Iron distribution in geopolymer with ferromagnetic rich precursor

GOMES Kelly Cristiane1a, LIMA G.S.T.1b, TORRES Sandro Marden1c, DE BARROS Silvio1d, VASCONCELOS Igor Frota2e,

BARBOSA Normando Perazzo3f

1 Núcleo de Estudos e Pesquisas em Materiais NEPEM-UFPB, Joao Pessoa/PB – Brazil 2 Departamento de Engenharia Metalúrgica e de Materiais-UFC, Fortaleza/CE - Brazil

3 Laboratório de Ensaios de Materiais e Estruturas LABEME-UFPB, Joao Pessoa/PB – Brazil akcgomes1411@gmail.com, bgabrieldsa@hotmail.com, csandromardentorres@yahoo.co.uk,

dsilvio.debarros@gmail.com, eifvasco@ufc.br, f nperazzo@ct.ufpb.br

Keywords: Iron, geopolymers, mössbauer.

Abstract. Geompolymers and their engineering applications have attracted significant attention of

the scientific community. This is due to properties such as good thermal stability and high resistance

to aggressive environments. Most studies on this subject are based on traditional precursor materials

such as calcined kaolinite clay (metakaolinite) and fly ash. The iron content is significant, reaching

around 10% in metakaolinite, for instance. The role of iron in geopolymers still lacks systematic

investigation. This can be attributed to the limitations presenting nuclear magnetic resonance

(NMR) spectroscopy, which is a widely used technique to study geopolymers structure.

Ferromagnetic elements such as iron, which is often present in some precursors, affect the magnetic

response of the material, compromising the proper analysis of its structure by NMR results. Iron

content in some industrial residues may be several times higher as it is often found in metakaolinite.

This work presents x-ray diffraction, infrared and Mössbauer spectroscopy studies on the

distribution of iron species in iron oxide/hydroxide-rich precursor, which was used to synthesized

geopolymers.

Introduction

Geopolymers are inorganic polymers, constituted of alternating SiO4 and AlO4 tetrahedra

chains, connected by a shared oxygen atom and balanced by cations [1-4]. Some precursor materials

used to produce geopolymers contain great amounts of iron. Although the presence of iron might

play important roles in the structure and properties of geopolymers, substitutions of Al by Fe was

not yet fully studied even it may occur in clays [5-7]. For instance, fly ash, with iron content of

about 10%, stands out among these commonly used iron-rich precursor materials and up to 40% for

some low-calcium ferronickel slag materials [8].

However, the presence of large amounts of iron in geopolymers results in NMR data that are

less useful to characterize geopolymer structure. This is given to the fact that the ferromagnetic

nature of iron may compromise data analysis. In this sense, 57

Fe Mössbauer Spectroscopy is an

alternative technique that favours analyses where NMR fails to provide accurate data [9], because it

is based on resonant excitation of nuclear energy levels which are sensitive to iron local

coordination and electronic structure.

Studies on geopolymers are mostly based on traditional precursor materials like metakaolinte

(Fe2O3 content of 2%), fly ash (Fe2O3 content of 10%) and blast furnace slag (Fe2O3 content of

0.5%). Nevertheless, recent studies have shown that precursors with iron content higher than the

usually found in fly ashes may be activated in alkaline environment [6-8] with applications in

engineering. This work aims to investigate the implications of the presence of iron in the

geopolymeric structure.

Materials Science Forum Vol. 643 (2010) pp 131-138© (2010) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/MSF.643.131

All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of thepublisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 150.165.163.41-10/12/09,18:19:28)

Experimental

The chemical composition of the precursor material (in natura and calcined states) is presented in

Table 01. Molar ratios and curing regimes for the synthesized geopolymers are shown in Table 02.

The precursor materials (SL1 – in natura and SL2 – calcined) and corresponding synthesized

geopolymers (GP-SL1 – in natura precursor and GP-SL2 – calcined precursor) were ground and

sieved through a 45 µm sieve. The samples were characterized by powder x-ray diffraction (XRD),

multiple internal reflection Fourier transform infrared spectroscopy (MIR-FTIR) and Mössbauer

spectroscopy (ME). XRD measurements were performed with a Siemens D5000 diffractometer

while MIR-FTIR spectra were obtained from KBr dilluted platelets by a Bomem spectrometer

(2/198mg sample to KBr ratio). Mössbauer spectra were measured in standard transmission

geometry using a constant acceleration spectrometer with a radioactive source of 57

Co in Rh matrix

at 298 K. The spectra were evaluated using the fitting software Normos. All isomer shift values

quoted are relative to metallic iron (α-Fe).

Table 01: Chemical composition of precursors

SiO2 Al2O3 Fe2O3 K2O Na2O CaO TiO2

SL1 35.58 6.09 56.67 0.03 0.19 0.06 0.81

SL2 32.80 5.82 60.02 0.04 0.02 0.06 0.79

Table 02: Synthesis parameters and curing regimes.

S/A+F H/M M/S Regimes de Cura (Tempo acumulado)

24(h) 48(h) 168(h)

GP-SL1 1.96 9.5 0.12 1.96 9.5 0.12

GP-SL2 1.70 9.7 0.11 1.70 9.7 0.11

Results and Discussion

X-Ray Diffraction. Figure 1 and 2 show diffraction patterns obtained from the geopolymeric

precursors (in natura and calcined) and from corresponding synthesized geopolymers. Table 03

summarizes the crystalline phases present in each sample.

The crystalline phases present in the in natura precursor are kaolinite, quartz, goethite and hematite.

Quartz and hematite are the only phases present in the calcined precursor and respective

geopolymer, evidencing the kaolinite-to-metakaolin and goethite-to-hematite transformation (Fig.

1).

Patterns for the samples SL1 and GP-SL1 suggest the occurrence of kaolinite amorphization as seen

by the halo between 10o2 and 20

o2 with the disappearance of the kaolinite peak. The only iron-

bearing phases present are hematite and goethite.

132 Functional and Structural Materials

Figure 1. XRD pattern for geopolymeric precursors in natura (SL1) and calcined (SL2); (Q) Quartz,

(K) Kaolinite, (G) Goethite, (H) Hematite; (R) Internal Standard Rutile

Figure 2. XRD pattern for respective geopolymers (GP-SL1 e GP-SL2); (Q) Quartz, (G) Goethite,

(H) Hematite; (R) Internal Standard Rutile

Materials Science Forum Vol. 643 133

Table 03: Phase content of geopolymeric precursors and geopolymers before and after calcinations

Precursor Materials Phases in Natura Phases after calcination

(700oC/2h) P

recu

rso

rs

SL1

Quartz

Kaolinite

Goethite

Hematite

SL2 Quartz

Hematite

Geo

po

lym

ers

GP-SL1

Quartz

Goethite

Hematite

GP-SL2 Quartz

Hematite

Diffraction patterns for samples SL2 and GP-SL2 do not show the presence of kaolinite, and the

halo between 10o2 and 20

o2 was also present. In these samples, hematite is the only iron-bearing

phase, as both kaolinite and goethite undergo amorphisation with the calcintion process.

Infrared Spectroscopy. Infrared spectra of unreacted precursor materials in natura (SL1) and its

synthesized geopolymer (GP-SL1). The bands at about 3695 cm-1

and 3620 cm-1

can be attributed to

the axial asymmetric and symmetric deformation mode of the hydroxyls associated to Al atoms and

free water [9-10]. The 1450 cm-1

and 450 cm-1

bands are attributed to T-O-Si links (T=Si or Al) [9-

10]. These bands, which are clearly evident in the spectrum for the in natura precursor material

(SL1), were broadened in the spectrum after alkaline activation (GP-SL1).

Figure 02: Transmission spectra in the infrared region. Precursor material in natura (SL1) and its

respective geopolymer (GP-SL1); Calcined precursor material (SL2) and its respective geopolymer

(GP-SL2).

134 Functional and Structural Materials

This observation can be an indication of the effect of such activation on the structure of

geopolymers, with possible increase of the peak is around 960cm-1

, resulting on the broadening of

the peaks around 1000cm-1

. The former peak has been called “the main band” for geopolymers and

is considered to give indications of the gel structure of geopolymers [9]. It is also possible to

observe some degree of amophisation of the kaolinite due to the alkaline activation process,

evidenced by the disappearance of both the 3695 cm-1

and 3620 cm-1

and the 855 cm-1 bands,

attributed to the axial asymmetric and symmetric modes of Al-OH and octahedral Al.

Fig. 03 also shows the spectra for the unreacted calcined material (SL2) and their respective

geopolymer counterpart (GP-SL2). The amorphisation of kaolinite due to the thermal treatment can

be evidenced by the disappearance of the Al-OH and octahedral Al (855cm-1

) peaks. Also, the loss

of the definition of T-O-Si bands (1055cm-1

) can be identified as well as a slight shift of this band

towards the 960cm-1

in all activated samples. This observation seems to suggest the formation of a

geopolymer gel structure [9].

Mössbauer Spectroscopy. Figures from 3 to 6 show the Mössbauer spectra obtained from

precursors and synthesized geopolymers and Table 04 shows the fitting results.

Figure 3: Mössbauer spectra: unreacted in natura geopolymeric precursor (SL1).

Materials Science Forum Vol. 643 135

Figure 4: Mössbauer spectra: geopolymer synthesised from the in natura precursor (GP-SL1)

Figure 5. Mössbauer spectra: calcined geopolymeric precursor (SL2)

136 Functional and Structural Materials

Figure 6. Mössbauer spectra: geopolymer synthesised from the calcined precursor (GP-SL2).

Table 04: Hyperfine Parameters obtained from fitting of Mössbauer spectra.

Sextet 01 Sextet 02 Dublet

IS

(mm/s)

QS

(mm/

s)

BHF

(T)

RA

(%)

IS

(mm/

s)

QS

(mm/

s)

BHF

(T)

RA

(%)

IS

(mm/

s)

QS

(mm/

s)

LW

(mm/

s)

RA

(%)

SL1 0,37 -0.21 51.2 66 0.33 -0.30 33.9 30 0.38 0.61 0.39 4

GP-SL1 0,37 -0.20 51.2 66 0.22 -0.49 33.7 26 0.32 0.59 0.59 7

SL2 0,37 -0.21 51.3 100

GP-SL2 0,37 -0.21 51.5 100

IS= Isomer Shift

QS = Quadrupole Splitting

BHF = Hiperfine Magnetic Field

RA = Relative Spectral Area

LW = Line Width

The spectra for samples SL1 and GP-SL1 (top left and right, respectively) shows the presence of at

least three different iron sites. The Analysis of hyperfine parameters obtained from the fitting curves

and listed in Table 04 allows the association of the magnetic subspectra to hematite (sextet 1, red)

[10] and also to goethite (sextet 2, blue) [12-14]. In addition, the hyperfine parameters of the

doublet (green line) are consistent with structural octahedral Fe+3

in alumina-silicate structure such

as in kaolinite [15]. As can be seen in table 4, a slight increase from 4 to 7% the doublet's spectral

area due to the alkaline activation of the un-reacted precursor in natura. This observation might

indicate an increase of the iron in octahedral sites of alumina-silicate structure as a result of the

alkaline activation, corresponded to an increased amount of iron-doped kaolinte within the

geopolymer structure. The Mössbauer spectra for samples SL2 and GP-SL2 (bottom left and right,

Materials Science Forum Vol. 643 137

respectively) show that sole presence of hematite without measurable amounts of goethite. This

confirms XRD results which indicate that hematite is the only iron-bearing phase present in these

samples.

Summary

The distribution of iron in geopolymers made with iron-rich precursors has been investigated. There

has been indication of Fe-O bonds within the geopolymer matrixes. Iron (Fe+3

) occupancies were

primarily octahedral in all mixes. The Mössbauer spectra analyses indicate that the structure of Fe+3

was associated to the crystalline phases: hematite and goethite. Also, for the uncalcined system, the

hyperfine parameters associated to the doublet suggest the presence of structural Fe+3

within an

alumina-silicate structure. This observation suggests a replacement of Al+3

by Fe+3

in octahedral

sites, most likely within the kaolinite phase.

Acknowledgements

To CNPq and CAPES for some of the authors’ scholarships.

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