from the analysis ofthese results is that ash L is more reactive than ash Munder the conditions in which the present study was con-ducted, despite the fact that the two types of ash have verysimilar chemical compositions, grading, reactive silica con-tents and so on. They dier chiey with respect to theircrystalline phase content, which is obviously closely relatedto their silica and vitreous alumina contents (see Table 1).

A comparison of the 29Si NMR spectra of the two yashes activated with 8 M NaOH (see Fig. 4, samples LAcand MAc) shows that the spectrum for sample LAc has ahigher degree of reaction than sample MAc; in otherwords, the former has a more orderly and more thermody-namically stable structure. Nonetheless, the deconvolutionof the two spectra reveals that the basic signals are thesame in both, although their intensity varies (see Table5). This means that the silicon and aluminium tetrahe-dra occupy the same positions in both materials as theydo in the alkaline aluminosilicate gel, but in dierentproportions.

The spectrum signals most clearly aected by the 1:20HCl acid solution attack are those appearing at 104,98, 94 and 88 ppm; i.e., the signals associated withSi environments surrounded by 1, 2, 3 and 4 aluminiumatoms [6,15,16]. Table 6 presents the area in per cent underthese peaks (materials LAcHCl and MAcHCl) after nor-malization to take account of the insoluble residue remain-ing after the samples are attacked with HCl (calculationsbased on Eq. (1)). The table shows, among other thingsthat over 50% of the above signals are aected by the dis-solution of the material in an acid medium, and may there-fore be associated with the reaction products. In any event,attention is drawn to the fact that, unlike the resultsobtained with XRPD (Rietveld quantication), the infor-mation in Table 6 is semiquantitative only, given the limi-tations of the NMR technique.

If it is assumed, then, that the signals appearing at 104,

Chabazite Sodalite Amorphousa Total

18.2 0.7 67.1 10023.39 35.44

22.47 30.32

3.0 1.2 67.4(3) 99.1 + 0.9c = 100 30.30 60.96

f reaction.

uel 85 (2006) 19601969 196798, 94 and 88 are due primarily to the reaction prod-ucts formed during the alkali activation of the ash, the Si/Al ratio in the gel (zeolite precursor) formed can be deter-mined from the Engelhardt equation (2) [15]. According tothat procedure, ash L activated with an 8 M NaOH solu-tion and cured for 7 days at 85 C generates an alkalinealuminosilicate with an Si/Al ratio of 1.71, while ash Msubjected to identical treatment forms a silicoaluminatewith an Si/Al ratio of 1.56

Engelhardt equation Si=AlRMN P4

n0ISinAlP4

n0n4ISinAl

n 0; 1; 2; 3; 4 2

shows, it is absolutely indispensable for a certain amountof Al to be initially present in the system for the rst bondsto form.

This fact explains why ash M, with a smaller reactiveAl2O3 content (14.1%), has a lower degree of reactionthan ash L (20.5%, see Table 7).

Finally, the eectiveness of the methods used in the pres-ent study to determine the degree of reaction of y ash inalkaline media can be inferred from the data in Table 7.This table summarizes the a values obtained with the threetechniques used: selective acid attack, Rietveld XRD quan-tication and 29Si NMR (the calculations based on theNMR data are performed using the Si/Al ratio found withEngelhardt equation (Eq. (2)) and assuming in all cases

. / Fwhere ISi(nAl) is the intensity of the Si-associated compo-nent surrounded by nAl and (4-n)Si.

From the standpoint of the control mechanisms prevail-ing at any given time, there are several stages to the chem-ical reaction that takes place to form this alkalinesilicoaluminate. Initially, it involves dissolution [2,6,21](the high concentration of OH ions in the alkaline med-ium severs the covalent SiOSi, SiOAl and AlOAlbonds present in the vitreous phase of the ash, releazingthe silicon and aluminium ions into the medium where theyform SiOH and AlOH groups). In a subsequent stagethese monosilicates and aluminates condense to form SiOAl and SiOSi bonds, giving rise to an alkaline alumi-nosilicate gel characterized by its three-dimensionalstructure.

The presence of the alkaline cations taken up by the sys-tem is essential to this chemical process, inasmuch as theycompensate for and balance out the electric imbalance gen-erated in the structure by the replacement of Si4+ with Al3+

atoms.Similar processes have been schematically described in

zeolite [2224], geocement [2,3], and geopolymer [8,18]chemistry. The dierence between zeolite synthesis andthe production of cement from alkali activated y ash liesessentially in some of the reaction conditions. When pow-dery y ash is mixed with a small volume of alkaline solu-tion, the paste formed quickly hardens into a solid. Undersuch circumstances, there is neither sucient time norspace for the gel (reaction product) to develop into the sort

Table 6Normalization of 29Si MAS-NMR spectra

Chemical shifts (1 ppm)

82 84 88 94LAc 2.36 5.98 11.78 24.25LAcHClNormalized IR=35.44% 1.80 3.16LAcLAcHClNormalized 2.36 5.89 9.98 21.09(%) Solubilized 100 100 84.7 87MAc 5.1 20.9 21.1MAcHClNormalized IR=61.34% 3.75 2.75MAcMAcHClNormalized 5.1 17.15 18.35(%) Solubilized 100 82.1 87.0

1968 A. Fernandez-Jimenez et alof well-crystallized structure generated during zeolite for-mation; rather, the product obtained is an alkaline alumi-nosilicate gel, also called a zeolite precursor andgeopolymer too.

Kinetically speaking, SiOAl bonds are favoured overSiOSi bonds in the synthesis of these materials, and asa result the Si/Al ratio available in the medium (i.e., dis-solved) plays an instrumental role in the formation ofone aluminosilicate gel or another (initially a three-dimen-sional structure is formed by the bonding of SiO4 or AlO4tetrahedra across oxygen atoms). The gel formed may,however, change in composition over time [6]. In fact, theSiOSi bonds are more thermodynamically stable thanthe SiOAl conguration, which is why zeolite precursorshave been observed to grow progressively richer in siliconwith time [6,25]. However, as the mechanism proposed

Table 7Summary of degrees of reaction

LAc MAc

aaHCl attack = 64.5 38.7aXRPD Rietveld quantication = 62.6 41.3r = (Si/Al)NMR 1.7 1.6bR = SiO2/Al2O3 1.9 1.8c[Al2O3]vitreous 20.5 14.1[SiO2]NMR = R [Al2O3]vitreous 39.7 24.7aNMR = [Al2O3]vitreous + [SiO2]NMR 60.1 39.1a Degrees of reaction.b R = SiO2/Al2O3 = 1.135 (Si/Al).c Al2O3 vitreous = Al2O3 content in glass phase of y ash. Value deter-

mined by XRPD Rietveld quantication (see [4]).

Total (%)

98 103 108 115 11831.62 19.01 5.00 100%4.56 5.90 9.42 8.19 2.39 35.4227.06 13.11 4.42 8.19 2.3985.6 69 27.0 16.2 9.7 1002.33 8.16 21.9 16.25 6.26 61.424.67 8.04 12.2 16.25 6.2691 50

uel 85 (2006) 19601969that all the reactive aluminium has fully reacted and thatthere is a surplus of silicon). As Table 7 shows, the resultsobtained with the three methods are in quite good agree-ment taken into account the errors associated to the dier-ent analyses, a nding that substantiates their validity.

5. Conclusions

1. The three methods used in the present studychemicalanalysis with selective solutions, Rietveld X-ray powderdiraction quantication and nuclear magnetic reso-nanceis eective to determine the degree of reactionof y ash in alkaline media. The committed error byusing any of the dierent methods is lower than 5%.However the most accurate data are obtained through

Rietveld and chemical attack. Nevertheless the mostimportant fact to be remarked is that the combined uti-lization of the three methods allows the quantication ofthe crystalline and amorphous reaction products formedas a consequence of the alkaline activation of y ashes.

2. A certain minimum percentage of reactive alumina mustbe present in the initial materials to set o the reactions.Fly ashes with similar reactive silica produce dierentreaction degrees because of a dierent content of reac-tive alumina. If the reactive alumina content is high(case of y ash L), a high degree of reaction is achievedand a high amount of crystalline zeolites are produced(18.9% for y ash L).

[8] Van Jaarsveld JGS, Van Deventer JSJ. Eect of the alkali metalactivator on the properties of y ash based geopolymers. Ind EngChem Res 1999;38(10):393241.

[9] Arjuan P, Silbee MR, Roy DM. Quantitative determination of thecrystalline and amorphous phases in low calcium y ashes. In:Proceeding of the 10th international congress of the chemistry ofcement, vol. 3, Gothenburg, Sweden, June 26 1997.

[10] Rietveld HM. A prole renement method for nuclear and magneticsstructures. J Appl Crystallogr 1969;2:6571.

[11] McCusker LB, Von Dreele RB, Cox DE, Louer D, Scardi P. Rietveldrenements guidelines. J Appl Crystallogr 1999;32:3650.

[12] Larson AC, Von Dreele RB. Los Alamos National LaboratoryReport No. LA-UR-86-748; 1994.

[13] Thompson P, Cox DE, Hasting JB. Rietveld renement of Debye

A. Fernandez-Jimenez et al. / Fuel 85 (2006) 19601969 1969Acknowledgements

Funding for this research was provided by the Director-ate General of Scientic Research under project COO-1999-AX-038; a post-doctoral contract associated withthe study was awarded by the CSIC conanced by theEuropean social bottom (REF. I3P-PC2004L). Theauthors wish to thank I. Sobrados and J. Sanz for theirhelp with the MAS-NMR studies.

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Quantitative determination of phases in the alkaline activation of fly ash. Part II: Degree of reactionIntroductionExperimental methodsCharacterization of raw materialsAlkali activation of fly ashSelective chemical attack: reference testsTechniquesXRPD

ResultsDiscussionConclusionsAcknowledgementsReferences