Geopolymers are a class of inorganic polymer
formed by the reaction between an alkaline solution and an
aluminosilicate source or feedstock. The hardened material has an
amorphous 3-dimensional structure similar to that of an
aluminosilicate glass. However unlike a glass these materials are
formed at low temperature and as a result can incorporate an
aggregate skeleton and a reinforcing system if required, during the
forming process.
Reactants
The reactants are an alkali metal hydroxide/silicate solution
(often referred to as the chemical activator) and an
aluminosilicate fine binder (typically with a median particle size
in the range 1 micron to 30 microns). The binder or feedstock needs
to have a significant proportion of the silicon and aluminium ions
held in amorphous phases.
The most common activator is a mixture of water, sodium
hydroxide and sodium silicate but other alkali metal systems or
mixtures of different alkalis can be used, as can any waste source
of concentrated alkali. The solution needs to be concentrated or
the end product will be a crystalline zeolite rather than a
geopolymer.
Commonly used binders include class F flyash, ground granulated
slags or metakaolin, but any fine amorphous aluminosilicate
material can be used.
Process
As with conventional organic polymerization, the process
involves forming monomers in solution then thermally triggering
them to polymerize to form a solid polymer.
The geopolymerization process involves three separate but
inter-related stages.
During initial mixing the alkaline solution
DISSOLVES silicon and aluminium ions from the
amorphous phases of the feedstock. The binder is the primary
feedstock but any amorphous phases in the aggregate skeleton (stone
or sand particles) will also react during this stage.
In the sol so formed, neighbouring silicon or aluminium
hydroxide molecules then undergo a CONDENSATION
reaction where adjacent hydroxyl ions from these near neighbours
condense to form an oxygen bond linking the molecules, and a free
molecule of water; OH- +
OH- -> O2- +
H2O
(Ref : Hench L L, “Sol-Gel Silica. Properties,
Processing and Technology Transfer”, Noyes Publications,
1998)
The “monomers” so formed in solution can be
represented in 2-dimensions by;-
- Si – O – Al – O -
(poly[silalate]),
or, - Si – O – Al – O – Si
– O - (poly[silalate-siloxi]),
etc,
where each oxygen bond, formed as a result of a condensation
reaction, bonds the neighbouring Si or Al tetrahedra.
The application of mild heat (typically ambient or up to 90
degrees C) causes these “monomers” and other silicon
and aluminium hydroxide molecules to POLY-CONDENSE
or polymerize, to form rigid chains or nets of oxygen bonded
tetrahedra.
Higher “curing” temperatures produce stronger
geopolymers. As each hydroxyl ion in the tetrahedral is capable of
condensing with one from a neighbouring molecule it is
theoretically possible for any one silicon ion to be bonded via an
oxygen bond to 4 neighbouring silicon or aluminium ions, so forming
a very rigid polymer network. Aluminium ions in such a network
require an associated alkali metal ion (usually Na) for charge
balance.
Hardened Material
The resultant products are;-
-
a rigid chain or net of geopolymer
-
a pore solution composed of water (from the catalytic water
initially incorporated in the mix recipe plus water generated as a
result of the condensation reactions), excess alkali metal ions and
unreacted silicon hydroxide. In the case of sodium based activators
this pore solution can be considered as a weak solution of sodium
metasilicate, with a pH of about 12. It forms a continuous nano or
meso porosity throughout the geopolymer unless removed during
poly-condensation.
The physical properties of the
hardened geopolymer are influenced by the Si/Al ratio of the
geopolymer network. Below a Si/Al ratio of 3:1, the resultant 3D
nets are rigid, suitable as a concrete, cement or waste
encapsulating medium. As the Si/Al ratio increases above 3,
the resultant geopolymer becomes less rigid and more flexible or
"polymer-like". With higher Si/Al ratios, up to 35:1, the
resultant crosslinked 2D chains are more suited as an adhesive or
sealant, or as an impregnating resin for forming fibre mat
composites.
History
This concept of geopolymerization was first described by Joseph
Davidovits in his numerous papers and patents on the subject (Ref:
www.geopolymer.org), and
gave a fresh insight into this class of inorganic polymer.
Davidovits developed the notion of a geopolymer (a Si/Al
inorganic polymer) to better explain these chemical processes and
the resultant material properties. To do so required a major shift
in perspective, away from the classical crystalline hydration
chemistry of conventional cement chemistry towards that of organic
chemistry. To date this shift has not been well accepted by
practitioners in the field of alkali activated cements who still
tend to explain such reaction chemistry in portland cement
terminology.
