16601968

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Quantitative determination of phases in the alkaline activation

of ﬂy ash. Part II: Degree of reaction

A. Ferna´ndez-Jime´nez

a,*

, A.G. de la Torre

b

, A. Palomo

a,*

, G. Lo´pez-Olmo

b

,

M.M. Alonso

a

, M.A.G. Aranda

b

a_{Eduardo Torroja Institute (CSIC), c/Serrano Galvache, n}

4, 28080, Madrid, Spain

b_{Department of Inorganic Chemistry, University of Mala´ga, Campus Teations s/n, 29071, Mala´ga, Spain}

Received 7 March 2006; received in revised form 11 April 2006; accepted 13 April 2006 Available online 15 May 2006

Abstract

A working procedure was developed for determining the degree of reaction of fly ash subjected to alkali activation (with 8 M NaOH) at mild temperatures. Since the reaction products dissolve in HCl, the residue left after this acid attack contains only the fraction of the original ash that failed to react with the basic solution. This residue was analysed with Rietveld XRPD quantification and NMR and the findings were compared to the results of the analyses run on the activated ash to obtain a very precise quantification of all of the (crys-talline, vitreous and amorphous) phases present in the systems studied.

Keywords: Fly ash; Alkali-activation; Rietveld

1. Introduction

The present paper describes and interprets experimental data gathered to quantify the phases formed as a result of the alkali activation of fly ash (a powdery by-product of the coal-fired steam generation of electric power). The study discussed here is actually a logical extension of research begun some time ago [1] aiming to establish a procedure for determining the reactive capacity of fly ash when mixed with a highly concentrated alkaline medium in the produc-tion of ‘‘alkaline cements’’[2,3]. In a previous paper[4], the authors described a methodology for quantifying the crys-talline and vitreous components of fly ash with different analytical and instrumental techniques (selective chemical attack), X-ray power diffraction (XRPD), and magic-angle spinning nuclear magnetic resonance (MAS-NMR). That research showed, among other things, the effectiveness of the various techniques used to reach the objectives

pur-sued. The study likewise provided a fuller understanding of the vitreous phase (fundamental element that controls alkali reactivity) of ﬂy ash, as well as a very precise method for determining its SiO2and Al2O3content.

Any further research in this direction necessitated the exploration of ways to identify and quantify the reaction products forming during the alkali activation of ﬂy ash. And that, essentially, is the speciﬁc objective of the present study. 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 method and MAS-NMR).

A technique similar to the HCl attack used here was proposed earlier by Granizo et al.[5]. Their procedure con-sisted in dissolving the reaction products obtained in the alkali activation of metakaolin in HCl 1:9 (solution in vol-ume) followed by an attack with 5% Na2CO3solution. The

method has more recently been modiﬁed to optimize results

[6], and now consists of a 1:20 HCl attack at laboratory temperature.

Finally, the alkali activation of ﬂy ash can be brieﬂy described as a physical–chemical process in which this

*

Corresponding authors.

E-mail address:[email protected](A. Ferna´ndez-Jime´nez).

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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< 100C) for a short period of time to produce a material with good binding properties[7,8]. The main reaction product formed in this process is an X-ray amorphous alkaline aluminosil-icate gel characterized by somewhat limited short-range structural order [6]. In addition, Na-Herschelite-type zeo-lites and hydroxysodalite are formed as secondary reaction products[1,6].

2. Experimental methods

2.1. Characterization of raw materials

Two different fly ashes type F were used in this study. The chemical compositions as well as, the amount of fly ash retained in different sieves were given in the previous paper [4].Table 1 is a summary of data, concluded from that mentioned previous paper, concerning the chief com-ponents of fly ashes of the two types of ash: reactive silica, alumina and vitreous silica content, as well as the majority crystalline phase (quartz and mullite). The differences between thereactiveSiO2andvitreousSiO2shown inTable

1 might be due to the very diﬀerent analytical techniques used for the quantiﬁcation (in principle reactive and vitre-ous silica should be the same):reactive SiO2is determined

by chemical attack (quartz and mullite could be slightly attacked during this process), whilevitreousSiO2is

calcu-lated by XRPD (see Ref.[4]).

2.2. Alkali activation of ﬂy ash

The two types of ash studied were activated with an 8 M NaOH solution having ‘‘alkaline solution/ash’’ ratios of 0.4 and 0.56 in mass for ﬂy ashes L and M, respectively (the dif-ferent ‘‘alkaline solution/ash’’ ratios used had the purpose of getting the same workability values in the pastes). The paste obtained was placed in air-tight plastic bags and kept at 85C 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 85C), the alkali activated ﬂy ash was attacked with 1:20 HCl to determine the amount

of ash that had been converted to ‘‘cement’’ and the por-tion that had not reacted with the alkaline solupor-tions; in short, to determine the degree of reaction (a), since this attack provokes the dissolution of the chief reaction prod-ucts of the alkali activation of ﬂy ash (alkaline aluminosil-icate gel and zeolites) in the acid, while the fraction of ash not 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.

The experimental procedure followed in the acid attack consisted in adding 1 g of activated fly ash to a beaker con-taining 250 ml of (1:20) HCl. The mixture was stirred with a plastic rotor for three hours, after which it was filtered and washed with de-ionized water to a neutral pH. The insoluble residue was first dried at 100C and then calcined at 1000C; the degree of reaction,a, was found by deter-mining weight loss. These trials were repeated at least three times to guarantee reproducibility.

In addition, one of the residues obtained after attacking the sample of activated ﬂy ash L with 1:20 HCl (but not calcinated at 1000C) was re-activated with 8 M NaOH for three days at 85C. The material so obtained had been then subjected to all the steps mentioned before for the chemical attack performed after the ﬁrst activation process, these are: acid attack, washing process, drying and calcina-tions process.

2.4. Techniques

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

2.4.1. XRPD

Sample preparation[4]. Standard a-Al2O3was

synthe-sized as follows: 6 g of c-Al2O3(99.997% from Alfa) was

ground in an agate ball mill at 200 rpm for 30 min. The resulting powder was placed in a Pt crucible and heated at 1200C for 4 h. The oxide was allowed to cool to 150C over 5 h and ground at room temperature in an agate mortar for 5 min. The sample was subjected to a sec-ond thermal treatment at 1300C for 6 h and then cooled as above. This standard was then ground in an agate mor-tar for 5 min and sieved (<35lm) prior to weighing. Each of the mixtures used for analysis was carefully ground in

Table 1

Main components of ﬂy ashes

Reactive SiO2 a

(%) Vitreous phaseb Vitreous phasec Vitreous SiO2 c

Vitreous Al2O3 d

(%) Quartzc(%) Mullitec(%)

Fly Ash L 42.17 64.94 80.6 40.53 20.45 6.3 11.7

Fly Ash M 45.07 54.28 65.7 40.05 14.11 10.8 22.6

a

Value determined as speciﬁed 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 quantiﬁcation (see[4]). d

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an agate mortar for 10 min, adding acetone to facilitate particle dispersion, and then heated at 60C. Around 30% (wt) of corundum was added, with the exact quantity recorded for each measurement. The samples were gently loaded (vertically) onto an aluminium X-ray sample holder.

X-ray data collection [4]. Laboratory XRPD measure-ments were performed for all mixtures on a Siemens D5000 automated diﬀractometer with Cu Ka1,2 radiation

(1.5418 A˚ ) and a secondary curved graphite monochroma-tor. The readings were taken in vertical Bragg–Brentano

(h/2h) geometry (flat reflection mode) between 18 and 70(2h) at 0.03steps, measuring 15 s per step. The sam-ples were rotated at 15 rpm during acquisition to improve powder averaging, which is essential to have accurate intensities and hence good phase analysis. The D5000 dif-fractometer optic consisted of a system of primary Soller foils between the X-ray tube and the 2-mm fixed aperture slit. One 2-mm scattered-radiation slit was placed down-stream of the sample, followed by a system of secondary Soller slits and a 0.2-mm detector slit. The X-ray tube oper-ated at 40 kV and 30 mA.

2-Theta, deg

Counts

20.0 30.0 40 50.0 60.0 70.0

X10E 3

-2.0

0.0

2.0

4.0

6.0

8.0

SiO

2

Al

2

O3

Mu

Mu;Ma

Mu;chabazite

Al

2

O3

Al

2

O3

Al

2

O3

Al

2

O3

Al

2

O3

Al

2

O3

Al

2

O3

chabazite chabazite

chabazite

chabazite chabazite chabazite;Ma

sodalite

(a) LAc

40.0

20.0 30.0 40.0 50.0 60.0 70.0

--2.0

0.0

2.0

4.0

6.0

8.0

Al

2

O3

Al

2

O3

;

Mu

Sodalita

Mu

Si

O2

Al

2

O3

Al

2

O3

Al

2

O3

Al

2

O3

Al

2

O3

SiO

2

Mu

Mu;Ma Al

2

O3

CaCO

3

Chabacita Chabacita;CaCO

3

Chabacita;Mut

Mu

(b) MAc

Si

O2

Fig. 1. Rietveld plots (18–70/2h) showing the observed (crosses) and calculated (solid line) powder patterns, and the difference between them (bottom line), for fly ash activated with 8 M NaOH and cured at 85C for 7 days: (a) sample LAc and (b) sample MAc. The marks denote the Bragg peaks of the different phases; the main diffraction peaks of each phase are highlighted (a-Al2O3was used as standard).

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X-ray data analysis. Rietveld refinement was used to analyse the powder patterns [10,11], applying GSAS soft-ware[12] with a pseudo-Voigt function[13]and including a correction for asymmetry due to axial divergence [14]. The crystal structures used to calculate the powder patterns were taken from the Inorganic Crystal Structure Database (ICSD). The collection codes for the various structures were: corundum 73725; a-quartz 63532; mullite 66263; maghemite 87119; calcite 80869; albite 68193; chabazite 201584 and sodalite 72059. Neither the positional nor the thermal vibration parameters were refined. The parameters optimized were: background coefficients, cell parameters, zero-shift error, peak shape parameters (including aniso-tropic terms if needed), and phase fractions.

29

Si MAS-NMR.29Si MAS-NMR spectroscopic

charac-terization was conducted with a Bruker apparatus, model MSL-400. The resonance frequency used in this study was 79.5 MHz and the spinning rate was 4 kHz. The measure-ments were taken at laboratory temperature with TMS as the external standard. The error in the chemical shift values was estimated to be lower than 1 ppm. Magnetic materials were removed from the samples prior to NMR spectra recording by exposing the sample to a strong magnetic ﬁeld.

3. Results

The results of characterizing the ﬂy ash used in this research have been discussed in previous publications

[1,4] and are therefore not included here. Nonetheless, a brief description is provided below of some of the similar-ities and differences 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 and M, have similar elemental chemical characteristics (see Table 1 of the previous part I[4]) and even similar particle size distributions (see Table 2 of the previous part I [4]). They differ substantially, however, in terms of their respec-tive mineralogical compositions (seeTable 1). Ash L, for instance, has a mullite content of approximately 12%, and a 7% quartz content, while ash M contains 22% mullite and 11% quartz. These differences in mineralogical compo-sition have a sizeable impact on the reactive alumina (vitre-ous alumina) content of the ash; and this in turn is closely associated with the intensity of alkali activation.

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

denominated LAc and MAc). Alkali activation transforms the ﬂy 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 the halo from the original ash[4], it is shifted towards slightly higher 2h values. The alkaline activation of the ash also generates new crystalline phases.

Such new crystalline phases formed during the fly ash reaction were identified as Na-Herschelite-type zeolites (majority) and hydroxysodalite. These phases were identi-fied by comparing the signals on the diffractograms with the reported patterns of the phases in the PDF database. Since there is no structural description of Na-Herschelite (formula: NaAlSi2O6Æ3H2O) in the ICSD (Inorganic

Crys-tal Structure Database), the structure of the Na-chabazite crystal (a zeolite practically identical to Na-Herschelite) was used in the Rietveld analysis. The stoichiometry used for the sodium chabazite was that described in ICSD record 201584: Na0.92Al0.92Si2.081O6Æ3.25H2O. Similarly,

ICSD record 72059, which describes a sodalite with the empirical formula Na2(Al1.5Si1.5O6)Æ0.5(OH)Æ0.5H2O

was used for analyzing the pattern of hydroxysodalite.

Table 2gives the results obtained with XRPD quantiﬁ-cation of the diﬀerent mineral phases present in the materi-als (including the amorphous fraction) by using the Rietveld method.

Once characterized, cements LAc and MAc were attacked with a 1:20 HCl solution to dissolve the reaction products formed in the alkali activation process (alkaline aluminosilicate gel + zeolites). The results of this attack are shown in Table 3.

Table 2

XRPD Rietveld quantiﬁcation of activated ash and 1:20 HCl-insoluble residues

Sample Quartz (%) Mullite (%) Maghemite (%) Chabazite (%) Sodalite (%) CaCO3(%) Amorphous (%)a

LAc 4.4 8.7 0.9 18.2 0.7 – 67.1

LAcHCl 9.2 23.9 0.9 – – – 66.0

LAc2HCl 6.4 17.7 1.8 – – – 74.1

MAc 8.9 18.2 0.4 3.0 1.2 0.9 67.4

MAcHCl 15.9 33.7 0.4 – – – 49.4

a _{Amorphous = vitreous phase from ﬂy ash + amorphous phase formed during the activation process.} Table 3

Results of 1:20 HCl cold acid attack

Name Characteristic (%) Insoluble in HCl

(%) Soluble = reaction degreea

LAca NaOH-activated L

LAcHCl Attacked with HCl 35.44 64.5 LAc2b _{NaOH-activated LAcHCl}

LAc2HCl LAc2 attacked with HCl 85.60 (Ref.[2]) 14.4

Normalized to IR=35.44% 30.33 69.6

MAca NaOH-activated M

MAcHCl MAc attacked with HCl 61.34 38.66 a

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

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According to these results, the degree of reaction in ash L, when activated under the conditions speciﬁed above, is on the order of 64.5%, or around 25% higher than ash M. In addition, the insoluble residues obtained as a result of the HCl attack (presumably unreacted ash: vitreous phase + quartz + mullite) were characterized using the Rietveld reﬁnement technique (see Fig. 2, (a) sample LAcHCl and (b) MAcHCl).

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

its farthest limit (to quantify the effectiveness of the ash activation process). This residue was mixed with an 8 M NaOH solution (‘‘alkaline solution/solid’’ = 0.4) and the resulting paste was cured at 85C for 3 days. This sample (LAc2) was subsequently attacked with HCl acid. Accord-ing to the results shown inTable 3, the degree of reaction in this second attack was 14.4%, for a rise in the degree of reaction of approximately 5% (14.4·35.44/100). This means that the conditions for the first alkali activation pro-cess (7 days at 85C) were sufficiently intense for most of

2-Theta, deg

Counts

20.0 30.0 40.0 50.0 60.0 70.0

X10E 3

-2.0

0.0

2.0

4.0

6.0

8.0

S

iO

2

M

a

A

l2

O3

S

iO

2

M

u

M

u;M

a

M

u

M

u

Al

2

O3

A

l2

O3

A

l2

O3

A

l2

O3

A

l2

O3

;

Mu Al

2

O3

Al

2

O3

M

u

(a) LAcHCl

Al

2

O3

A

l2

O3

Al

2

O3

2-Theta, deg

Counts

X10E 4

0.0

0.5

1.0 SiO

2

A

l2

O3

S

iO

2

M

u

Mu;Ma

M

u

M

u

Al

2

O3

A

l2

O3

A

l2

O3

A

l2

O3

A

l2

O3

;

M

u Al

2

O3

Al

2

O3

Mu

(b) MAcHCl

Cri

st

2

l2

3

2

3

2

l

23

Fig. 2. Rietveld plots (18–70/2h) showing the observed (crosses) and calculated (solid line) powder patterns, and the difference between them (bottom line), for the HCl-insoluble residues in the activated ash: (a) sample LAcHCl and (b) sample MAcHCl. The marks denote the Bragg peaks of the different phases; the main diffraction peaks of each phase are highlighted (a-Al2O3was used as standard).

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the vitreous phase of the ﬂy ash to react. After this second attack, the total degree of reaction came to 69.6%, while the insoluble residue amounted to 30.33%. Given that the crystalline phases in the original ash (quartz, mullite, and so forth, which accounted for 20% of the total material) are inert in the alkali activation process, it may be inferred that, after the second acid attack, the degree of reaction of the total ash liable to alkali activation came to 86%.

As in the preceding cases, the HCl-insoluble residue from the LAc2 sample was characterized using Rietveld XRPD quantiﬁcation. The pattern obtained is shown inFig. 3.

Fig. 4 gives the results of 29Si MAS-NMR analysis of the alkali activated ashes (LAc and MAc) and their HCl-insoluble residues (LAcHCl and MAcHCl). The spectra for the initial ﬂy ashes are likewise shown on the ﬁgure for readier interpretation of the results. The most promi-nent feature on these29Si NMR spectra for the initial ashes is 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 NaOH for 7 days are more distinct. The results of their deconvo-lution, conducted in accordance with the constant band width criterion, are shown in Table 4. The well-deﬁned peaks appearing on these spectra at104, 99, 94 and 88 ppm are associated with the presence of Q4(nAl) units

[15], wheren= 1, 2, 3, 4. In addition, the signals appearing at values greater than or equal to108 ppm were associ-ated with Q4(0Al) units, which can in turn be attributed to the unreacted fraction of the ash (especially quartz). Finally, the signal appearing at84 ppm may correspond to possible residues of the products formed during alkali

activation. Alternatively, it may be attributed to hydroxy-sodalite [16], a mineral which, despite its characteristically zeolitic composition, does not exhibit the properties typi-cally associated with these materials. Hydroxysodalite has a signal at84.8 ppm.

The spectra for the insoluble residues, however, have a wider signal centred at 108 ppm associated with Q4(0Al) units which, as mentioned above, are indicative of the presence of quartz (a phase that is essentially inert to alkali activation). The deconvolution of this signal reveals the presence of other less intense signals associated with the mullite[17] contained in the initial ash (signal at 87/88 ppm) as well as with another phase or phases. While not wholly deﬁned (signals at 92, 98 and 103 ppm), these latter elements may correspond either to an unreacted fraction of the vitreous material or to a small group of incipient mullite or quartz crystals exhibit-ing low crystallinity.

4. Discussion

The alkali activation of ﬂy ash is a physical–chemical process that transforms a powdery ash into a material with good cementitious properties [1,6–8,18]: high mechanical strength, excellent bonding to reinforcement steel [19], and so on. The interaction of the ash during this process generates an alkaline aluminosilicate gel as the main reac-tion product. This three-dimensional, XRPD-amorphous compound may be regarded to be a ‘‘zeolite precursor’’. Crystalline zeolites such as Na-Herschelite and hydroxy-sodalite are also found to appear as secondary reaction products.

2-Theta, deg

Counts

20.0 30.0 40.0 50.0 60.0 70.0

X10E 3

-2.0

0.0

2.0

4.0

6.0

8.0

Al

2

O3

Al

2

O3

; Mu

Al

2

O3

Mu Mu Mu

Al

2

O3

Al

2

O3

Si

O2

Al

2

O3

Si

O2

Mu

;M

a

Al

2

O3

Ma

g

Mu

;M

ag

Al

2

O3

LAc2HCl

3

Fig. 3. Rietveld plots (18–70/2h) showing the observed (crosses) and calculated (solid line) powder patterns, and the difference between them (bottom line), for the HCl-insoluble residue in the sample of ash L after alkali re-activation, LAc2HC. The marks denote the Bragg peaks of the different phases; the main diffraction peaks of each phase are highlighted (a-Al2O3was used as standard).

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The 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). A comparison of the patterns inFigs. 1 and 2readily shows that the reaction products disappear after the acid attack,

leaving a residue that contains only the unreacted phases of the original ﬂy ashes.

The results of quantifying the various phases found in samples LAc, MAc, LAcHCl, LAc2HCL and MAcHCl are given inTable 5. These values (the ones corresponding

Fig. 4.29Si MAS-NMR spectra of (a) the initial ash; (b) 8 M NaOH-alkali activated ash cured at 85C for 7 days; (c) the HCl-insoluble residue.

Table 4

Results of deconvolution of29_{Si MAS-NMR spectra for L and M ﬂy ash and alkali activated ﬂy ash pastes}

Sample 82 84 88 94 98 104 108 112 118

L Pos. (ppm) 84 88.3 93.6 98.6 103.4 108 115

Width 7.15 7.15 7.15 7.15 7.15 7.15 7.15

Integration (%) 5.32 11.0 16.98 20.66 19.46 20.9 5.61

LAc Pos. (ppm) 81.18 84.3 88.5 93.7 98.9 104.5 109

Width 3.62 3.62 3.62 3.62 3.62 3.62 3.62

Integration (%) 2.36 5.98 11.78 24.25 31.62 19.01 5.00

LAcHCl Pos. (ppm) 87 93.7 99.2 103.5 108 112 118

Width 6.43 6.43 6.43 6.43 6.43 6.43 6.43

Integration (%) 5.10 8.93 12.86 16.66 26.59 23.12 6.75

M Pos. (ppm) 89 95.2 99.4 104 109 116

Width 8.52 8.52 8.52 8.52 8.52 8.52

Integration (%) 14.25 7.76 9.15 17.58 34.17 16.84

MAc Pos. (ppm) 82.7 87.6 92.5 98.11 103.6 108.5

Width 4.93 4.93 4.93 4.93 4.93 4.93

Integration (%) 5.1 20.9 21.1 27.0 16.2 9.7

MAcHCl Pos. (ppm) 87.65 92.63 97.60 102.7 108.3 113 117

Width 5.70 5.70 5.70 5.70 5.70 5.70 5.70

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to the samples attacked with HCl) were normalized to the insoluble residue content with Eq.(1).

FR¼ ðFXRPDIRHClÞ=100 ð1Þ

whereFR= residual phase (HCl-insoluble phase);FXRPD=

XRPD-quantiﬁed phase in % (see Table 2); IRHCl=

HCl-insoluble residue (seeTable 3).

In the LAc and MAc samples, what is quantiﬁed as the amorphous phase is the sum of the unreacted vitreous frac-tion of the ash plus the aluminosilicate gel formed during alkali activation with sodium hydroxide. Subtracting the non-crystalline fraction of the samples attacked with HCl (fraction associated only with the vitreous phase of the ash) from that sum gives an estimate of the amount of alu-minosilicate gel formed. For LAc this value would be 67.1– 23.4 = 43.7. Adding this sum to the amount of zeolite formed (18.9%) yields the total percentage of reaction products (degree of reaction,a) or 62.6%, which is practi-cally the same as the value obtained with the HCl attack procedure (LAcHCl, a= 64.5, see Table 3). For ash M the results are 37.1% of alkaline aluminosilicate gel and a 41.3% (37.1 + 3.0 + 1.2) degree of reaction or a, which is also quite similar to that measured directly with the HCl attack method, 38.5%.

In the ash L sample subjected to double activation (LAc2) to attain the maximum degree of reaction in the system, only a small amount of amorphous material is observed to react after the second alkali activation (see

Table 5). In other words, the reaction is essentially com-pleted when 20% of the non-crystalline phase found in the original ash is still ‘‘intact’’. Table 5 also shows that the quartz and mullite content is clearly lower than in the initial ash[4], suggesting that both minerals may undergo partial attack in the aggressive conditions prevailing in this reaction. This ﬁnding is consistent with previous research by the authors[20], in which a scanning electronic micro-scopic study detected alterations in the surface texture of mullite crystals, indicative of an attack by the surrounding alkaline medium.

Another important fact deduced from the analysis of these results is that ash L is more reactive than ash M under the conditions in which the present study was con-ducted, despite the fact that the two types of ash have very similar chemical compositions, grading, reactive silica con-tents and so on. They diﬀer chieﬂy with respect to their

crystalline phase content, which is obviously closely related to their silica and vitreous alumina contents (seeTable 1). A comparison of the 29Si NMR spectra of the two ﬂy ashes activated with 8 M NaOH (seeFig. 4, samples LAc and MAc) shows that the spectrum for sample LAc has a higher degree of reaction than sample MAc; in other words, the former has a more orderly and more thermody-namically stable structure. Nonetheless, the deconvolution of the two spectra reveals that the basic signals are the same in both, although their intensity varies (see Table 5). This means that the silicon and aluminium tetrahe-dra occupy the same positions in both materials as they do in the alkaline aluminosilicate gel, but in diﬀerent proportions.

The spectrum signals most clearly affected by the 1:20 HCl acid solution attack are those appearing at 104, 98, 94 and 88 ppm; i.e., the signals associated with Si environments surrounded by 1, 2, 3 and 4 aluminium atoms[6,15,16].Table 6presents the area in per cent under these peaks (materials LAcHCl and MAcHCl) after nor-malization to take account of the insoluble residue remain-ing after the samples are attacked with HCl (calculations based on Eq. (1)). The table shows, among other things that over 50% of the above signals are affected 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 results obtained with XRPD (Rietveld quantification), the infor-mation inTable 6is semiquantitative only, given the limi-tations of the NMR technique.

If it is assumed, then, that the signals appearing at104, 98,94 and88 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 to that procedure, ash L activated with an 8 M NaOH solu-tion and cured for 7 days at 85C generates an alkaline aluminosilicate with an Si/Al ratio of 1.71, while ash M subjected to identical treatment forms a silicoaluminate with an Si/Al ratio of 1.56

Engelhardt equation ðSi=AlÞ_RMN¼

P4

n¼0ISiðnAlÞ P4

n¼0n4ISiðnAlÞ

n¼0;1;2;3;4 ð2Þ

Table 5

Normalization of XRPD Rietveld results

Sample Quartz Mullite Maghemite Chabazite Sodalite Amorphousa Total

LAc 4.4 8.7 0.9 18.2 0.7 67.1 100

LAcHClbIR = 35.44a= 64.5 3.26 8.47 0.32 23.39 35.44

LAc2HClbIR = 30.33a= 69.6 1.94 5.37 0.54 – – 22.47 30.32

MAc 8.9 18.2 0.4 3.0 1.2 67.4(3) 99.1 + 0.9c_{= 100}

MAcHClb_{IR = 61.34}_a_{= 38.66} _9.75 _20.67 _0.24 _– _– _30.30 _60.96

a _{Amorphous = vitreous phase in ﬂy ash + amorphous zeolite precursor.}

b _{Values in italics represent total ash content recalculated from the degree of reaction.} c

(9)

where ISi(nAl) is the intensity of the Si-associated

compo-nent surrounded bynAl 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 alkaline silicoaluminate. Initially, it involves dissolution [2,6,21]

(the high concentration of OHions in the alkaline med-ium severs the covalent Si–O–Si, Si–O–Al and Al–O–Al bonds present in the vitreous phase of the ash, releazing the silicon and aluminium ions into the medium where they form Si–OH and Al–OH groups). In a subsequent stage these monosilicates and aluminates condense to form Si– O–Al and Si–O–Si bonds, giving rise to an alkaline alumi-nosilicate gel characterized by its three-dimensional structure.

The presence of the alkaline cations taken up by the sys-tem is essential to this chemical process, inasmuch as they compensate 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 [22–24], geocement [2,3], and geopolymer [8,18]

chemistry. The difference between zeolite synthesis and the production of cement from alkali activated fly ash lies essentially in some of the reaction conditions. When pow-dery fly ash is mixed with a small volume of alkaline solu-tion, the paste formed quickly hardens into a solid. Under such circumstances, there is neither sufficient time nor space for the gel (reaction product) to develop into the sort of well-crystallized structure generated during zeolite for-mation; rather, the product obtained is an alkaline alumi-nosilicate gel, also called a ‘‘zeolite precursor’’ and ‘‘geopolymer’’ too.

Kinetically speaking, Si–O–Al bonds are favoured over Si–O–Si bonds in the synthesis of these materials, and as a result the Si/Al ratio available in the medium (i.e., dis-solved) plays an instrumental role in the formation of one aluminosilicate gel or another (initially a three-dimen-sional structure is formed by the bonding of SiO4or AlO4

tetrahedra across oxygen atoms). The gel formed may, however, change in composition over time[6]. In fact, the Si–O–Si bonds are more thermodynamically stable than the Si–O–Al conﬁguration, which is why zeolite precursors have been observed to grow progressively richer in silicon

with time [6,25]. However, as the mechanism proposed shows, it is absolutely indispensable for a certain amount of Al to be initially present in the system for the ﬁrst bonds to form.

This fact explains why ash M, with a smaller reactive Al2O3 content (ﬃ14.1%), has a lower degree of reaction

than ash L (ﬃ20.5%, seeTable 7).

Finally, the effectiveness of the methods used in the pres-ent study to determine the degree of reaction of fly ash in alkaline media can be inferred from the data in Table 7. This table summarizes theavalues obtained with the three techniques used: selective acid attack, Rietveld XRD quan-tification and 29Si NMR (the calculations based on the NMR data are performed using the Si/Al ratio found with Engelhardt equation (Eq. (2)) and assuming in all cases that all the reactive aluminium has fully reacted and that there is a surplus of silicon). AsTable 7shows, the results obtained with the three methods are in quite good agree-ment taken into account the errors associated to the differ-ent analyses, a finding that substantiates their validity.

5. Conclusions

1. The three methods used in the present study—chemical analysis with selective solutions, Rietveld X-ray powder diffraction quantification and nuclear magnetic reso-nance—is effective to determine the degree of reaction of fly ash in alkaline media. The committed error by using any of the different methods is lower than 5%. However the most accurate data are obtained through

Table 7

Summary of degrees of reaction

LAc MAc

a_a

HCl attack= 64.5 38.7

aXRPD Rietveld quantiﬁcation= 62.6 41.3

r= (Si/Al)NMR 1.7 1.6

b_{R = SiO}

2/Al2O3 1.9 1.8

c_[Al

2O3]vitreous 20.5 14.1

[SiO2]NMR= R·[Al2O3]vitreous 39.7 24.7 aNMR= [Al2O3]vitreous+ [SiO2]NMR 60.1 39.1

a _{Degrees of reaction.} b_{R = SiO}

2/Al2O3= 1.135 (Si/Al). c_Al

2O3vitreous= Al2O3content in glass phase of ﬂy ash. Value

deter-mined by XRPD Rietveld quantiﬁcation (see[4]). Table 6

Normalization of29Si MAS-NMR spectra

Chemical shifts (±1 ppm) Total (%)

82 84 88 94 98 103 108 115 118

LAc 2.36 5.98 11.78 24.25 31.62 19.01 5.00 100%

LAcHClNormalized IR = 35.44% 1.80 3.16 4.56 5.90 9.42 8.19 2.39 35.42

LAc–LAcHClNormalized 2.36 5.89 9.98 21.09 27.06 13.11 4.42 8.19 2.39

(%) Solubilized 100 100 84.7 87 85.6 69 – – –

MAc 5.1 20.9 21.1 27.0 16.2 9.7 100

MAcHClNormalized IR = 61.34% 3.75 2.75 2.33 8.16 21.9 16.25 6.26 61.4

MAc–MAcHClNormalized 5.1 17.15 18.35 24.67 8.04 12.2 16.25 6.26

(10)

Rietveld and chemical attack. Nevertheless the most important fact to be remarked is that the combined uti-lization of the three methods allows the quantification of the crystalline and amorphous reaction products formed as a consequence of the alkaline activation of fly ashes. 2. A certain minimum percentage of reactive alumina must be present in the initial materials to set off the reactions. Fly ashes with similar reactive silica produce different reaction degrees because of a different content of reac-tive alumina. If the reacreac-tive alumina content is high (case of fly ash L), a high degree of reaction is achieved and a high amount of crystalline zeolites are produced (18.9% for fly ash L).

Acknowledgements

Funding for this research was provided by the Director-ate General of Scientiﬁc Research under project COO-1999-AX-038; a post-doctoral contract associated with the study was awarded by the CSIC coﬁnanced by the European social bottom (REF. I3P-PC2004L). The authors wish to thank I. Sobrados and J. Sanz for their help with the MAS-NMR studies.

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