Analysing and Manipulating the Nanostructure
of Geopolymers
J.L. Provis, A. Hajimohammadi, C.A. Rees, and J.S.J. van Deventer1
Abstract. Geopolymer concretes are currently being commercialised in Australia
and elsewhere around the world, with a view towards enhancing the sustainability
of the world’s construction industry. The fundamental geopolymer binder is an
aluminosilicate gel which displays key structural features on every length scale
from Ångstroms up to centimetres, meaning that multiscale analysis is key to the
development of a detailed understanding of geopolymer formation and performance. Here, we present results from investigations of geopolymer nanostructure,
focusing on the use of infrared spectroscopy as an analytical tool. The effects of
different combinations of precursors in geopolymer formation provides critical
information, in particular with regard to the rate of reaction and its impact on the
final distribution of elements and structures within the geopolymer binder. Formulations are designed so that the same composition is obtained by the use of precursors which release their constituent elements at very different rates under alkaline
attack during geopolymerisation, and this provides essential information regarding
the role of different elements in forming strong and durable geopolymer structures. Seeding the geopolymer mixture with very low doses of oxide nanoparticles
presents several unexpected effects, both in terms of reaction kinetics and also in
altering the nature of the zeolitic crystallites formed within the predominantly
X-ray amorphous geopolymer binder.
1 Introduction
A geopolymer is a type of alkali-activated aluminosilicate cement which can have
comparable or superior mechanical, chemical and thermal properties when compared to Portland-based cements, and with significantly lower CO2 production [1].
This has led to geopolymers receiving increasing attention in the scientific literature over the past decade; however, much about these materials is still not well
understood [2, 3]. Geopolymers are generally synthesised by the reaction between
J.L. Provis, A. Hajimohammadi, C.A. Rees, and J.S.J. van Deventer
Department of Chemical & Biomolecular Engineering, University of Melbourne, Australia
e-mail: [email protected]
http://www.chemeng.unimelb.edu.au/geopolymer/
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J.L. Provis et al.
an aluminosilicate source (often fly ash, metakaolin and/or blast furnace slag) and
an alkali metal hydroxide or silicate solution. The geopolymer binder structure
consists of tetrahedral Si-O and Al-O bonds arranged in a predominantly X-ray
amorphous gel network, where the tetrahedral Al sites are charge-balanced by an
alkali cation.
The first stage of geopolymerization is the release of aluminate and silicate species from a solid source, induced by alkali attack on an aluminosilicate material.
First, the surface of the solid contacts the activating solution, and hydrolysis reactions begin to occur, with the formation of oligomers and finally polycondensation
to form a three-dimensional aluminosilicate network [4]. Soluble silicates are frequently used in geopolymer production to aid dissolution of the aluminosilicate
starting material and enhance the mechanical properties of the binder [5, 6].
The primary aim of this paper is to analyse the role of gel nucleation in the
formation of geopolymers, particularly by the use of high-surface area seed particles to modify the nucleation process [7], and also by designing geopolymer mixes
with different reaction rates but the same composition. Spatially-resolved infrared
spectroscopy will be used to identify the effect of the kinetics on the distribution
of silicate and aluminate species within the geopolymer gel [8].
2 Materials and Methods
To synthesise geopolymers for seeding experiments, 20.8g of a 6M NaOH solution was mixed with 60g of fly ash (Gladstone Power Station, Queensland, Australia, oxide composition and detailed characterisation given in [6, 9]) and stirred
mechanically for no more than 2 minutes. Additional samples were prepared with
the same composition, but with 0.01g of Al2O3 nano-particles (NanoScale Materi2
als, USA, mean particle size 200nm and specific surface area 275m /g) dispersed
in the activating solution immediately before mixing with the fly ash, to act as potential nucleation sites. XRD analysis was performed after 100 days at 30ºC.
Geopolymers for infrared spectroscopy study were synthesised using a ‘onepart’ (just add water) procedure [10]: solid washed geothermal silica (96% SiO2,
from the Cerro Prieto geothermal power station, Mexico) and reagent-grade sodium aluminate (Aldrich) were blended to give the desired Si/Al ratios, and then
mixed with water at a molar ratio of H2O/Na2O = 12. ATR-FTIR spectra of onepart mix geopolymers were collected using a Varian FTS 7000 FT-IR spectrometer, with a Specac MKII Golden Gate single reflectance diamond ATR attachment
with KRS-5 lenses and heater top plate. Absorbance spectra were collected from
-1
-1
4000-400 cm at a resolution of 2 cm and a scanning speed of 5 kHz with
32 scans.
3 Results and Discussion
3.1 Seeded Nucleation
Fig.1 shows the X-ray diffraction data obtained from seeded and unseeded geopolymer formulations after 100 days’ curing at 30°C. The most striking aspect of
Analysing and Manipulating the Nanostructure of Geopolymers
115
these diffractograms is that the presence of less than 0.01% by mass of nanosized
alumina seed particles has entirely changed the nature of the zeolite product
observed, from the ‘normal’ (widely observed) faujasite-type structure of zeolite
Na-X to the unusual edingtonite-type Na-F [7]. Zeolite Na-F has previously been
reported predominantly as the result of ion exchange from a product synthesised in
the potassium form [11, 12].
Fig. 1 X-ray diffractograms of fly ash geopolymers with and without nanoparticle seeds.
Selected peaks are marked - F: zeolite Na-X (faujasite), M: mullite, Q: quartz, ZF: zeolite
Na-F. Other peaks are due to unreacted mullite, quartz and iron oxides. Data from [7]
In addition to this change in the nature of the reaction product, seeding also
shows a strong effect on the reaction kinetics. The induction period prior to the
onset of gel formation which is observed in most hydroxide-activated fly ash geopolymer syntheses [13] was entirely absent in the seeded system [7]. This is consistent with an explanation based around nucleation behaviour; nucleation of the
geopolymer gel in hydroxide-activated geopolymers usually takes place on the fly
ash particle surfaces [14], but the availability of the seed particle surfaces means
that the energy barrier associated with this nucleation process is greatly reduced.
Nucleation therefore takes place directly around the seed particles, which direct
the geopolymer structure in a different manner to the fly ash surfaces, resulting in
the formation of a different type of zeolite in the geopolymer gel.
3.2 Time-Resolved Infrared Spectroscopy
Fig. 2 shows the results of infrared spectroscopy carried out at different times in
the geopolymerisation process. The progression from the original powder mix to a
hardened geopolymer structure is made more clearly visible by the removal of the
water background from these spectra.
-1
Bands at 1060 cm are assigned to stretching of Si-O-Si bonds at the surface of
-1
the unreacted silica particles [15], and bands at 1100, 800 and 475 cm relate to
stretching, bending and rocking of the Si-O-Si bonds within the network of the
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(b)
(a)
8
8
6
6
4
4
2
2
0
0
1000
800
1200
-1
Wavenumber (cm )
600
1200 1000 800
600
-1
Wavenumber (cm )
Fig. 2 ATR-FTIR spectra of one-part geopolymer samples with H2O/Na2O = 12 and (a)
Si/Al = 1.5; (b) Si/Al = 2.5. Numbers refer to the geopolymer age in days, at a temperature
of 40°C. Data from reference [10]
-1
unreacted particles of geothermal silica [16]. Bands at 545, 625 and 700 cm are
related to vibrations in the unreacted solid aluminate [17]. The position of the
main Si-O-T (T: tetrahedral Si or Al) stretching band gives an indication of the
length and angle of the Si-O bonds in the silicate network [6, 13].
-1
Initially, the main Si-O-T stretching band occurs at 1060 cm , shifting to lower
wavenumbers in both samples over time. This can indicate changes in the silicate
network including an increase in non-bridging oxygen on silicate sites, charge balancing by sodium cations in the system [18] or increasing the substitution of tetrahedral Al in the silicate network [19].
Over time, there is also a reduction the in intensity of the main Si-O-Si band,
indicating that the solid silica is dissolving. At the same time, a new band starts
-1
forming at about 950cm and the intensity of this band increases over time. This
particular band is associated with the Si-O-T stretch within the newly formed geopolymer network [6, 13], and it appears to grow at a similar rate in both samples
depicted in Figure 2, accounting for the fact that the geothermal silica peak is larger in Fig. 2b due to the higher concentration of Si in this sample. Spectra such as
these, and reaction kinetic analysis derived from in situ time resolved FTIR analysis [13], provide the opportunity to observe in detail the reaction processes taking
place during geopolymerization, and the similarities and differences between ‘traditional’ and one-part geopolymer formulations.
Analysing and Manipulating the Nanostructure of Geopolymers
117
4 Conclusions
The data presented in this paper show that there is clearly a strong degree of kinetic control in the formation of geopolymer gels by alkaline activation of fly
ashes. The addition of seed particles and the manipulation of alumina release rates
can each give control of geopolymer structure on a length scale of nanometres to
microns. Such manipulation is likely to be important in the future development of
geopolymer technology, particularly in optimizing mixes for specific applications
including rapid controllable strength development. The application of experimental techniques such as time-resolved infrared spectroscopy is of significant value
in the study of complex materials such as geopolymers, as these techniques provide the opportunity to obtain structural information which would not normally be
accessible by more ‘standard’ methods of analysis.
Acknowledgments. This work was funded partially through a studentship awarded to
Ailar Hajimohammadi by the Centre for Sustainable Resource Processing through the Geopolymer Alliance, partly through Discovery Project funding from the Australian Research
Council, and partly through the Particulate Fluids Processing Centre, a Special Research
Centre of the Australian Research Council.
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