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Structure, Design and Applications of Geopolymeric Materials
Ioanna Giannopoulou1,a and Dimitrios Panias1,b
1National Technical University of Athens, School of Mining and Metallurgical Engineering,
Laboratory of Metallurgy, 9 Heroon Polytechniou str., Zografos Campus, 15780, Athens –
Greece.
ammmpgi@central.ntua.gr, bpanias@metal.ntua.gr
Keywords: Geopolymerization, Inorganic Polymers
Abstract
The term “geopolymer” was firstly applied to describe a family of alkaline aluminosilicate binders
formed by the alkali activation of aluminosilicate minerals. The formation of geopolymeric
materials is the result of a complicated heterogeneous chemical reaction occurring between Al-Si
solid materials and strongly alkaline silicate solutions. The geopolymerization reaction is
exothermic and takes place under atmospheric pressure at temperatures below 100oC. Despite of the
intense research on the geopolymerization of different aluminosilicate materials and the
development of a wide range of geopolymeric materials, the exact mechanism that takes place
during geopolymerization is not fully understood. The most proposed mechanism for
geopolymerization includes the following four stages, which proceed in parallel and thus, it is
impossible to be distinguished:
(i) Dissolution of Si and Al from the solid aluminosilicate materials in the strongly alkaline
aqueous solution.
(ii) Formation of Si and / or Si-Al oligomers in the aqueous phase.
(iii) Polycondensation of the oligomers to form a three-dimensional aluminosilicate framework.
(iv) Bonding of the solid particles into the geopolymeric framework and hardening of the whole
system into a final solid polymeric structure.
This paper will describe in detail the structure of the geopeolymeric materials, the parameters that
has to be taken into consideration for their designing and their potential applications.
Introduction
Geopolymers is a new family of synthetic aluminosilicate materials formed by alkali activation of
solid aluminosilicate raw materials [1]. The term “alkali activation” refers to the chemical
dissolution of aluminosilicate raw materials in a strongly alkaline environment caused by an
aqueous solution of sodium or potassium hydroxide. Geopolymers belong to the family of inorganic
polymers, which are macromolecules linked by covalent bonds and having -Si-O-M-O- backbone,
where M denotes principally aluminum and secondarily other metals such as iron [1,2]. The
difference between geopolymers and the other inorganic polymers lies in the kind of silicon and
aluminum precursors used for their synthesis. Normal inorganic polymers are synthesized with the
sol – gel process, which employs silicon and aluminum alkoxides in alcohol–water solution as
precursors. In such systems the alkoxide groups are removed stepwise by hydrolysis under acidic
or basic catalysis and replaced by hydroxyl groups, which then form -M-O-M- linkages, where M
denotes Si or Al. Thus, branched polymeric chains grow and interconnect and through the gelation
process form a network that spans the entire solution volume. Inorganic polymers, such as
Poly(aluminosiloxanes), are polymers containing an -Si-O-Al-O- backbone synthesized by the
sodium salt of a poly(dimethylsiloxane) and aluminium chloride as Si and Al precursors,
respectively. Geopolymers, on the other hand, are synthesized by alkali activation of solid
aluminosilicate raw materials utilizing as activator a strong alkaline aqueous solution of sodium or
potassium silicate and sodium or potassium hydroxide [3]. In this case the Si precursor is the
sodium or potassium silicate solution as well as the dissolved silicon from the aluminosilicate raw
material. The Al precursor is only the dissolved aluminum from the aluminosilicate raw material,
although in some cases the aqueous activator phase is doped with aluminum cations [4,5].
Therefore, the synthesis systems of geopolymers are composed only from pure inorganic chemicals
in contrast with the synthesis systems of typical inorganic polymers that are composed primarily
from organometallic compounds.
The geopolymeric systems have gained the scientific interest during the last two decades. This is
attributed to the large variety of solid aluminosilicate raw materials that can be used for the
synthesis of geopolymers. Among the potential solid aluminosilicate raw materials, industrial
minerals, such as kaoline, feldspars, bentonite, perlite, etc. [6-8], as well as solid industrial by-
products, such as fired-coal fly ash, alumina red mud, tailings from bentonite and perlite
exploitation, metallurgical slag, building demolition materials, etc. [9-13], are the most important
raw materials. The latter class of potential raw materials is extremely attractive, mainly for
environmental reasons. Indeed, the European Union has identified the harmful effects caused by
industrial wastes and promotes in the Member States the establishment of a legal framework to
protect the human health and the environment against these effects. Through that framework, the
European Union among the others encourages the recovery and re-use of waste in order to conserve
natural resources. The geopolymerization technology has the potential to utilize the solid industrial
aluminosilicate wastes as raw materials for the production of alternative construction materials with
excellent mechanical properties and unique thermal properties.
Structure of geopolymeric materials
The most proposed mechanism for the synthesis of geopolymers includes the following four stages
[2,7,11,14], which proceed in parallel and thus, it is impossible to be distinguished:
(i) Dissolution of Si and Al from the solid aluminosilicate materials in the strongly alkaline
aqueous solution.
In the presence of water the surface metal ions of the aluminosilicate oxides may coordinate H2O
molecules and form hydroxylated surface sites that are well known as Silanol (>Si-OH) and
Aluminol (>Al-OH) groups. These groups comprise the surface active sites, where the hydroxide
ions of the alkaline solution act chemically to form surface chemical species. Under a complicated
mechanism, silicon and aluminium ions are released from the surface species into solution, where
they form aqueous species through the complexing action of hydroxide ions completing in this way
the dissolution process. The Si and Al dissolution from the starting materials can be described by
the chemical Equation (1).
(SiO2, Al2O3) + 2MOH + 5H2O → Si(OH)4 + 2Al(OH)4- +2M+ (1)
Where M denotes Na or K.
In aqueous solutions, the chemical dissolution of Al-Si minerals and generally of materials of
aluminosilicate composition is favoured in the range of high pH values, given that the dissolution
rate of these materials increases significantly as the solution pH is increased. Moreover, the
dissolution rate of Al-Si solid materials is strongly depended on the size and the specific surface
area of particles, as it concerns for a typical heterogeneous chemical reaction.
(ii) Formation of Si and / or Si-Al oligomers in the aqueous phase.
As Si and Al concentrations in the aqueous phase increase gradually, certain chemical reactions
take place between the formed hydroxy-complexes. Reactions result in the formation of the
geopolymers precursors that are oligomer species (polynuclear hydroxy-complexes) consisting of
polymeric bonds of Si-O-Si and Si-O-Al type, as is described by chemical Equations (2) - (4).
Si(OH)4 + Si(OH)4 ⇔ (OH)3Si-O-Si(OH)3 + H2O (2)
Si(OH)4 + Al(OH)4- ⇔ (OH)3Si-O-Al(-)(OH)3 + H2O (3)
2Si(OH)4 + Al(OH)4- ⇔ (OH)3Si-O-Al(-)(OH)2-O-Si(OH)3 + 2H2O (4)
The existence of soluble silicates in the alkaline aqueous phase of the geopolymeric system
enhances the formation of oligomer species. Soluble silicates in the aqueous phase increase
essentially the concentration of Si, shifting mainly Equation (2) to the direction of Si-O-Si species
formation, as well as Equations (3) and (4) to the direction of Si-O-Al oligomers formation. Thus,
alkaline silicate solutions used in the synthesis of geopolymers provide the system with the
necessary silicate oligomers for the development of the geopolymeric framework.
(iii) Polycondensation of the oligomers to form a three-dimensional aluminosilicate framework.
The increase of oligomers concentration in the aqueous phase involves their polycondensation,
which in turn lead to the development of a three dimensional framework consisted of SiO4 and / or
AlO4 tetrahedra linked alternately by sharing common oxygen ions, as it is presented by chemical
Equations (5a) and (5b).
n[(OH)
3
Si-O-Si(OH)
3
] → (-Si-O-Si-O-)
n
+ 3nH
2
O (5a)
O O
n[(OH)
3
Si-O-Al
(-)
(OH)
3
] → (-Si-O-Al
(-)
-O-)
n
+ 3nH
2
O (5b)
O O
Polycondentation reaction involves the chemical bonding of geopolymers precursors (oligomers) by
simultaneous removal of water molecules. This procedure is well known as polymerization.
Oligomers may react in every hydroxyl ion site, forming macromolecular chains and /or rings that
result in a three dimensional framework. As long as aluminum ion Al3+ participates into the
geopolymeric framework in IV-fold coordination with respect to oxygen (Equations (3) – (5)), a
negative charge imbalance is created and therefore, the adsorption of diluted cations (Na+, K+, Li+,
Ca++, Ba++, NH4+, H3O+, etc.) in the framework cavities, near the sites of aluminum ions, is essential
to maintain the electric neutrality in the matrix.
(iv) Bonding of the solid particles into the geopolymeric framework and hardening of the whole
system into a final solid polymeric structure.
Since the geopolymeric framework is developed in the aqueous phase, it comes across the active
surface sites of the solid particles, where it is possible to react bonding the undissolved particles in
the final geopolymeric structure, according to the chemical Equation (6).
>T-OH + ΗO-
(
-Si-O-Al-O-
)
n
→ >T-O-
(
-Si-O-Al-O-
)
n
+ H
2
O
(
6
)
O O OO
Where >T denotes surface Si or Al sites.
The active surface sites of the solid particles, which are presented as >T-ΟΗ in Equation (6), are the
silanol (>Si-OH) and aluminol (>Al-OH) groups. It is possible for a macromolecular chain or a ring
of the geopolymeric framework to create a bond of >Si-O-Si and >Al-O-Si type on these sites,
bonding in this way the undissolved particles in the polymeric framework. Thereinafter, hardening
of the polymeric matrix, which occurs as the excess of water is removed from the geopolymeric
matrices during the curing procedure, may lead to durable and tough materials.
The geopolymeric materials are composite materials. Their typical microstructure, as it has been
observed by Scanning Electron Microscopy, is depicted in Figure 1.
(a) (b)
Figure 1. Typical microstructure of geopolymeric material as it is seen by SEM. (a) Geopolymer
produced from fired coal fly ash (b) Geopolymer produced from ultrafine perlite.
It is clearly observed that non-dissolved spherical fly ash particles (Fig. 1a) or elongated perlite
particles (Fig. 1b) are enclosed within an amorphous aluminosilicate matrix acting as a binder.
Therefore, the geopolymeric materials are composed from solid particles chemically connected with
an inorganic polymeric binder formed during the geopolymerization process. The mechanical
strength of materials is strongly related to the strength of the chemical bonds formed on the
interface solid particle / inorganic polymer. This interface is normally the surface where the
geopolymeric materials fail.
The inorganic polymeric binder has primarily an aluminosilicate chemical composition, although
impurities such as calcium and iron oxides are the most usually occurring chemical constituents of
geopolymeric systems. The polymeric binder is normally amorphous or semi-crystalline, as it is
clearly seen in Figure 2. Figure 2(a) shows typical XRD diagrams of geopolymeric materials as
well as of an aluminosilicate raw material used for the formation of these materials. It can be
observed the slight shift of the broad hump registered between 2θ=20o and 30o, attributed to the
amorphous phase of the raw material, towards higher values 2θ=25o-35o.
(a) (b)
Figure 2. (a) Typical XRD diagrams and (b) FTIR diagrams of geopolymers synthesized from an
aluminosilicate by-product produced during the exploitation of Greek bentonites.
This typical shift indicates the dissolution of the primary amorphous phase and the formation of a
new amorphous phase in the geopolymeric material. The formation of this new amorphous phase is
Raw material
Geopolymers
Raw material
Geopolymers
also observed in Figure 2(b), where the typical FTIR spectrums of geopolymeric materials, as well
as of the raw material used for their formation, are shown. The most characteristic FTIR band in the
geopolymeric systems appears in the wavenumbers region 990-1090cm-1 and is attributed to the
asymmetric stretching vibration of T-O-Si, where T denotes Si or Al [11]. The shift of this peak
towards lower wavenumbers indicates the dissolution of the amorphous aluminosilicate phase of the
raw material and the formation of a new amorphous gel in which the backbone is consisting of
polymeric chains with smaller length in relation to the ones of the raw material.
The structure of the new amorphous aluminosilicate gel formed during the geopolymerization can
be revealed with the application of 29Si Nuclear Magnetic Resonance spectroscopy as it is seen in
Figure 3(a). The 29Si MAS-NMR spectrum reveals the degree of structural order in geopolymers
[15]. In Figure 3(a), it is seen the 29Si MAS-NMR spectrum and its deconvolution, which means the
analysis of spectrum in several component peaks. The most intense peaks are located at around -
88ppm and -94ppm. The first one denotes the existence of an Q4(4Al) aluminosilicate backbone
having as structural unit silicon tetrahedra [SiO4]4- surrounded by four aluminum tetrahedra
[AlO4]5. The latter peak denotes the existence of an Q4(3Al) aluminosilicate backbone. The peaks
at around -99ppm, -105ppm and -110ppm denote the presence of Q4(2Al), Q4(1Al) and Q4(0Al)
structural units in the aluminosilicate gel formed during geopolymerization. Peaks at less than -
84ppm are normally attributed to monomeric or oligomeric units consisting of silanol groups. The
general conclusion is that the backbone of the geopolymeric materials is consisting of silicon
tetrahedra surrounded by four, three, two, one or zero aluminum tetrahedra. The relative amount of
Q4(4Al), Q4(3Al), Q4(2Al), Q4(1Al) and Q4(0Al) structural units is strongly dependent on the Si:Al
molar ratio in the gel, which in turn is dependent on the amount of easily dissolved aluminum in the
utilized solid raw materials as well as on the chemical activity of NaOH in the initial strong alkaline
aqueous solution of the geopolymeric synthesis. Increased aluminum content in the aluminosilicate
gel, which means low Si:Al molar ratio, improves the cross-linking of the growing polymers having
as a result the formation of a three-dimensional structure. High Si:Al molar ratio in the gel promotes
the formation of a two-dimensional structure with the form of layers interconnected imperfectly
with cross-linking, as it is seen in Figure 3(b).
(a) (b)
Figure 3. (a) 29Si MAS-NMR spectrum for a geopolymeric system based on fired coal fly ash.
(b) 2D structure in a geopolymeric system based on red mud and metakaoline.
Design of geopolymeric materials
The geopolymeric systems are composed from two phases, the solid one and the aqueous one which
is called activator. The solid phase is an aluminosilicate material, which contains easily dissolved
silicon and aluminum in a strongly alkaline aqueous solution. This material could be a typical
aluminosilicate industrial mineral, such as kaoline, feldspars, bentonite, perlite, etc. or a solid
industrial by-product, such as fired-coal fly ash, alumina red mud, tailings from bentonite and
perlite exploitation, metallurgical slags, building demolition materials, etc. The aqueous phase is a
strong alkaline sodium hydroxide and sodium silicate solution. The sodium hydroxide concentration
in the aqueous phase has to be high enough so that the hydroxide promoted dissolution of the solid
phase to take place under substantially high rates. This value can be obtained experimentally by
performing specifically designed dissolution’s tests. On the other hand, the sodium silicate
concentration in the activator must be high enough so that the mass ratio SiO2: Na2O to be higher
than 1 accelerating with this way the polycondensation phenomena (Figure 4) and forming larger
rings, complex structures and at the end polymers [11].
0
10
20
30
40
50
60
70
80
00,511,522,533,5
SiO
2
/Na
2
O, mass ratio
Apparent Relative Concentration, %
Complex
Structures,
Polymers
Larger Rings
Monosilicate
Chains &
Cy c lic T rime r s
Figure 4. Soluble silicate species equilibria in 1m aqueous silicate solution.
The solid and aqueous phases are mixed together in the highest allowable Solid:Liquid ratio so that
the obtained viscous paste to have good workability and therefore to be easily molded in plastic
molds. The molded materials are vibrated in a vibration table in order for the entrapped air bubbles
to be efficiently removed and then are cured under low temperatures (<100oC) in a controlled
humidity environment. The curing duration is strongly depended on the curing temperature and
varies normally from some hours to some days. In general, the higher the curing temperature, the
lower the curing duration.
Properties and applications of geopolymeric materials
Although some factors governing the formation of geopolymers are still not completely understood,
the physical, chemical and mechanical properties of these materials indicate that they offer
attractive options towards a wide range of industrial applications. The Si:Al molar ratio in the
geopolymeric structures determines essentially the properties and therefore, the application fields of
geopolymers. Depending on the Si:Al ratio, it is possible to obtain products with different
characteristics and consequently, for different industrial applications [16], as they are summarized
in Table 1.
Geopolymers with three-dimensional structure can substitute ceramic, cement and concrete
products. They present excellent mechanical properties concerning mainly to the compressive
strength that, in certain cases, overcomes largely the respective one of the Portland cement, the
tensile strength, which is twice to three times that of Portland cement and the Mohs hardness that
ranges from 4 to 7. Additionally, they have normally low apparent porosity or nano-porosity, which
gives them very low water permeability ranging between 10-9-10-12 cm/s and thus, very good
resistance in freezing-thawing cycles. Moreover, geopolymers are exceptional fire resistant, heat
resistant, endothermic materials that can be used for the fire protection of constructions from steel
reinforced concrete, such as buildings and road tunnels. Recently, geopolymers are examined as
potential technological solutions for the management of toxic and radioactive waste materials.
Extent laboratory and pilot-plant researches suggest that geopolymeric composites are well suited
for the disposal of toxic and radioactive wastes (mine tailings, slugs, etc.) offering a safe chemical
encapsulation of the contaminants and presenting structural long-term stability with respect to
adverse environmental conditions. Properties, such as low water permeability, small shrinkage,
good resistance to freezing-thawing cycles and high resistance to acid attack, favor the use of
geopolymeric materials as solidification and immobilization systems for toxic and radioactive
wastes.
Table 1: Dependence of the geopolymers structure and applications on the molar ratio Si:Al
Applications
Si:Al ratio Polymeric
character
Low
technology
High
technology
Si:Al=1:1
- Tiles
- Ceramics
- Fire
protection
Si:Al=2:1
3D
Network
- Cements
- Concretes
- Radioactive
and toxic
wastes
management
Si:Al=3:1
- Foundry
equipment
- Fire resistant
fiber glass
composites
- Tooling for
aeronautics
- Heat
resistant
composites
Si:Al>3:1 - Sealants for
industry
- Tooling for
aeronautics
20:1<Si:Al
and
Si:Al>35:1
2D
Cross-
link
- Fire and heat
resistant
fiber
composites
Geopolymers with two-dimensional cross-linking structure poses even better heat and fire resistant
properties in relation to the three-dimensional structure materials. This is attributed to their unique
structure that allows physically and chemically bonded water to migrate and evaporate without
damaging the material. In order for their mechanical properties to be improved, 2D cross-linked
geopolymers are reinforced by different types of fibers (carbon, glass, minerals or steel) to produce
advanced composite materials. Provided that geopolymeric composites are able to withstand at high
temperatures exhibiting refractory properties, they can find specific applications in high technology
industrial areas. The fiber reinforced geopolymeric materials can be used as fire safety advanced
materials into the transportation (air, sea, rail, car, etc.), nuclear and pharmaceutical industrial areas
as well as for the production of moulds for thermoplastic materials and metals casting, containers
for hazardous chemicals and radioactive wastes, components for high performance engines,
lightweight materials and prototype automobile components.
As it is evident from the aforementioned applications, geopolymers gain increasingly attention as
viable alternative to conventional materials in numerous industrial areas. The most important
advantages that render them attractive are related mainly to:
(i) The low production cost, considering that they are based on aluminosilicate materials, which
both, occur naturally in abundance on the crust of the Earth as clay minerals and derive from
industrial wastes as fly ash, blast-furnace slag, red mud etc.
(ii) The energy-effectiveness of the production procedure, since geopolymers are cured and
hardened at relatively low temperature. As it is documented, the energy consumed for
geopolymeric tiles fabrication is less than 16% of that of conventional ceramic bodies.
(iii) The environmental contribution provided that: (a) a number of waste materials can be
transferred into added value innovative products, (b) greenhouse gas emissions are essentially
reduced during geopolymeric materials production (the geopolymeric materials manufacture
emits 80% less CO2 than that of Portland cement) (c) geopolymeric composites can be utilized
for safe stabilization and immobilization of radioactive and toxic wastes.
Conclusions
Geopolymers are novel materials, which are rapidly developed during the last decades. They are
produced by geosynthesis that involves the alkali activation of solid aluminosilicate raw material.
Geopolymers belong to the family of inorganic polymers, which are macromolecules linked by
covalent bonds and having -Si-O-Al-O- backbone. Factors such as the molar ratio Si/Al of the
starting materials, the concentration of the alkali metal silicate solution, the water content in the
synthesis and the extent of Si and Al dissolution have a significant correlation with the structure and
consequently, with the properties of the resulted geopolymeric materials. Despite that the
geopolymerisation chemistry and mechanism have not yet clearly understood, geopolymers have
gained increasingly attention as viable alternative to conventional materials in numerous industrial
areas. Their most important potential applications are the substitution of ceramic, cement and
concrete products in construction industry, the fire protection of buildings and road tunnels, the
management of toxic and radioactive wastes and the production of advanced composite materials
for high technology applications into transportation, nuclear, pharmaceutical and aeronautic
industrial areas. The main advantages of geopolymeric materials are their low production cost, the
energy-effective production procedure and their environmental friendly character.
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