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

Microporous and Mesoporous Materials 91 (2006) 111–119 www.elsevier.com/locate/micromeso

The role played by the reactive alumina content in the alkaline activation of fly ashes A. Ferna´ndez-Jime´nez a b

a,*

, A. Palomo a, I. Sobrados b, J. Sanz

b

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

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

Abstract This study explores the relationship between the chemical composition of fly ashes and the microstructural characteristics and mechanical properties of the cementitious materials resulting from the alkali activation of fly ashes (AAFA). Three reactive systems were prepared by mixing three F ashes, with an 8 M NaOH solution and stored at 85 C. The main reaction product formed in three systems is an 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 (2–5 h), an Al-rich gel was formed with silicon in tetrahedral Q4(4Al) units (intermediate phase), yielding low mechanical strengths. When the curing time increase (7 days) the gel changes into a more stable Si-rich phase with a greater mechanical strength. Finally, it has been shown that the amount of the reactive aluminium plays an important role in the aluminosilicate gel formation, from a kinetic point of view.  2005 Elsevier Inc. All rights reserved. Keywords:

29

Si and

27

Al MAS-NMR; Fly ash; Alkali activation; Pre-zeolite binder

1. Introduction The alkali activation of alumino-silicate materials is a chemical process that transforms partially or totally amorphous, vitreous and/or metastable structures into compact cementitious skeletons [1–5]. As a result of the reaction that takes place between fly ashes and alkalis under mild thermal conditions (60–90 C), the major reaction product is an amorphous alkaline aluminosilicate gel [6–8]. The 29Si MAS-NMR analysis [6] of these gels showed the formation of three-dimensional networks that constitutes the cementitious material that connects unreacted fly ash spheres. In this aluminosilicate gel the Si is found in a variety of Q4(nAl) environments. On the other hand there is a striking parallelism between the reaction mechanism involved in zeolite formation from alkali activation metakaolins and that regulating the alkali *

Corresponding author. Tel.: +34 91 302 0440; fax: +34 91 302 6047. E-mail address: [email protected] (A. Ferna´ndez-Jime´nez).

1387-1811/$ - see front matter  2005 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2005.11.015

attack of type F fly ashes. In this regard, Davidovits [9] and Palomo et al. [6,7] concluded that the long term formed products are ultimately related to zeolite phases. The essential difference between the two types of material lies in the properties they exhibit: the final product of zeolite synthesis is an adsorbent, catalytic powder, whereas the material obtained with AAFA is cementitious, with high mechanical strength and considerable stability. Recent surveys have shown that the Si/Al ratio of the aluminosilicate gel obtained from the alkali activation of fly ashes depends heavily on the chemical composition of the starting material, nature and concentration of alkali activator, synthesis temperature, and thermal curing time [6,7,10–13]. Nonetheless, many questions persist about the reactivity of fly ashes in strong alkaline environments. For this reason, the aim of our study has been the analysis of the formation of different pre-zeolite gels as a function of the curing time for different fly ashes. For this purpose, a number of different techniques have been used for the structural and microstructural characterization of prepared

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112

materials. Taken into account the vitreous nature of starting ashes as well as the amorphous characteristic of the formed gels, the MAS-NMR spectroscopy is particularly adapted to this work.

of powder samples were recorded on a Philips diffractometer PW 1730, with Cu Ka radiation. Magnetic materials were removed from the samples prior to the NMR spectra acquisition by exposure the samples to a strong magnetic field. 29Si and 27Al MAS-NMR spectra of purified samples were performed with an MSL-400 Bruker apparatus. The resonance frequencies used in this study were 79.5 and 104.3 MHz, with spinning rates of 4 kHz and 12 kHz. All measurements were taken at room temperature with TMS 3þ (tetramethylsilane) and AlðH2 OÞ6 as external standards. The estimated errors in chemical shift values were lower than 0.5 ppm. A JEOL JSM 5400 scanning electron microscope (SEM), equipped with a LINK-ISIS energy dispersive (EDX) analyzer, was used for micro-analysis.

2. Experimental 2.1. Characterization of the starting materials Three fly ash materials (called P, L and M) from three different Spanish steam power plants were used in this study. All three were F type ashes (ASTM classification) consisting primarily of SiO2 and Al2O3. Chemical compositions (determined according to Spanish Standard UNE 80215-88), the reactive silica (determined according to UNE 80-225-93), and the reactive alumina contents (determined according to Ref. [14,15]), are shown in Tables 1 and 2. In Table 2, the relative amount of the vitreous phase in three ashes is also given.

2.5. Mechanical properties The mechanical properties of resulting products were studied on prismatic mortar specimens prepared by mixing sand, fly ash, and the activating solution. The sand used has a 95% quartz content (CEN EN 196-1). The ‘‘fly ash /sand’’ ratio was 2:1. The fresh mixtures were poured into metal prismatic moulds (4 · 4 · 16 cm) and kept in a stove at 85 C under relative humidity >95% for up to 168 h. The mortar prisms were then subjected to flexural and compressive test failure as described in the Spanish Standard UNE80-101-88.

2.2. Alkali activation of the ash The three ashes were activated with an 8 M solution of NaOH. The ‘‘solution/ash’’ ratio used in each case was chosen to obtain a paste of standard consistency (UNE– EN 196-3); i.e., 0.33, 0.4 and 0.56 by mass/weight for ashes P, L and M, respectively. The pastes obtained were cured at 85 C and 98% relative humidity for different periods of time (2, 5, 8 and 20 h and 7 days). After each experiment, the material was removed from the stove, cooled to laboratory temperature, ground and then mixed with small amounts of acetone to prevent the activation progress.

3. Results 3.1. Degree of reaction (alkali activation)

Alkali activated materials were subjected to an acid attack with a 1:20 solution of HCl to determine the ‘‘degree of reaction’’ attaint in each sample (Table 3). The acid solution dissolves the reaction products formed by alkaline activation of ashes (aluminosilicate gel and zeolites) but does not interact significantly with the unreacted fly ash [6,16]. After filtering, the concentration of the Al dissolved was determined by ICP-MS on a Spectromass 2000 instrument (Table 4).

Table 3 gives the degrees of reaction found by subjecting the materials to attack with 1:20 solution of HCl [6,16]. The results showed that the degree of reaction increased with curing time. However, while this parameter rises substantially in the case of ashes L and P, the degree of reaction 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 incorporated into reaction products increased with reaction time. However, in the case of the ash M the amount of incorporated Al is lower than in ashes L and P after 8 h of reaction.

2.4. Techniques

3.2. Structural characterization

2.3. Chemical attack

27

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

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

Table 1 Chemical analysis of fly ashesa Oxides (%)

SiO2

Al2O3

Fe2O3

CaO

MgO

Na2O

K2O

SO3

Fly ash P Fly ash L Fly ash M

54.42 51.51 59.89

26.42 27.47 27.67

7.01 7.23 3.02

3.21 4.39 3.45

1.79 1.86 1.22

0.59 0.70 0.94

3.02 3.46 1.01

0.01 0.15 0.51

a

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

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Table 2 Reactive SiO2 and Al2O3 content of fly ashes Fly ash P Fly ash L Fly ash M a b

Vitreous phasea (%)

Reactiveb SiO2 (%)

Reactivea Al2O3 (%)

Reactiveb SiO2 + aAl2O3 (%)

Si/Al (atomic ratio)

61.08 64.94 54.28

45.05 42.17 45.07

18.04 22.46 12.60

63.09 64.63 57.67

1.42 1.64 2.38

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

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

Degree of reaction

a

Ash

Thermal curing time 2h

5h

8h

20 h

7 days

P (%) L (%) M (%)

33.85 36.01 34.56

36.36 41.20 37.57

38.83 44.10 38.35

42.91 48.00 38.77

44.50 64.90 38.54

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

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

Dissolved Al2O3

a

Ash

Thermal curing time 2h

5h

8h

20 h

7 days

P (%) L (%) M (%)

8.60 8.66 8.69

9.6 10.0 9.2

9.7 11.5 9.8

10.7 15.1 9.5

14.3 19.9 11.5

high iron content of starting ashes (7% for ash P and L and 3% for M) was considerably reduced with the exposure of samples to a strong magnetic field. The partial elimination of iron oxides decrease problems derived from paramagnetism in NMR spectra. Nonetheless in P sample, the iron could was not completely eliminated, which led to a higher broadening of components and a lower signal/noise ratio than in other pastes. This fact explains that satellite bands detected in MAS-NMR spectra of P sample are much more important (not shown). 29 Si MAS-NMR of spectra ashes activated with the NaOH solution and thermally cured at 85 C between 2 h and 7 days are also given in Fig. 1 (B, C, D, E and F rows). As the reaction time increases the intensity of signals attributable to the reaction products grows at expenses of those of starting ashes. The most relevant change is observed in the early stages of the reaction. Between 2 and 8 h of curing at 85 C, the most intense signal detected is located at

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

main reaction products are always amorphous to XRD, the structural characterization of resulting products has been undertaken with MAS-NMR spectroscopy and SEM. XRD analysis has been used to confirm the crystalline character of formed phases. The most relevant data refers to the detection of the characteristic humps of amorphous phases in X-ray patterns (not shown) [16,17]. However, long reaction times give rise to the formation of minor crystalline phases, identified as—herschelite (JCPDF-19-1178) and hydroxysodalite (JCPDF 11-401)—zeolites [16,18]. 3.2.1. 29Si MAS-NMR spectroscopy Fig. 1 depicts the 29Si MAS-NMR spectra of original fly ashes (P, L, and M) and the reaction products formed at different curing times. The wide signals generally observed in 29Si MAS-NMR spectra of starting materials are an indication of the heterogeneous character of ashes. (spectra in row A, Fig. 1). In these spectra peaks detected at 84, 94, 98 and 103 ppm are attributed to the different Si environments of the ash [6,16]. The peak at around 87/ 88 ppm, in turn, is identified as mullite [19], a crystalline phase present in all ashes. Finally, the peaks with chemical shifts above 108 ppm are attributed to Q4(0Al) environments in quartz (108/109 ppm) and cristobalite (113/114 ppm) [20,21]. It should be mentioned, in connection with the deconvolution of the alkali activated ash spectra, that the relatively

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

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around 86/88 ppm. This signal is associated with the formation of a tectosilicate rich in aluminium with a predominance of Q4(4Al) silicon units [21,22]. The signals detected at lower values (82/80 and 79/77 and 72/70 ppm) are associated with the presence of less condensed, monomer and dimer species, that decrease as the reaction progresses. The signals appearing above 88 ppm overlap with those of unreacted ash, making difficult their assignment. A comparison of the 29Si MAS-NMR spectra of pastes resulting from three alkali-activated ashes reveals that the final and therefore the most thermodynamically stable compound formed is similar in all cases (7 days at 85 C, row F, Fig. 1). Nonetheless, the kinetics of the formation of this compound varies with the nature of the starting materials, as Fig. 1 shows. In all cases, spectra can be primarily attributed to the formation of an aluminosilicate gel with a higher Si content than that of the gel previously detected. The spectra obtained after 7 days of reaction are formed by five components associated with the presence of silicon surrounded by none, one, two, three or four aluminium tetrahedron in the silico-aluminate gel, whose signals appear at around 110, 104, 98, 93, and 88 ppm (with an error of ±1 ppm). In some cases signals are detected at 74.5 and 81.0 ppm that are attributed to the presence of residual less condensed species, probably monomer or dimer units with silanol groups. For intermediate reaction times, 29Si NMR spectra (rows C and D) are formed by components of the two formed aluminosilicates. 3.2.2. 27Al MAS-NMR spectroscopy The 27Al MAS-NMR spectra for both the starting ash and the samples activated for different times are given in Fig. 2. NMR patterns of starting fly ashes contain two wide signals, one centred at +53.86 ppm associated with tetrahedral aluminium (AlT) and a second small signal centred at +4.5 ppm, attributed to octahedral aluminium (AlO). The last component is mainly associated with the presence of mullite in the starting fly ash [20,21]. During alkali activation, the tetrahedral aluminium signal is observed to shift first from +53.9 to 60, and finally from +60 to +59 ppm, indicating that the aluminium always remains tetrahedrally coordinated. This component has been ascribed to aluminium surrounded by four silicon tetrahedra, which is characteristic of Al in zeolites precursors (Alq4(4Si) environments). 3.2.3. SEM study The three studied ashes are formed by hollow or compact spheres of different sizes, with a smooth and regular texture (like most of type F ashes). In Fig. 3(a)–(c) micro-morphological aspects of ashes activated with 8 M NaOH and cured at 85 C for 7 days are shown. In all cases an amorphous aluminosilicate gel is formed, which constitutes the cementitious material detected between unreacted ash spheres. The large number of spherical particles observed in M pastes corroborate the low degree of reac-

Fig. 2. 27Al MAS-NMR of starting ashes and AAFA resulting products in: (a) fly ash P; (b) fly ash L; (c) fly ash M. Row A stands for starting ashes; row B for samples cured 2 h at 85 C; row C for samples cured 5 h at 85 C for; 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.

tion attained in this material (see Table 3). EDX technique was used to find the average composition of the formed gel. Deduced Si/Al values were 1.8–2.0 for sample L, 2.0–2.3 for sample P and 1.4–1.5 for sample M. The crystalline deposits, usually found inside of the partially unreacted ash spheres or between particles, correspond to the zeolites detected with XRD. An example of these crystalline phases (white arrow) can be seen in Fig. 3(b). This compound is herschelite crystals having a Si/Al ratio of 2.2 and a Na/Al ratio of 1.02. 3.3. Mechanical strength The development of compressive strength in alkaline activated fly ash mortar prisms is plotted against reaction time in Fig. 4. The most visible change is produced for short reaction times. The increment of mechanical strength is similar for the three types of ashes at 5 h, but changes significantly for longer reaction times. Between 5 and 20 h, the compressive strength increases considerably in L and P ashes but remains almost constant in M pastes. After 168 h (7 days) of thermal curing, the highest mechanical strength value was obtained in the ash P (80 MPa), followed by the ash L (72 MPa). Strength developed in ash M, 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-

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115

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.

Compressive strength (MPa)

100 P L M

80 60 40 20 0 0

20

40

60

80 100 120 140 160 180 200 Time (hours)

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

solution of starting materials and the formation of aluminosilicate gels. A similar mechanism was detected during the formation of zeolites from the alkaline attack of the kaolinite [22]. The dissolution stage, begins immediately after the alkali solution comes into contact with the fly ash. In this stage, the high OH concentration of the alkaline medium favours the break of covalent Si–O–Si, Si–O– Al and Al–O–Al bonds present in the vitreous phase of the ash, releasing the silicon and aluminium ions into the solution, where they form species with a high number of Si–OH and Al–OH groups. During the gelation stage, ionic species present in the solution (monosilicates and monoaluminates units) condense to form Si–O–Al and Si–O–Si bonds, giving rise to a three-dimensional aluminosilicate gel with alkaline cations compensating the deficit charges associated with Al for Si substitution. In these gels, the formation of Al–O–Al bonds between contiguous tetrahedra is not favoured (Loewenstein’s rule). In the early stages of the reaction, the speed of formation of dissolved monomers is greater than the speed of precipitation of the gel.

Actually, the rate of dissolution of ashes strongly depends on the amount and composition of ashes. Table 1 shows that the three ashes used in this study have very similar total silica and alumina contents, however not all silica and alumina are reactive. Taken into account that mullite and quartz are considered to be inert, our study will be focused on the amount of silica and alumina reactive (see Table 2). The reactive silica content is likewise similar, but the reactive alumina content of the vitreous phase differs appreciably in the three analyzed ashes. This explains differences detected in calculated ‘‘(Si/Al)Reactive’’ ratios. In Fig. 5(a), the evolution of the reaction degree is given as a function of the reaction time. In this figure, it can be observed the existence of two stages in the alkaline activation of ashes. During the first few hours the reaction degree is quite similar; however, as the reaction progresses differences become evident. In P and L pastes the second stage is clearly observed; however, in M pastes, the reaction almost does not progress. The analysis of Fig. 5(b) shows that during the first 5 or 8 h of reaction the three ashes release the same amount of Al (10%); however, after this stage, different amounts of Al are released. In the fly ash L, with an initial amount of reactive alumina of 18.04% (see Table 2), the amount of Al released into dissolution increase appreciably as the reaction progress. In the case of P samples the amount of Al released is considerably lower in the second stage; finally, in M samples, a very slow activity is detected after 8 h of reaction. This observation is explained by assuming that most of reactive alumina of the M ash has been consumed in the first stage of reaction (75% of the reactive alumina). These results provide support to the hypothesis that a certain minimum amount of reactive Al is always necessary to favours the formation of aluminosilicate gels. These results agree with those reported by Van Deventer et al. in a large number of mineral aluminosilicates [23]. These authors deduced the

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28

60

Unreacted [Al 2 O3 ] (% in mass from fly ash)

Reaction degree (%)

70

50 40 30 P L M

20 10 0

P L M

24 20 16 12

(M)

8

(P)

4

(L)

0 0

2h

5h

8h

20 h

7d

0

Time of thermal curing

(a)

(b)

2h

5h

8h

20 h

7d

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 maximum quantity of aluminium that can react in each ash (see Tables 1 and 2).

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

a b c

20 h

7 days

7 days

c (%)

b

c (%)

b

c (%)

b

c (%)

c

1.42 1.64 2.38

70.4 61 42

1.40 1.42 1.44

71.4 70.4 69.5

1.48 1.62 1.48

67.6 61.7 67.6

1.85 1.71 1.57

54.05 58.4 63.7

2.0–2.3 1.8–2.0 1.4–1.5

(Si/Al)Reactive

P L M

8h

a

Si/AlNMR

Si/AlNMR

Si/AlNMR

Si/AlEDX

See Table 2. 104 ppm Q4(1Al) signal, 108 ppm signal disregarded. Value determined by microanalysis (SEM/EDX).

existence of a correlation between the quantities of Si and Al dissolved in the reactional medium. According to this idea, the synchronous dissolution of Si and Al would explain in our case that the speed with which the unattacked silica of the M ash dissolves is drastically reduced in the second stage of the reaction, as a consequence of the reactive Al absence. In alkaline attack of starting ashes, Al–O bonds are more readily broken than Si–O bonds; from this fact, the rate of reaction will be very high when the amount of reactive aluminium passes through a maximum in the solution. According to this fact, the probability c1 of the formation of Al–O–Si bonds is higher than that of Si–O–Si in aluminosilicate gels (see Table 5) at the early stages of activation [20,21]. This favours the incorporation of Al in the aluminosilicate gels formed (see spectra B and C in Fig. 1). From this fact, the signal detected around 86/88 ppm has been associated with the formation of an Al-rich gel [6,7], in which four Al surrounds a Si tetrahedron and four Si surrounds an Al. A similar conclusion may be deduced from the 27Al MAS-NMR spectra (see Fig. 2). The AlT signal shifts towards more positive positions, characteristic of tetrahedral aluminium in zeolites (AlQ4(4Si)). According to this fact, the gel detected for short reaction times would 1 c = probability of the formation of Si–O–Al bonds in the gel, is derived from c = (1/r) where r = Si/Al. A value of c = 1 indicates that all the bonds are Si–O–Al, whilst c = 0.55 indicates that 55% are Si–O–Al bonds and 45% Si–O–Si bonds.

display Si/Al ratios near to 1 (S4R-type intermediate reaction compound) [7]. In addition to the most prominent signal Si(4Al), 29Si NMR spectra show other less intense peaks at lower chemical shift values which are associated with the presence of less condensed species produced during alkaline activation of ashes. Finally, signals detected at 94, 98 and 104 ppm, correspond to the unreacted vitreous phase, and signals appearing at around 108 and 112 ppm are due to the Q4(0Al) units of the quartz and cristobalite, present in the starting ash. As the alkaline activation progresses, aluminosilicate precipitates covering partially ash particles (see Fig. 3(b)). However, SEM images have shown that aluminosilicate gels formed at short reaction times are not homogeneous: some ash particles, either because of their composition or because the particles size, react earlier than others (Figs. 3 and 4). Mechanical properties of alkaline activated ashes depend strongly on the characteristics of the continuous precipitate that interconnect unreacted ash particles in resulting prepared composites. In particular, the absence of the continuity in particles connection between particles should reduce considerably mechanical performances of mortars. Differences on the amount of the deposited aluminosilicate explain different compressive strengths measured in materials prepared for short reaction times in three studied ashes (Fig. 4). The coverage of ash particles with formed aluminosilicates produces also a substantial slowdown of the reaction, retarding the dissolution of silicon and aluminium required

(a)

(b)

(a)=Q4(4Al)

2

5

(b)=Q4(2Al)

8 20 Time (hours)

Fig. 6. Evolution of signals intensity of

168

40 35 30 25 20 15 10 5 0

FA L

(a)

Area (%)

FA P

(b)

(a)=Q4(4Al)

2

5

(b)=Q4(2Al)

8 20 Time (hours)

168

40 35 30 25 20 15 10 5 0

117

FA M (a) (b)

(a)=Q4(4Al)

2

5

(b)=Q4(2Al)

8 20 Time (hours)

168

29

Si MAS-NMR Q4(4Al) and Q4(2Al) components as function of the reaction time in the three analyses ashes.

to the formation of gels. As the reaction progresses, further amounts of SiO2 and Al2O3 are dissolved, favouring the evolution of the initial Gel 1 (Al-rich phase) into a new Gel 2 (Si-rich phase). This assertion is supported by the changes observed in the 29Si NMR spectra. The intensity of Si(3Al), Si(2Al), Si(1Al) and Si(0Al) signals increases at expenses of the Si(4Al) signal. Moreover, the line width of 29Si MAS-NMR components becomes smaller with reaction time, indicating that the new-formed phase is more regular. The Si/Al ratio of the silicon-enriched gel gradually approaches to 1.8, giving rise to (D6R-type) structures [7]. In Fig. 6, the intensity of Q4(4Al) and Q4(2Al) NMR signals (representative of the two formed gels) are plotted versus reaction time. In all cases, the intensity of the Q4(2Al) band increases at expenses of that Q4(4Al). It is observed that the ash L, with the highest Al reactive amount, present the quickest transformation, the P 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 Engelhard’s equation [21] P n I n ðSin AlÞ P ðSi=AlÞNMR ¼ n ¼ 0; 1; 2; 3; 4 0:25 n nI n ðSin AlÞ where In(SinAl) stands for the intensity of the component associated with silicon surrounded by nSi and (4  n)Al. The analysis of values given in Table 5, confirm the observation that the Si/Al ratio of the formed gel increases with the reaction time. In this table, the ‘‘(Si/Al)Reactive’’ values, deduced by EDX, corresponding to starting ashes and prezeolite gels formed after 7 days of reaction are given. Despite differences observed in values obtained with NMR and EDX, the same trends are observed. Taken into account experimental conditions required to form the cementitious material (very alkaline systems, very low ‘‘liquid/ solid’’ ratios, relatively short working times, low temperatures), the zeolite crystallization process is extremely unfavoured. According to this fact, the amount of zeolites detected in this work is very low. A deeper analysis of Table 5 shows an inverse relationship between the ‘‘(Si/Al)Reactive’’ values deduced in starting fly ashes and the Si/Al ratios of the alkaline aluminosilicate 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 absence of Al is considerably retarded [24,25]. According to these facts, composition of aluminosilicate gels obtained in L and P ashes are similar; however, slow kinetics prevent to attain this composition in M samples. The reaction kinetics depends on a series of intrinsic and extrinsic variables (particle size, chemical composition of the ash, pH of the medium, nature and concentration of the activator, curing time and temperature, etc.), which differ in analyzed systems. In the case of the M ash, the kinetic is considerably slowdown as a consequence of the fast consumption of the reactive Al2O3 in the first formed gel. In Fig. 7, it is analyzed the relationship between mechanical 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 the material increases during formation of the gel in first stage of the alkaline activation (coating of ash particles with an Al-rich aluminosilicate gel, Gel 1), but increases further as a result of the Si enrichment of the cementitious materials (formation of the Si enriched aluminosilicate gel, Gel 2). In the case of the ash M with the highest ‘‘(Si/Al)Reactive’’ ratio, the lowest mechanical strength (see Fig. 5) is obtained as a consequence of the smaller amount of Si incorporated into the aluminosilicate gel (low degree of reaction attained) (see Table 3). Focusing specifically on the present research, it can be concluded that the most suitable ashes for the manufacture of alkaline cement with

Compressive Stregth (MPa)

40 35 30 25 20 15 10 5 0

Area (%)

Area (%)

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90 P L M 80 70 Gel 2 Gel 1 60 50 40 30 20 10 0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 [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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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.

good cementitious properties are of type F, with (Si/ Al)Reactive ratios below 2. Results of this investigation have clearly shown that the availability of dissolved Al and Si at a given moment highly influences the kinetics of the alkaline activation of fly ashes. Other variables, as concentration of the activator, curing temperature and curing time, play also a significant role in kinetics of gels formation [10,13]. From a thermodynamic 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 component of the fly ash is dissolved. No mechanical strength development is observed during the dissolution process. • Stage 2 (Induction period): During the induction period a massive precipitation of a metastable Gel (named Gel 1) takes place, that produces the coating of unreacted fly ash particles. This gel display the singular characteristic of incorporate (into the microstructural framework) a big part of the reactive aluminium existing in the ash, but not all the silicon. The beginning of this stage is associated to the initial setting of the paste. In this case a real degree of reaction2 around 70–80% (the apparent degree of reaction3 has been estimated about 30–40%), is obtain but the mechanical strength development of the material is not important [6,7]. • Stage 3 (Silicon incorporation stage): Finally, stage 3 corresponds to a period in which Gel 1 is transformed into Gel 2. This new gel is a Si-rich material since it accommodates into the structural framework that silicon 2

Real degree of reaction (only the reactive—vitreous—component of the fly 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).

which is more slowly dissolved in the alkaline medium. Naturally, during the time in which Stage 3 is running the reaction degree continues advancing till reaching values >90% (real degree of reaction). At the same time, mechanical strength increases considerably. When the content of Gel 1 in the alkali activated fly ash is higher than the content of Gel 2, the mechanical strength development is low, 20–25 MPa, however when the content of Gel 2  Gel 1, then the mechanical strength gain notably increases to 80 MPa (see Fig. 8).

5. Conclusions The analysis of the mechanical strength, degree of reaction and microstructural characteristics of alkaline-activated ash pastes has shown that fly ashes that best perform under alkaline activation are: (i) ashes with a high reactive SiO2 and Al2O3 contents and (ii) ashes with ‘‘(Si/ Al)Reactive’’ ratios below 2. In all analyzed cases an alkaline aluminosilicate gel is formed as the major reaction product regardless of the composition of the ash. For short reaction times, formed gels are constituted by an Al-rich phase, in which Si tetrahedra are surrounded by four Al tetrahedra (Q4(4Al) units). As the reaction progresses, this phase evolves into a more stable Si-rich phase, in which a higher amount of Si occupy Q4(3Al) and Q4(2Al) environments. In these materials, the increment of Si/Al ratios, improve considerably mechanical properties of aluminosilicate gels formed. Acknowledgements This study was funded by the Spanish Department General of Scientific Research, under Project BIA2004-04835.

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