the role played by reactive alumina.pdf

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The role played by the reactive alumina content inthe alkaline activation of fly ashes

A. Fernandez-Jimenez a,*, A. Palomo a, I. Sobrados b, J. Sanz b

a Eduardo Torroja Institute (CSIC), P.O. Box 19002, 28080 Madrid, Spainb Instituto Ciencia de Materiales (CSIC), Cantoblanco, 28049 Madrid, Spain

Received 14 September 2005; received in revised form 24 October 2005; accepted 8 November 2005Available online 4 January 2006

Abstract

This study explores the relationship between the chemical composition of fly ashes and the microstructural characteristics andmechanical properties of the cementitious materials resulting from the alkali activation of fly ashes (AAFA). Three reactive systems wereprepared by mixing three F ashes, with an 8 M NaOH solution and stored at 85 C. The main reaction product formed in three systems isan amorphous alkaline aluminosilicate gel. This gel (zeolite precursor), has a three-dimensional framework, with Al occupying Al(4Si)and Si occurring in a variety of environments Q4(nAl). After short thermal activation periods (25 h), an Al-rich gel was formed withsilicon in tetrahedral Q4(4Al) units (intermediate phase), yielding low mechanical strengths. When the curing time increase (7 days) the gelchanges into a more stable Si-richphase with a greater mechanical strength. Finally, it has been shown that the amount of the reactivealuminium plays an important role in the aluminosilicate gel formation, from a kinetic point of view.2005 Elsevier Inc. All rights reserved.

Keywords: 29Si and 27Al MAS-NMR; Fly ash; Alkali activation; Pre-zeolite binder

1. Introduction

The alkali activation of alumino-silicate materials is achemical process that transforms partially or totally amor-phous, vitreous and/or metastable structures into compactcementitious skeletons[15]. As a result of the reaction thattakes place between fly ashes and alkalis under mild ther-mal conditions (6090 C), the major reaction product isan amorphous alkaline aluminosilicate gel [68]. The 29Si

MAS-NMR analysis[6]of these gels showed the formationof three-dimensional networks that constitutes the cementi-tious material that connects unreacted fly ash spheres. Inthis aluminosilicate gel the Si is found in a variety ofQ4(nAl) environments.

On the other hand there is a striking parallelism betweenthe reaction mechanism involved in zeolite formation fromalkali activation metakaolins and that regulating the alkali

attack of type F fly ashes. In this regard, Davidovits [9]andPalomo et al. [6,7] concluded that the long term formedproducts are ultimately related to zeolite phases. The essen-tial difference between the two types of material lies in theproperties they exhibit: the final product of zeolite synthe-sis is an adsorbent, catalytic powder, whereas the materialobtained with AAFA is cementitious, with high mechanicalstrength and considerable stability.

Recent surveys have shown that the Si/Al ratio of the

aluminosilicate gel obtained from the alkali activation offly ashes depends heavily on the chemical composition ofthe starting material, nature and concentration of alkaliactivator, synthesis temperature, and thermal curing time[6,7,1013]. Nonetheless, many questions persist aboutthe reactivity of fly ashes in strong alkaline environments.For this reason, the aim of our study has been the analysisof the formation of different pre-zeolite gels as a function ofthe curing time for different fly ashes. For this purpose, anumber of different techniques have been used for thestructural and microstructural characterization of prepared

1387-1811/$ - see front matter 2005 Elsevier Inc. All rights reserved.

doi:10.1016/j.micromeso.2005.11.015

* Corresponding author. Tel.: +34 91 302 0440; fax: +34 91 302 6047.E-mail address:[email protected](A. Fernandez-Jimenez).

www.elsevier.com/locate/micromeso

Microporous and Mesoporous Materials 91 (2006) 111119
mailto:[email protected]:[email protected]


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materials. Taken into account the vitreous nature of start-ing ashes as well as the amorphous characteristic of theformed gels, the MAS-NMR spectroscopy is particularlyadapted to this work.

2. Experimental

2.1. Characterization of the starting materials

Three fly ash materials (called P, L and M) from threedifferent Spanish steam power plants were used in thisstudy. All three were F type ashes (ASTM classification)consisting primarily of SiO2and Al2O3. Chemical composi-tions (determined according to Spanish Standard UNE 80-215-88), the reactive silica (determined according to UNE80-225-93), and the reactive alumina contents (determinedaccording to Ref.[14,15]), are shown inTables 1 and 2. InTable 2, the relative amount of the vitreous phase in threeashes is also given.

2.2. Alkali activation of the ash

The three ashes were activated with an 8 M solution ofNaOH. The solution/ash ratio used in each case waschosen to obtain a paste ofstandard consistency (UNEEN 196-3); i.e., 0.33, 0.4 and 0.56 by mass/weight for ashesP, L and M, respectively. The pastes obtained were cured at85 C and 98% relative humidity for different periods oftime (2, 5, 8 and 20 h and 7 days). After each experiment,the material was removed from the stove, cooled to labora-tory temperature, ground and then mixed with small

amounts of acetone to prevent the activation progress.

2.3. Chemical attack

Alkali activated materials were subjected to an acidattack with a 1:20 solution of HCl to determine the degreeof reaction attaint in each sample (Table 3). The acid solu-tion dissolves the reaction products formed by alkalineactivation of ashes (aluminosilicate gel and zeolites) butdoes not interact significantly with the unreacted fly ash[6,16]. After filtering, the concentration of the Al dissolvedwas determined by ICP-MS on a Spectromass 2000 instru-ment (Table 4).

2.4. Techniques

All materials were characterized with XRD, 29Si and27Al MAS-NMR and SEM/EDX. X-ray diffractograms

of powder samples were recorded on a Philips diffractome-ter PW 1730, with Cu Ka radiation. Magnetic materialswere removed from the samples prior to the NMR spectraacquisition by exposure the samples to a strong magneticfield. 29Si and 27Al MAS-NMR spectra of purified sampleswere performed with an MSL-400 Bruker apparatus. The

resonance frequencies used in this study were 79.5 and104.3 MHz, with spinning rates of 4 kHz and 12 kHz. Allmeasurements were taken at room temperature with TMS(tetramethylsilane) and AlH2O

36 as external standards.

The estimated errors in chemical shift values were lowerthan 0.5 ppm. A JEOL JSM 5400 scanning electron micro-scope (SEM), equipped with a LINK-ISIS energy disper-sive (EDX) analyzer, was used for micro-analysis.

2.5. Mechanical properties

The mechanical properties of resulting products werestudied on prismatic mortar specimens prepared by mixing

sand, fly ash, and the activating solution. The sand usedhas a 95% quartz content (CEN EN 196-1). The fly ash/sand ratio was 2:1. The fresh mixtures were poured intometal prismatic moulds (4 4 16 cm) and kept in a stoveat 85 C under relative humidity >95% for up to 168 h. Themortar prisms were then subjected to flexural and compres-sive test failure as described in the Spanish Standard UNE-80-101-88.

3. Results

3.1. Degree of reaction (alkali activation)

Table 3gives the degrees of reaction found by subjectingthe materials to attack with 1:20 solution of HCl [6,16].The results showed that the degree of reaction increasedwith curing time. However, while this parameter rises sub-stantially in the case of ashes L and P, the degree of reac-tion in ash M remained essentially unchanged after 8 h.

Table 4, shows the Al2O3 content in the liquid phase,expressed as a percentage of the Al2O3 content of ashes.These results indicate that the quantity of Al incorporatedinto reaction products increased with reaction time. How-ever, in the case of the ash M the amount of incorporatedAl is lower than in ashes L and P after 8 h of reaction.

3.2. Structural characterization

Detailed characterizations of the original ashes can befound in previous published works [7,11,15]. Since the

Table 1Chemical analysis of fly ashesa

Oxides (%) SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O SO3

Fly ash P 54.42 26.42 7.01 3.21 1.79 0.59 3.02 0.01Fly ash L 51.51 27.47 7.23 4.39 1.86 0.70 3.46 0.15Fly ash M 59.89 27.67 3.02 3.45 1.22 0.94 1.01 0.51

a Determined as stipulated in Spanish Standard UNE 80-215-88.

112 A. Fernandez-Jimenez et al. / Microporous and Mesoporous Materials 91 (2006) 111119


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main reaction products are always amorphous to XRD, the

structural characterization of resulting products has beenundertaken with MAS-NMR spectroscopy and SEM.XRD analysis has been used to confirm the crystalline

character of formed phases. The most relevant data refersto the detection of the characteristic humps of amorphousphases in X-ray patterns (not shown) [16,17]. However,long reaction times give rise to the formation of minor crys-talline phases, identified asherschelite (JCPDF-19-1178)and hydroxysodalite (JCPDF 11-401)zeolites[16,18].

3.2.1. 29Si MAS-NMR spectroscopy

Fig. 1depicts the 29Si MAS-NMR spectra of original flyashes (P, L, and M) and the reaction products formed atdifferent curing times. The wide signals generally observedin 29Si MAS-NMR spectra of starting materials are anindication of the heterogeneous character of ashes. (spectrain row A,Fig. 1). In these spectra peaks detected at 84,94, 98 and 103 ppm are attributed to the different Sienvironments of the ash [6,16]. The peak at around 87/88 ppm, in turn, is identified as mullite [19], a crystallinephase present in all ashes. Finally, the peaks with chemi-cal shifts above 108 ppm are attributed to Q4(0Al) envi-ronments in quartz (108/109 ppm) and cristobalite(113/114 ppm)[20,21].

It should be mentioned, in connection with the deconvo-

lution of the alkali activated ash spectra, that the relatively

high iron content of starting ashes (7% for ash P and Land 3% for M) was considerably reduced with the exposureof samples to a strong magnetic field. The partial elimina-tion of iron oxides decrease problems derived from para-magnetism in NMR spectra. Nonetheless in P sample, theiron could was not completely eliminated, which led to ahigher broadening of components and a lower signal/noiseratio than in other pastes. This fact explains that satellitebands detected in MAS-NMR spectra of P sample aremuch more important (not shown).

29Si MAS-NMR of spectra ashes activated with theNaOH solution and thermally cured at 85 C between 2 hand 7 days are also given inFig. 1(B, C, D, E and F rows).As the reaction time increases the intensity of signals attrib-utable to the reaction products grows at expenses of thoseof starting ashes. The most relevant change is observed inthe early stages of the reaction. Between 2 and 8 h of curingat 85 C, the most intense signal detected is located at

Table 2Reactive SiO2and Al2O3 content of fly ashes

Vitreous phasea (%) Reactiveb SiO2(%) Reactivea Al2O3(%) Reactive

b SiO2+aAl2O3 (%) Si/Al (atomic ratio)

Fly ash P 61.08 45.05 18.04 63.09 1.42Fly ash L 64.94 42.17 22.46 64.63 1.64Fly ash M 54.28 45.07 12.60 57.67 2.38

a

Determined by acid attack with HF 1% (see Refs. [13,14]).b Determined as stipulated in Spanish Standard UNE 80-225-93.

Table 3Degree of reaction (% of activated ash) at different curing timesa

Ash Thermal curing time

2 h 5 h 8 h 20 h 7 days

Degree of reaction P (%) 33.85 36.36 38.83 42.91 44.50L (%) 36.01 41.20 44.10 48.00 64.90M (%) 34.56 37.57 38.35 38.77 38.54

a Determined by acid attack with HCl 1:20 (see Ref. [15]).

Table 4Amounta of Al2O3 dissolved after acid attack on activated ashes atdifferent curing times

Ash Thermal curing time

2 h 5 h 8 h 20 h 7 days

Dissolved Al2O3 P (%) 8.60 9.6 9.7 10.7 14.3L (%) 8.66 10.0 11.5 15.1 19.9M (%) 8.69 9.2 9.8 9.5 11.5

a Expressed as percent (by wt.) of total ash.

Fig. 1. 29Si MAS-NMR spectra of starting ashes and AAFA resultingproducts in: (a) fly ash P; (b) fly ash L; (c) fly ash M. Row A stands forstarting ashes; row B for samples cured 2 h at 85 C; row C for samplescured 5 h at 85 C; row D for samples cured 8 h at 85 C; row E for

samples cured 20 h at 85 C; and row F for samples cured 7 days at 85 C.

A. Fernandez-Jimenez et al. / Microporous and Mesoporous Materials 91 (2006) 111119 113


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around 86/88 ppm. This signal is associated with theformation of a tectosilicate rich in aluminium with a pre-dominance of Q4(4Al) silicon units [21,22]. The signalsdetected at lower values (82/80 and 79/77 and72/70 ppm) are associated with the presence of less con-densed, monomer and dimer species, that decrease as the

reaction progresses. The signals appearing above88 ppm overlap with those of unreacted ash, making dif-ficult their assignment.

A comparison of the 29Si MAS-NMR spectra of pastesresulting from three alkali-activated ashes reveals that thefinal and therefore the most thermodynamically stablecompound formed is similar in all cases (7 days at 85 C,row F,Fig. 1). Nonetheless, the kinetics of the formationof this compound varies with the nature of the startingmaterials, asFig. 1shows. In all cases, spectra can be pri-marily attributed to the formation of an aluminosilicate gelwith a higher Si content than that of the gel previouslydetected. The spectra obtained after 7 days of reaction

are formed by five components associated with the pres-ence of silicon surrounded by none, one, two, three or fouraluminium tetrahedron in the silico-aluminate gel, whosesignals appear at around 110, 104, 98, 93, and88 ppm (with an error of 1 ppm). In some cases signalsare detected at 74.5 and 81.0 ppm that are attributed tothe presence of residual less condensed species, probablymonomer or dimer units with silanol groups. For interme-diate reaction times, 29Si NMR spectra (rows C and D) areformed by components of the two formed aluminosilicates.

3.2.2. 27Al MAS-NMR spectroscopy

The 27

Al MAS-NMR spectra for both the starting ashand the samples activated for different times are given inFig. 2. NMR patterns of starting fly ashes contain two widesignals, one centred at +53.86 ppm associated with tetrahe-dral aluminium (AlT) and a second small signal centred at+4.5 ppm, attributed to octahedral aluminium (AlO). Thelast component is mainly associated with the presence ofmullite in the starting fly ash [20,21].

During alkali activation, the tetrahedral aluminium sig-nal is observed to shift first from +53.9 to 60, and finallyfrom +60 to +59 ppm, indicating that the aluminiumalways remains tetrahedrally coordinated. This componenthas been ascribed to aluminium surrounded by four silicontetrahedra, which is characteristic of Al in zeolites precur-sors (Alq4(4Si) environments).

3.2.3. SEM study

The three studied ashes are formed by hollow or com-pact spheres of different sizes, with a smooth and regulartexture (like most of type F ashes). In Fig. 3(a)(c)micro-morphological aspects of ashes activated with 8 MNaOH and cured at 85 C for 7 days are shown. In all casesan amorphous aluminosilicate gel is formed, which consti-tutes the cementitious material detected between unreactedash spheres. The large number of spherical particles

observed in M pastes corroborate the low degree of reac-

tion attained in this material (seeTable 3). EDX techniquewas used to find the average composition of the formed gel.Deduced Si/Al values were 1.82.0 for sample L, 2.02.3for sample P and 1.41.5 for sample M.

The crystalline deposits, usually found inside of the par-tially unreacted ash spheres or between particles, corre-spond to the zeolites detected with XRD. An example ofthese crystalline phases (white arrow) can be seen inFig. 3(b). This compound is herschelite crystals having aSi/Al ratio of 2.2 and a Na/Al ratio of 1.02.

3.3. Mechanical strength

The development of compressive strength in alkalineactivated fly ash mortar prisms is plotted against reactiontime in Fig. 4. The most visible change is produced forshort reaction times. The increment of mechanical strengthis similar for the three types of ashes at 5 h, but changes sig-nificantly for longer reaction times. Between 5 and 20 h, thecompressive strength increases considerably in L and Pashes but remains almost constant in M pastes. After 168h (7 days) of thermal curing, the highest mechanicalstrength value was obtained in the ash P (80 MPa), fol-lowed by the ash L (72 MPa). Strength developed in ashM, is considerably lower, 31 MPa.

4. Discussion

It has been reported in previous papers [6,7] that the

alkali activation of fly ashes is a process comprising the dis-

Fig. 2. 27Al MAS-NMR of starting ashes and AAFA resulting productsin: (a) fly ash P; (b) fly ash L; (c) fly ash M. Row A stands for startingashes; row B for samples cured 2 h at 85 C; row C for samples cured 5 hat 85 C for; row D for samples cured 8 h at 85 C; row E for samplescured 20 h at 85 C; and row F for samples cured 7 days at 85 C.



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solution of starting materials and the formation of alumi-nosilicate gels. A similar mechanism was detected duringthe formation of zeolites from the alkaline attack of thekaolinite [22]. The dissolution stage, begins immediatelyafter the alkali solution comes into contact with the flyash. In this stage, the high OH concentration of the alka-line medium favours the break of covalent SiOSi, SiOAl and AlOAl bonds present in the vitreous phase of theash, releasing the silicon and aluminium ions into the solu-tion, where they form species with a high number of SiOHand AlOH groups. During the gelationstage, ionic speciespresent in the solution (monosilicates and monoaluminatesunits) condense to form SiOAl and SiOSi bonds, giv-ing rise to a three-dimensional aluminosilicate gel withalkaline cations compensating the deficit charges associatedwith Al for Si substitution. In these gels, the formation ofAlOAl bonds between contiguous tetrahedra is notfavoured (Loewensteins rule). In the early stages of thereaction, the speed of formation of dissolved monomers

is greater than the speed of precipitation of the gel.

Actually, the rate of dissolution of ashes stronglydepends on the amount and composition of ashes. Table1shows that the three ashes used in this study have verysimilar total silica and alumina contents, however not allsilica and alumina are reactive. Taken into account thatmullite and quartz are considered to be inert, our study willbe focused on the amount of silica and alumina reactive(seeTable 2). The reactive silica content is likewise similar,but the reactive alumina content of the vitreous phase dif-fers appreciably in the three analyzed ashes. This explains

differences detected in calculated (Si/Al)Reactive ratios.InFig. 5(a), the evolution of the reaction degree is given

as a function of the reaction time. In this figure, it can beobserved the existence of two stages in the alkaline activa-tion of ashes. During the first few hours the reaction degreeis quite similar; however, as the reaction progresses differ-ences become evident. In P and L pastes the second stageis clearly observed; however, in M pastes, the reactionalmost does not progress. The analysis ofFig. 5(b) showsthat during the first 5 or 8 h of reaction the three ashesrelease the same amount of Al (10%); however, after thisstage, different amounts of Al are released. In the fly ash L,with an initial amount of reactive alumina of 18.04% (seeTable 2), the amount of Al released into dissolutionincrease appreciably as the reaction progress. In the caseof P samples the amount of Al released is considerablylower in the second stage; finally, in M samples, a very slowactivity is detected after 8 h of reaction. This observation isexplained by assuming that most of reactive alumina of theM ash has been consumed in the first stage of reaction(75% of the reactive alumina). These results provide sup-port to the hypothesis that a certain minimum amount ofreactive Al is always necessary to favours the formationof aluminosilicate gels. These results agree with thosereported by Van Deventer et al. in a large number of

mineral aluminosilicates [23]. These authors deduced the

Fig. 3. SEM micrograph and microanalysis (EDX) of (a) P, (b) L and (c) M ashes, activated with 8 M NaOH and cured at 85 C for 7 days.

0 20 40 60 80 100 120 140 160 180 200

0

20

40

60

80

100

P

L

M

Compressivestrength(MPa)

Time (hours)

Fig. 4. Compressive strength as a function of the reaction time, for mortarprisms formed with alkali activated fly ashes.



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existence of a correlation between the quantities of Si andAl dissolved in the reactional medium. According to thisidea, the synchronous dissolution of Si and Al would

explain in our case that the speed with which the unat-tacked silica of the M ash dissolves is drastically reducedin the second stage of the reaction, as a consequence ofthe reactive Al absence.

In alkaline attack of starting ashes, AlO bonds aremore readily broken than SiO bonds; from this fact, therate of reaction will be very high when the amount ofreac-tivealuminium passes through a maximum in the solution.According to this fact, the probability c1 of the formationof AlOSi bonds is higher than that of SiOSi in alumi-nosilicate gels (seeTable 5) at the early stages of activation[20,21]. This favours the incorporation of Al in the alumi-nosilicate gels formed (see spectra B and C in Fig. 1). Fromthis fact, the signal detected around 86/88 ppm hasbeen associated with the formation of an Al-rich gel [6,7],in which four Al surrounds a Si tetrahedron and four Sisurrounds an Al. A similar conclusion may be deducedfrom the 27Al MAS-NMR spectra (seeFig. 2). The AlTsig-nal shifts towards more positive positions, characteristic oftetrahedral aluminium in zeolites (AlQ4(4Si)). According tothis fact, the gel detected for short reaction times would

display Si/Al ratios near to 1 (S4R-type intermediate reac-tion compound)[7]. In addition to the most prominent sig-nal Si(4Al), 29Si NMR spectra show other less intense

peaks at lower chemical shift values which are associatedwith the presence of less condensed species produced dur-ing alkaline activation of ashes. Finally, signals detectedat 94, 98 and 104 ppm, correspond to the unreactedvitreous phase, and signals appearing at around 108and 112 ppm are due to the Q4(0Al) units of the quartzand cristobalite, present in the starting ash.

As the alkaline activation progresses, aluminosilicateprecipitates covering partially ash particles (see Fig. 3(b)).However, SEM images have shown that aluminosilicategels formed at short reaction times are not homogeneous:some ash particles, either because of their composition orbecause the particles size, react earlier than others (Figs.3 and 4). Mechanical properties of alkaline activated ashesdepend strongly on the characteristics of the continuousprecipitate that interconnect unreacted ash particles inresulting prepared composites. In particular, the absenceof the continuity in particles connection between particlesshould reduce considerably mechanical performances ofmortars. Differences on the amount of the deposited alumi-nosilicate explain different compressive strengths measuredin materials prepared for short reaction times in three stud-ied ashes (Fig. 4).

The coverage of ash particles with formed aluminosili-cates produces also a substantial slowdown of the reaction,

retarding the dissolution of silicon and aluminium required

0 2 h 5 h 8 h 20 h 7 d0

4

8

12

16

20

24

28

(b)

(L)

(P)

(M)

Unreacted[Al2O

3]

(%i

nm

assfromf

lyash)

Time of thermal curing

PLM

0 2 h 5 h 8 h 20 h 7 d0

10

20

30

40

50

60

70

(a)

PL

MReactiondegree(%)

Time of thermal curing

Fig. 5. (a) Reaction degree versus time; (b) unreacted Al2O3 versus reaction time in the analyzed ashes. The horizontal lines represent the maximumquantity of aluminium that can react in each ash (see Tables 1 and 2).

Table 5Si/Al ratio of starting ash, and AAFA pastes at 8 h, 20 h and 7 days of reaction

Reaction product

Ash 8 h 20 h 7 days 7 daysa(Si/Al)Reactive c (%)

bSi/AlNMR c (%) bSi/AlNMR c (%)

bSi/AlNMR c(%) cSi/AlEDX

P 1.42 70.4 1.40 71.4 1.48 67.6 1.85 54.05 2.02.3L 1.64 61 1.42 70.4 1.62 61.7 1.71 58.4 1.82.0M 2.38 42 1.44 69.5 1.48 67.6 1.57 63.7 1.41.5

a SeeTable 2.b 104 ppm Q4(1Al) signal, 108 ppm signal disregarded.c Value determined by microanalysis (SEM/EDX).

1c= probability of the formation of SiOAl bonds in the gel, is derived

from c= (1/r) where r= Si/Al. A value of c= 1 indicates that all thebonds are SiOAl, whilst c = 0.55 indicates that 55% are SiOAl bonds

and 45% SiOSi bonds.



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to the formation of gels. As the reaction progresses, furtheramounts of SiO2 and Al2O3 are dissolved, favouring theevolution of the initial Gel 1 (Al-rich phase) into a newGel 2 (Si-rich phase). This assertion is supported by thechanges observed in the 29Si NMR spectra. The intensityof Si(3Al), Si(2Al), Si(1Al) and Si(0Al) signals increases

at expenses of the Si(4Al) signal. Moreover, the line widthof 29Si MAS-NMR components becomes smaller withreaction time, indicating that the new-formed phase ismore regular. The Si/Al ratio of the silicon-enriched gelgradually approaches to 1.8, giving rise to (D6R-type)structures [7]. In Fig. 6, the intensity of Q4(4Al) andQ4(2Al) NMR signals (representative of the two formedgels) are plotted versus reaction time. In all cases, the inten-sity of the Q4(2Al) band increases at expenses of thatQ4(4Al). It is observed that the ash L, with the highest Alreactive amount, present the quickest transformation, theP ash display intermediate transformations, and the M

ash shows very low activity after 8 h of reaction.The Si/Al ratio of the formed gels can be determined by

applying Engelhards equation[21]

Si=AlNMR

PnInSinAl

0:25P

nnInSinAl

n 0; 1; 2; 3; 4

where In(SinAl) stands for the intensity of the componentassociated with silicon surrounded by nSi and (4 n)Al.The analysis of values given inTable 5, confirm the obser-vation that the Si/Al ratio of the formed gel increases withthe reaction time. In this table, the (Si/Al)Reactive values,deduced by EDX, corresponding to starting ashes and pre-zeolite gels formed after 7 days of reaction are given. De-spite differences observed in values obtained with NMRand EDX, the same trends are observed. Taken into ac-count experimental conditions required to form the cemen-titious material (very alkaline systems, very low liquid/solid ratios, relatively short working times, low tempera-tures), the zeolite crystallization process is extremely unfa-voured. According to this fact, the amount of zeolitesdetected in this work is very low.

A deeper analysis ofTable 5shows an inverse relation-ship between the (Si/Al)Reactive values deduced in start-ing fly ashes and the Si/Al ratios of the alkalinealuminosilicate gels obtained after 7 days of reaction.

These observations result from two different facts: (i) the

more stable aluminosilicate gels finally formed display Si/Al ratios near 1.8; (ii) the incorporation of Si in absenceof Al is considerably retarded [24,25]. According to thesefacts, composition of aluminosilicate gels obtained in Land P ashes are similar; however, slow kinetics prevent toattain this composition in M samples. The reaction kinetics

depends on a series of intrinsic and extrinsic variables (par-ticle size, chemical composition of the ash, pH of the med-ium, nature and concentration of the activator, curing timeand temperature, etc.), which differ in analyzed systems. Inthe case of the M ash, the kinetic is considerably slowdownas a consequence of the fast consumption of the reactiveAl2O3in the first formed gel.

In Fig. 7, it is analyzed the relationship betweenmechanical strengths and the relative amount of Q4(4Al)versus Q4(3Al) + Q4(2Al) units in gels. From these results,it can be concluded that the mechanical strength of thematerial increases during formation of the gel in first stage

of the alkaline activation (coating of ash particles with anAl-rich aluminosilicate gel, Gel 1), but increases furtheras a result of the Si enrichment of the cementitious materi-als (formation of the Si enriched aluminosilicate gel, Gel 2).In the case of the ash M with the highest (Si/Al)Reactiveratio, the lowest mechanical strength (see Fig. 5) isobtained as a consequence of the smaller amount of Siincorporated into the aluminosilicate gel (low degree ofreaction attained) (see Table 3). Focusing specifically onthe present research, it can be concluded that the most suit-able ashes for the manufacture of alkaline cement with

2 5 8 20 16805

10

15

20

25

30

35

40

(a)

(b)

FA P

Area(%)

Time (hours)

(a)=Q4(4Al) (b)=Q4(2Al)

2 5 8 20 16805

10

15

20

25

30

35

40FA L

(b)

(a)

Area(%)

Time (hours)

2 5 8 20 16805

10

15

20

25

30

35

40FA M

(b)

(a)

Area(%)

Time (hours)

(a)=Q4(4Al) (b)=Q4(2Al) (a)=Q4(4Al) (b)=Q4(2Al)

Fig. 6. Evolution of signals intensity of 29Si MAS-NMR Q4(4Al) and Q4(2Al) components as function of the reaction time in the three analyses ashes.

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.00

10

20

30

40

50

60

70

80

90

Gel 2Gel 1

P L M

CompressiveStregth(MPa)

[Q4(2Al) +Q4(3Al)] / Q4(4Al)

Fig. 7. Mechanical strength versus Q4(2Al) + Q4(3Al)/Q4(4Al) ratios

deduced by NMR spectroscopy in the three fly ashes.



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good cementitious properties are of type F, with (Si/Al)Reactiveratios below 2.

Results of this investigation have clearly shown that theavailability of dissolved Al and Si at a given moment highlyinfluences the kinetics of the alkaline activation of fly ashes.Other variables, as concentration of the activator, curingtemperature and curing time, play also a significant rolein kinetics of gels formation [10,13]. From a thermody-namic point of view the process of activation of fly ashes

can be divided up to three main stages (see Fig. 8):

Stage 1(Dissolution stage): Most of the vitreous compo-nent of the fly ash is dissolved. No mechanical strengthdevelopment is observed during the dissolution process.

Stage 2(Induction period): During the induction period amassive precipitation of a metastable Gel (named Gel 1)takes place, that produces the coating of unreacted flyash particles. This gel display the singular characteristicof incorporate (into the microstructural framework) abig part of the reactive aluminium existing in the ash,but not all the silicon. The beginning of this stage isassociated to the initialsetting of the paste. In this casea real degree of reaction2 around 7080% (the apparentdegree of reaction3 has been estimated about 3040%),is obtain but the mechanical strength development ofthe material is not important[6,7].

Stage 3(Silicon incorporation stage): Finally, stage 3 cor-responds to a period in which Gel 1 is transformed intoGel 2. This new gel is a Si-rich material since it accom-modates into the structural framework that silicon

which is more slowly dissolved in the alkaline medium.Naturally, during the time in which Stage 3 is runningthe reaction degree continues advancing till reachingvalues >90% (real degree of reaction). At the same time,mechanical strength increases considerably. When thecontent of Gel 1 in the alkali activated fly ash is higherthan the content of Gel 2, the mechanical strength devel-opment is low, 2025 MPa, however when the content ofGel 2 Gel 1, then the mechanical strength gain nota-

bly increases to 80 MPa (see Fig. 8).

5. Conclusions

The analysis of the mechanical strength, degree of reac-tion and microstructural characteristics of alkaline-acti-vated ash pastes has shown that fly ashes that bestperform under alkaline activation are: (i) ashes with a highreactive SiO2 and Al2O3 contents and (ii) ashes with (Si/Al)Reactive ratios below 2.

In all analyzed cases an alkaline aluminosilicate gel isformed as the major reaction product regardless of thecomposition of the ash. For short reaction times, formedgels are constituted by an Al-rich phase, in which Si tetra-hedra are surrounded by four Al tetrahedra (Q4(4Al)units). As the reaction progresses, this phase evolves intoa more stable Si-rich phase, in which a higher amount ofSi occupy Q4(3Al) and Q4(2Al) environments. In thesematerials, the increment of Si/Al ratios, improve consider-ably mechanical properties of aluminosilicate gels formed.

Acknowledgements

This study was funded by the Spanish Department Gen-

eral of Scientific Research, under Project BIA2004-04835.

Fig. 8. Schematic description of mechanical properties evolution to the reaction time. The increment of mechanical performances is related to the Si/Al

ratio in the gel.

2 Real degree of reaction(only the reactivevitreouscomponent of thefly ash is taken into account for the calculations).3 Apparent degree of reaction (100% of the fly ash is considered to

contribute to the alkaline activation reactions).



9/9

Authors thank CSIC and European Social Fund for theI3P Contract (Ref. 13P-PC2004L) co-financed by the Euro-pean Social Fund.

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