Geopolymer Alliance

The Geopolymerization Process

Geopolymers

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.