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Fuel 86 (2007) 315–322

Review

Alkali-activated fly ash: Effect of thermal curing conditionson mechanical and microstructural development – Part II

G. Kovalchuk a, A. Fernandez-Jimenez b,*, A. Palomo b

a V.D. Glukhovskiy Scientific Research Institute on Binders and Materials, Kyiv, Ukraineb ‘‘Eduardo Torroja’’ Institute, CSIC, Madrid, Spain

Received 19 April 2006; received in revised form 6 July 2006; accepted 11 July 2006Available online 17 August 2006

Abstract

The development of mechanical strength and the mineral and microstructural characteristics of the alkali activated fly ash (AAFA),reveal the importance of the role played by the curing conditions prevailing during setting and hardening process: curing in a coveredmould (CCM) at 95 �C; dry curing (DC) at 150 �C, or steam curing (ST) at 95 �C. CCM yields the highest compressive strength (up to102 MPa after 8 h of curing) and can be readily used in any AAFA system. DC is only recommended for NaOH-based systems (lowSiO2/Al2O3 ratio), since waterglass-based mixes tend to retard reaction kinetics. SC, in turn, has an intermediate effect on strength devel-opment, between CCM and DC.� 2006 Elsevier Ltd. All rights reserved.

Keywords: Alkaline cements; Alkaline activation; Curing conditions

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3162. Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3163. Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 317

0016-2

doi:10.

* CoE-m

3.1. Mechanical strength. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3173.2. X-ray diffraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3173.3. FTIR spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3183.4. 29Si MAS-NMR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3183.5. Water content and pore structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319

4. Discussion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3205. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 322References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 322

361/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

1016/j.fuel.2006.07.010

rresponding author.ail addresses: george.kiev@mail.ru (G. Kovalchuk), anafj@ietcc.csic.es (A. Fernandez-Jimenez), Palomo@ietcc.csic.es (A. Palomo).

316 G. Kovalchuk et al. / Fuel 86 (2007) 315–322

1. Introduction

Alkaline cements are attracting increasing interest fortheir potential to enable the industry to operate withinthe limitations placed on CO2 emissions. Although firstdeveloped in 1957, such cements are still relatively poorlyknown. They exhibit excellent performance in many areas:early mechanical strength, fire and acid resistance and soon. And industrial trials have confirmed that these cementsmake good building materials [1–5].

The research described here aimed to attain a fullerunderstanding of the formation of alkaline aluminosilicategel from alkali-activated fly ash (AAFA) with a view tocontributing to the development of new groups of cementi-tious materials. The effect of the molar ratios of Na2O/Al2O3 and SiO2/Al2O3 in the initial system on the finalproduct has been recently studied by different groups ofauthors [6,7]. Today the mechanical strength of the mate-rial is known to be enhanced by a higher initial SiO2/Al2O3 ratio [7,8], which increases the number of strongSi–O–Si bonds in the final product. The amount of Al2O3

in the reagent system is also known to play an importantrole in the controlling process kinetics [8].

Curing conditions also have a significant effect onmicrostructural and mechanical strength development inmost cementitious systems. Mild curing temperaturesdetermine AFt and AFm phase formation on Portlandcement pastes, for instance. Such conditions should there-fore be expected to affect the setting and hardening ofcements with an alkaline pre-zeolitic gel, such as AAFA[7,8], since a structure of the synthesizing zeolites is knownto be very sensible to the conditions of synthesis [9,10].

Since AAFA pastes harden slowly at ambient tempera-ture, these systems are usually subjected to mild curingtemperatures. Most research has been conducted with cur-ing conditions of about 95% relative humidity and temper-atures ranging from 30 �C to 85 �C [11–16]. Accordingly,curing times may vary from several hours to several days,plus a few hours of additional conditioning at ambient tem-perature. Contrary to what has been observed in Portlandcement mortars/concretes, mild curing temperatures do notaffect AAFA-based material durability [17].

A comparative study of different curing conditions[12,13] showed that temperature and humidity play a keyrole in the development of the microstructure and conse-quently the properties of AAFA materials. Unsuitable cur-ing conditions may favour carbonation at a very early stage[18], in turn lowering pH levels and as a result substantiallyretarding the ash activation rate and mechanical strength

Table 1Chemical composition of the main solid constituents

SiO2 Al2O3 Fe2O3 CaO Mg

Fly ash 54.42 (45.05)b 26.42 7.01 3.21 1.79Silica fume 92.02 0.70 0.39 – –

a IR – insoluble residue, LOI – loss on ignition.b Reactive silica determined as indicated in Spanish Standard UNE 80-225-9

development. The only way to prevent such initial carbon-ation is by controlling the environmental curing regime(high relative humidity). Another study found that raisingthe curing temperature from 45 �C to 65 �C increased therate of mechanical strength development fivefold; and a10-fold rise was recorded between 65 �C and 85 �C [16].There is, however, a threshold value beyond which strengthincreases at a slower rate.

Finally, autoclaving, another possible curing procedure[12], favours the formation of highly crystallized zeolitessuch as analcime, zeolite-P, Na-chabazite and so on. Opti-mal strength properties are achieved with this procedurewhen the conditions cause moderate amounts of crystallinezeolites to precipitate in the material.

The objective pursued in this research, in light of theforegoing, was to define the curing and compositional con-ditions that optimize the polymerization reactions takingplace during the alkali activation of fly ash.

2. Experimental

Typical class F fly ash from a Spanish power plant wasused in this study (chemical composition shown in Table1): 78.86% of the particles were smaller than 45 lm andthe Blaine specific surface was 202 m2/kg. Water glass (sol-uble sodium silicate) with a silicate modulus of 3.35 and adensity of 1350 kg/m3 (8.2% Na2O, 27% SiO2 and 64.8%H2O), supplied by Panreac S.A., was used as the main alka-line activator. The waterglass was adjusted to the necessaryNa2O/Al2O3 ratio by adding 98% pure NaOH pellets.Small amounts of silica fume were similarly used to controlthe SiO2/Al2O3 ratio. The chemical composition of thechief solid components is given in Table 1.

The composition of the initial mixes was calculated onthe basis of a series of pre-established total ratios betweenthe main oxides (see Table 2 and [6]). The water/solid ratio(W/S) was constant throughout, at 0.18.

The ‘‘hot moulding’’ procedure was used to preventquick setting [12,13]. The moulds were filled, covered witha plastic membrane to prevent water from evaporating andthen subjected to the following thermal treatments:

• Curing in covered moulds (CCM) also name in a sealed

mould: The moulds prepared were covered by a individ-ual plastic bag totally sealed to prevent water evapora-tion and then introduced in an oven for 8 h at 95 �C.This curing procedure, successfully applied in the firstpart of this survey [6], is readily adaptable to actualindustrial conditions.

O SO3 Na2O K2O IRa LOI Total

0.01 0.59 3.02 0.78 2.19 99.44– – – – 6.28 99.45

3.

Table 2Initial composition of alkali-activated fly ash mixes

Sample Molar ratio of mix Water/solid ratio (W/S)

SiO2/Al2O3 Na2O/Al2O3

S4N1 4 1 0.18S3N1 3.5 1 0.18S4N0 4 0.5 0.18S3N0 3.5 0.5 0.18

Table 3Compressive strength of AAFA paste prisms (MPa)

Molar ratio Compressive strength after

SiO2/Al2O3 Na2O/Al2O3 Curing incovered moulds(CCM)

Drycuring(DC)

Steamcuring(SC)

S4N1 4 1 102.1 31.8 71.0S3N1 3.5 1 88.9 57.1 55.5S4N0 4 0.5 64.1 27.6 76.0S3N0 3.5 0.5 50.4 45.6 36.4

G. Kovalchuk et al. / Fuel 86 (2007) 315–322 317

• Dry curing (DC): The covered moulds were first storedin an oven for 2 h at 95 �C, after which the prismswere de-moulded and subjected to the following ther-mal ramp: from room temperature to 150 �C in 3 h,6 h at 150 �C, and cooling to ambient temperature in3 h.

• Steam curing (SC): The covered moulds were first storedin an oven for 2 h at 95 �C after which the prisms werede-moulded and subjected to the following thermalramp: from room temperature to 95 �C in 3 h, 6 h at95 �C, and cooling to ambient temperature in 3 h. Sys-tem RH was at a constant 100% throughout. This curingprocedure, commonly used in the precast concreteindustry, has also been successfully employed in previ-ous studies [12,13].

Since the duration of the isothermal phase was 6 h in allprocedures (after 2 h of pre-curing in covered mould), anyreal differences among them should be sought in the waterenvironment: in the CCM procedure, only the mixingwater contributed to hydration; in the SC procedure, theprisms were always in contact with a constant amount ofexcess water; and in the DC procedure water was lostdue to the relatively high curing temperature.

The mechanical strength of the (1 · 1 · 6-cm) pasteprisms was determined 24 h after curing. The materialswere studied for mineral composition and microstructuralcharacteristics with XRD, FTIR, 29Si MAS-NMR andmercury intrusion porosimetry. The X-ray diffraction pat-terns of the powdered samples were recorded on a PhilipsPW 1730 diffractometer using CuKa radiation. The testswere run in a 2h range of 5–60� at a scanning rate of 2�/min, with a deliverance slit of 1�, an anti-scatter slit of 1�and a receiving slit of 0.01 mm. The FTIR spectra wererecorded on an ATI Mattson spectrometer. Pellets wereprepared from a mix of 1 mg of sample and 300 mg ofKBr. The spectral range was 400–1600 cm�1 and the reso-lution, 1 cm�1. The 29Si MAS-NMR spectra of the purifiedsamples were taken on an MSL-400 Bruker spectrometer.The resonance frequency used in this study was104.3 MHz, with a spinning rate of 12 kHz. All measure-ments were taken at room temperature with TMS (tetra-methylsilane) as the external standard. Finally, porestructure (total porosity and average pore diameter), downto a minimum pore diameter of 0.0067 lm, was studiedwith a Micrometrics Autopore II 9220 porosimeter at acontact angle of 141.3� and surface tension of 485 din/cm2.

Samples were weighed immediately after curing andafter subsequent drying for 24 h at 105 �C, determininghumidity as the difference between the two measurements.

3. Results

3.1. Mechanical strength

The compressive strength values are given in Table 3. Itmay be deduced from that Table 3 that curing conditionshave a considerable impact on mechanical strength devel-opment, although values also differ with mix composition.

Generally speaking, the highest strength in all the mixeswas attained with CCM, while the best result was recordedfor sample S4N1. This sample (molar ratios of Na2O/Al2O3 = 1.0 and SiO2/Al2O3 = 4.0) developed compressivestrengths of up to 102 MPa after curing. Under this curingprocedure, compressive strength was also observed to risewith alkali content and the per cent of soluble silica.

The lowest strength, by contrast, was found for dry-cured prisms (DC), especially when SiO2/Al2O3 was 4.0(irrespective of the alkali content). Rapid water loss duringearly curing would, then, appear to be a key factor in thedecline in strength, while the alkalis (Na2O/Al2O3 ratio)seem to have no significant influence on strength develop-ment in the DC curing procedure.

Finally, with the steam curing procedure (SC), strengthdevelopment was optimum for mixes with a SiO2/Al2O3

molar ratio of 4.0 (see Table 3), regardless of the Na2O/Al2O3 ratio.

The above mechanical strength values for the cementi-tious systems defined in Table 2 led to the choice ofS4N1 and S3N1 as the mixes most suitable (constantNa2O/Al2O3 ratio of one and variable SiO2/Al2O3 ratio,ranging from 3.5 to 4.0) for conducting a detailed studyof the effect of curing conditions on mineralogical andmicrostructural properties.

3.2. X-ray diffraction

The S3N1 and S4N1 XRD patterns for the curing pro-cedures studied are given in Fig. 1. Viewed as a whole, thediffractograms show that the prevailing reaction product isan amorphous compound. Nonetheless, minor amounts ofcrystalline phases are also present in the samples. One of

0 10 15 20 25 30 35 40 45

C

CC

M

M

QM

M

MS

SS

SSSS

H**

*

*

HC

SG

G G

Q

Q

Q

Q MM

MM

M

MM

M

M M

M

M

MS3N1-CCM

S3N1-DC

S3N1-SC

S4N1-CCM

S4N1-DC

S4N1-SC

Fly ash

2 Θ5

Fig. 1. XRD patterns for AAFA systems: S = hydroxysodalite;H = herschelite; C = thermonatrite; Q = quartz; M = mullite; * = instru-mental artefact. 1500 1250 1000 750 500 250

S3N1-CCM

S3N1-DC

S3N1-SC

S4N1-CCM

S4N1-DC

S4N1-SC

Fly ash

Wavenumber cm-1

1

2 34 5

M

Q

a b

900 800 700 600 500Wavenumber cm-1

Fig. 2. FTIR spectra for AAFA systems; Q = quartz; M = mullite.

318 G. Kovalchuk et al. / Fuel 86 (2007) 315–322

the most relevant deductions drawn from these diffracto-grams is that while small amounts of hydroxysodaliteappeared in all the samples studied (d = 0.635, 0.365,0.317, 0.260, 0.212, 0.176 nm), the percentage differeddepending on the curing conditions (steam curing < curingin covered moulds < dry curing). Moreover, more hydroxy-sodalite was detected in mix S3N1 than S4N1, a findingthat may be associated with the slower zeolite crystalliza-tion kinetics in systems with a very high silica content.Traces of herschelite (Na-chabazite) and sodium carbonatewere also found in certain samples.

The existence of thermonatrite crystals (Na2CO3 Æ H2O)is an indication that carbonation may take place, depend-ing on the curing conditions. Where this occurs only partof the alkaline material is fixed as a zeolite-like gel product.

3.3. FTIR spectroscopy

The infrared spectra for all the samples selected, as wellas the spectrum for the original ash, are plotted in Fig. 2.One of the most relevant findings is the shift observed inthe broad, intense T–O band (T@Si or Al) located in the1000–1100 cm�1 range. In the initial ash, this band is cen-tred at around 1060 cm�1 but as a result of the formationof new reaction products associated with ongoing alkaliactivation, it moves to lower frequencies. In sampleS4N1, regardless of the curing procedure, it is found at1003 cm�1 (peak 1 in Fig. 2). In mix S3N1, however, theband shifts to 986, 997 and 1003 cm�1 for DC-, CCM-and SC-cured samples, respectively. According with previ-ous paper [19], the position of this band depends on the Al/Si ratio of the reaction product. The substitution of a Si+4

for an Al+3 involves a reduction of the T–O–T angle, andtherefore the appearance of the signal at a lower frequency,due to the smaller bonding force and the fact that the Al–Obond is longer than the Si–O bond. The bond force con-stant for the modal Al–O–Si is smaller than for the modeof the Si–O–Si bond.

The bands appearing between 800 and 500 cm�1 areassociated with secondary building unit (SBU) tetrahedralvibrations and aluminosilicate system fragments. In the lit-erature [19–21], these external bands are attributed to thepresence of rings with a variable number of units, whichmay in turn bond to form three-dimensional structures.Increases in the number of members in a ring cause thebands to shift toward lower frequencies. The bands in the650–720 region are assigned to a symmetric stretchingvibration within the TO4 tetrahedra (peaks 2, 3 and 4 inFig. 2(b)). In the systems studied here, these bands areattributed to the presence of certain Al-rich structures suchas hydroxysodalite (phase previously identified in the XRDstudies). The medium sized band between 650 cm�1 and500 cm�1 (peak 5 in Fig. 2(b)) is characteristic of D6R-typestructures such as zeolites of the chabazite family (a hersch-elite-like zeolite likewise detected in the XRD study ofthese materials). This band is especially intense in thedry-cured samples.

3.4. 29Si MAS-NMR

The 29Si MAS-NMR spectra of the original fly ashand the reaction products formed during ash activation(mix S4N1) under different curing conditions are depictedin Fig. 3. The wide signal observed in the 29Si MAS-NMR spectrum of the initial ash is an indication of itsheterogeneous and glassy nature [8,15]. Note (withregard to the profile deconvolution of the differentspectra shown in Fig. 3) that the iron content of theoriginal ash (�7%) was considerably reduced by exposingthe samples to a strong magnetic field, a technique usedto help eliminate paramagnetism-related problems inNMR spectra.

The 29Si MAS-NMR spectra of the S4N1 samples(pastes subjected to all three different curing procedures)

4Al

3Al

2Al

1Al

0 5 10 15 20 25 30 35

Steam Curing

Percentage (%)

Q4 (n

Al)

4Al

3Al

2Al

1Al

0 5 10 15 20 25 30 35

Dry Curing

Percentage (%)

Q4 (n

Al)

4Al

3Al

2Al

1Al

0 5 10 15 20 25 30 35

Curing in Covered Mould

Percentage (%)Q

4 (nA

l)

Fig. 3. 29Si NMR-MAS spectra for AAFA systems.

Table 4S4N1 water content and pore structure parameters

Property measured after curing Curing procedure

Curing incovered moulds

Drycuring

Steamcuring

Water content (%) 10.9 0.0 22.5Total porosity (%) 15.81 19.37 20.09Average pore

diameter (lm)0.776 0.372 0.239

G. Kovalchuk et al. / Fuel 86 (2007) 315–322 319

exhibit different degrees of structural order. The mostintense signal detected, located at around �88 ppm, is asso-ciated with the formation of an aluminium-rich tectosili-cate with a predominance of Q4(4Al) silicon units[8,16,22]. Signals appearing at around �94, �99, �105and �110 ppm (with an error of ±1 ppm) are associatedwith the presence of silicon respectively surrounded bythree, two, one or zero aluminium tetrahedra in the silicoa-luminate gel. In some cases signals detected at less than�84.0 ppm are attributed to the presence of scantly con-densed residual species, probably monomer or dimer unitswith silanol groups. Finally, signals higher than �110 ppmare associated with quartz-type crystalline phases.

The results obtained from spectrum deconvolution showthat the percentage of Q4(4Al) units present in the samplevaries in keeping with the following sequence: dry cur-ing > steam curing > curing in covered moulds. Furtherto previous results [8], this signal (Q4(4Al) a �88 ppm) isattributable to the formation of a low strength, Al-rich alu-minosilicate gel (Gel 1). This initially meta-stable phaseevolves with time into a more stable Si-rich phase (Gel 2)[8,16] in which larger amounts of Si occupy the Q4(3Al),

Q4(2Al) and Q4(1Al) environments. In aluminosilicate gels,the higher the Si content, the better the mechanicalperformance.

3.5. Water content and pore structure

The water content and pore structure of the S4N1 sam-ples cured under the different procedures were likewisestudied. The results are given in Table 4 and Fig. 4.

These results confirm the marked difference between dryand wet curing conditions. In the former, water evaporates

0.01 0.1 10 100 10000.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

CCM

DC

SC

Lo

g d

iffe

ren

tial

intr

usi

on

(m

g/L

)

Pore diameter (μm)1

Fig. 4. Mix S4N1: differential pore size distribution for the various curingprocedures.

Fig. 5. Correlation between compressive strength and total porosity, bycuring procedure.

320 G. Kovalchuk et al. / Fuel 86 (2007) 315–322

rapidly and completely, affecting hydration insofar as onlythe bonded water is involved in that process during the first2 h of the reaction. In steam curing, on the contrary, anabundance of excess water supports hydration throughoutcuring. Curing in covered moulds creates intermediatesituations.

Unlike OPC pastes, in which over 80% of the pores havea diameter smaller than 1 lm, AAFA pastes have a poredistribution ranging from 0.5 and 5 lm (see Fig. 4),although the large proportion of very small pores (under0.01 lm) in dry-cured samples may be an indication thatwater evaporated completely from the pores during curing.But it should also be mentioned that dry curing was foundto be the only curing type leading to the appearance oflarge pores in the range between 10 and 50 lm which seemsto be the reason of the mechanical strength drop for dry-cured materials.

The lowest total porosity value was found for samplescured in covered moulds and the highest for steam-curedsamples.

4. Discussion

During alkaline activation, the glassy component of flyash is dissolved and certain aluminosilicate gels are formed.These reactions depend on a series of intrinsic and extrinsicvariables, including: particle size, ash chemical composi-tion, pH of the activating solution, nature and concentra-tion of the activator, and curing time and temperature[6–8,15–18]. This paper is concerned with the influence ofcuring conditions on AAFA mechanical and microstruc-tural properties, for this is a key step in the industrialmanufacture of these materials. Despite its importance,however, the question has been insufficiently studied.

The effect of curing conditions on reaction product car-bonation was addressed in a previous paper [18]. Unsuit-able curing conditions may lead to speedy carbonationand concomitantly lower pH levels in the system, in turnoccasioning significant declines in the rate of ash activation

and mechanical strength development. And carbonationcan only be prevented by subjecting the paste to high envi-ronmental humidity throughout the curing process.

Given, then, that water is a key element in alkaline acti-vation reactions, curing regime humidity has an obviouseffect on the structural and mechanical properties ofAAFA pastes, mortars or concretes [13,18]. In this surveyall the mixes studied were pre-cured in covered mouldsfor 2 h (regarded to suffice for setting) at 95 �C prior tofinal curing under the various conditions described. Theproperties of the AAFA materials finally obtained con-firmed curing procedure to be a key factor in determiningwater fixation in the hardened material.

CCM was the procedure that yielded the strongest mate-rial. Since the water supply for the CCM materials wasintermediate with respect to the dry- and steam-curedmaterials, strength development may be interpreted assum-ing the mixing water to be the only water effectivelyinvolved in the activation reactions. When these reactionstake place the resulting material develops a very densestructure with the lowest porosity possible. In fact goodcorrelation is observed, generally speaking, betweenmechanical strength and total porosity in AAFA materialscured under the various procedures (see Fig. 5).

Conversely, the dry-cured materials developed the low-est mechanical strength, along with a substantial propor-tion of large pores (between 10 and 50 lm). Theexplanation may be the loss of moisture in the system dur-ing dry curing and the resultant shortage of water to fullydissolve the glassy component of fly ash (first stage ofcementitious gel formation) under these conditions.

The highest humidity levels were achieved under steamcuring conditions. In this case the excess water availablethroughout the curing process should allow the reactionsto advance far enough for the materials to develop highcompressive strength. And indeed, mechanical strengthsof up to 76 MPa were recorded. At the same time, this cur-ing procedure affects porosity, particularly in the capillarypore range (Fig. 4) associated with the excess water.

1.35 1.40 1.45 1.50 1.55 1.60 1.65 1.700

20

40

60

80

100

120Sample S4N1

DC

CCM

SC

Co

mp

ress

ive

Str

eng

th (

Mp

a)

Si/Al ratio

Fig. 6. S4N1 mechanical strength vs Si/Al ratio.

G. Kovalchuk et al. / Fuel 86 (2007) 315–322 321

The XRD, FTIR and NMR results show that the micro-structure of the main reaction product is similar in all thesystems studied, regardless of the curing procedure used:a three-dimensional aluminosilicate gel with alkaline cat-ions compensating the negative charges associated withAl–Si substitutions. The presence of minor crystallinephases (zeolites or sodium hydrocarbonate), however,establishes certain differences with respect to the effective-ness of the various curing procedures in accelerating theactivation reactions. Herschelite is found only in theCCM samples, for instance, and zeolite formation maybe regarded to be a sign of reactivity inasmuch as the alu-minosilicate gel, thermodynamically speaking, shouldeventually convert into a zeolite [16,23,24], (the crystalliza-tion of hydroxysodalite, which cannot be regarded to be anormal zeolite, is favoured by locally high alkaline condi-tions). Moreover, the detection of mere traces of carbon-ates in only one of the hardened materials can beinterpreted to mean that none of the curing conditionsstudied facilitates carbonation.

While the FTIR spectra show no significant variationsin the band associated with T–O stretching vibrations (seeFig. 2, band 1), relevant changes are visible between 800and 500 cm�1, the area of the spectrum associated withsecondary building unit (SBU) tetrahedral vibrations.Further to aluminosilicate theory, the changes occasionedby the formation of three-dimensional structures as aresult of ring inter-connection generates variations in thenumber, shape and positions of the bands in this areaof the spectrum, which are likewise modified by variationsin the Si/Al ratio. According to the literature [19–21],both when the number of members of a ring rises andwhen the number of [AlO4]5� tetrahedra declines, theband shifts toward lower frequencies. The most intenseband in this area in the DC samples, centred at around�734 cm�1, shifts toward lower frequencies in the othercuring systems (693 in SC and 689 in CCM). In otherwords, the DC samples would be Al-rich structures, whilethe SC and CCM samples would have a higher Si/Alratio, associated with a more mature stage of the gel.Studies by Fernandez-Jimenez and Palomo [18] showeda gel type 2 to be responsible for the higher mechanicalstrength values obtained for these samples. As discussedbelow, these findings concur with the Si/Al ratios deter-mined by NMR.

A review of the spectra in Fig. 3 reveals that curing con-ditions have a significant effect on the degree of reaction ofthe materials, which would justify the differences observedin mechanical performance. The most prominent signal forthe dry-cured gel, found at �88 ppm, is associated with theformation of an Al-rich phase. In both steam-cured sam-ples and gels cured in covered moulds, where the waterlevel was high, the intensity of the Si(3Al), Si(2Al), Si(1Al) and Si(0Al) signals was observed to increase as theintensity of the Si(4Al) signal decreased. The Si/Al ratioof the gels formed can be found from Engelhardt’s equa-tion [22], where In(SinAl) stands for the intensity of the

component associated with silicon units surrounded bynSi and (4 � n)Al.

ðSi=AlÞNMR ¼

XnInðSinAlÞ

0:25X

nnInðSinAlÞ

n ¼ 0; 1; 2; 3; 4

In Fig. 6, the mechanical strength of the S4N1 mix isplotted against its Si/Al ratio. These results concur closelywith previous findings [8]: when the Si/Al ratio in the gel islow, the material develops relatively low mechanical strength

(Al-rich aluminosilicate gel), whereas mechanical strengthrises during the transformation of the Al-rich into a Si-richaluminosilicate gel. Anyway the bibliography [25] limits themaximum development of strengths when these gels haverelationship Si/Al about 2.

Mechanical strength drops when the SiO2/Al2O3 ratiorises from 3.5 to 4.0, regardless of the value of the Na2O/Al2O3 ratio. This is very likely due to some shrinkage-tak-ing place in waterglass-based systems during drying [26].Consequently, dry curing might be more appropriatelyrecommended for NaOH-based systems (such as S3N1and S3N0).

Nonetheless, the small difference in strength betweensteam-cured mixes with different Na2O/Al2O3 ratios, andmixes cured in covered moulds (Table 3) is an indicationthat the enhanced migration of alkalis in moisture-ladensystems greatly improves their effectiveness as fly ash acti-vators, even when the alkaline concentration in the internalmatrix is low. Steam curing is therefore recommended forlow alkaline mixes, particularly when they carry a high sil-ica content.

5. Conclusions

Curing conditions determine the water available duringalkali-activated fly ash matrix hardening. Consequently,they play an essential role in the development of a mate-rial’s microstructural characteristics (such as porosityand phase composition), kinetics and degree of reactionand their respective macroscopic properties (mechanical

322 G. Kovalchuk et al. / Fuel 86 (2007) 315–322

strength, shrinkage and so forth). Curing in coveredmoulds, which yields a very dense, primarily amorphousmicrostructure with excellent mechanical strength (up to102 MPa after 8 h), can be successfully used with any typeof alkali-activated fly ash mix. Dry curing is only recom-mended for NaOH-based systems (i.e., systems with alow SiO2/Al2O3 ratio), since waterglass-based mixes tendto retard reaction kinetics. Moreover, the existence of largepores (10–50 lm) lowers the strength of dry-cured systems.Steam curing, in turn, has an intermediate effect onstrength development, between CCM and DC.

Acknowledgements

One of the authors of this study benefited from a NATOScientific Committee 2003 Science Fellowships Programmepost-doctoral grant. Spanish Scientific Research Councilsupport was provided in the form of an I3P contract(Ref. 13P-PC2004L) co-financed by the European SocialFund Finally, this research was funded by the SpanishDirectorate General for Scientific Research (Ministry ofEducation and Science) under project BIA2004-04835.

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