Geopolymer Background

A French scientist, Professor Joseph Davidovits, coined the term “geopolymer” in the 1970’s. At the time, Davidovits was studying the binders and mortars used in ancient structures, such as the Great Pyramids, in order to determine why they had not deteriorated over time. Although Davidovits is often referred to as the “father” of geopolymers, the basic chemistry regarding aluminosilicate mineral binders has been known for at least half a century

Geopolymers are formed when aluminium and silicon based materials are chemically fused, resulting in a solid ‘concrete-like’ substance. Waste materials such as fly ash, blast furnace slag, glass, paper, construction waste and contaminated soils can be used as raw materials for the manufacture of geopolymers. It may also be possible to incorporate other wastes such as fine iron ore or ‘red mud’ into the geopolymeric product.

Geopolymer materials derived from fly ash can be poured like ordinary concrete, and are suited to a variety of applications including construction products (e.g. replacements for besser bricks and hebel bricks), road building, refractory and ceramic components. The technology is well suited for application in the closing-out stages of mine tailings management projects.

Geopolymer technology can be used as a tool for cost-effective environmental management, because it has the potential to reduce and add value to existing waste streams. This opens up significant market opportunities including in the building and construction products industry.

Chemistry

Definitions: The term ‘geopolymer’ is used relatively freely in order to describe systems that would not always be classified as strictly geopolymers in accordance with the original writings of Davidovits. In simple terms:

(a) An inorganic polymer is a broad name given to any material where the three dimensional chemical structure consists of inorganic metal polymer chains, e.g when metals such as phosphorus, lithium, magnesium, etc. form part of the polymer chain.

(b) A geopolymer is a special class of inorganic polymer, whereby the chemical structure of the material is an aluminosilicate consisting of amorphous and semi-crystalline phases ie. the structure consists of Si-O-Al polymer chains only.

Terminology: Geopolymeric structures consist of a polymeric silicon-oxygen-aluminium framework (Figure x). In order to describe the formation and underlying structure of a geopolymer, Davidovits proposed a new scientific notation and terminology based on Si and Al building blocks described as: (1) Poly(sialate) with [-Si-O-Al-O-] as repeating unit, (2) Poly(sialate-siloxo) with [-Si-O-Al-O-Si-O-] as repeating unit and (3) Poly(sialate-disiloxo) with [-Si-O-Al-O-Si-O-Si-O-] as repeating unit.

Figure x: Proposed three-dimensional structure of a metakaolinite based geopolymer

Basic Chemical Principles of Geopolymerisation: The geopolymerisation process requires the presence of aluminium and silicon-containing minerals that are reactive enough to allow the silicon and aluminium components to be partly dissolved. These dissolved and partly dissolved minerals then react with each other under highly alkaline conditions to form a cement-like binder. Fly ash is a useful starting material for synthesis of geopolymeric products because of its high content of readily soluble glassy amorphous silica and aluminium phases.

It should be noted that a significant difference between ordinary pozzolanic cements and geopolymers is that the latter use a totally different reaction pathway in order to attain structural integrity. Whereas pozzolanic cements generally depend on the presence of calcium, geopolymers do not utilise the formation of calcium-silica-hydrates (CSH) for matrix formation and strength. Instead, geopolymers utilise the polycondensation of silica and alumina precursors and a high alkali content to attain structural strength.

Properties

Fly ash-based geopolymers possess exceptional physical and chemical properties compared with other binder systems such as concrete and lime-based processes. This is because the underlying chemistry is totally different from that of cement. In particular, fly ash-based geopolymers have:

• A high capacity for toxic metal immobilisation • Acid and chemical resistance • High fire resistance • Abrasion and scratch resistance • High early compressive strength • Extremely low thermal conductivity • Impermeable to water • Extreme durability

Chemically the geopolymer framework is stable in most corrosive environments and performs exceptionally well under highly saline conditions. Its resistance to sulphuric and other mineral acids is total and its thermal stability proven up to 1200 °C. These properties, combined with low permeability and a capacity to immobilise heavy metal ions, favours geopolymerisation technology as an alternative to presently used binder and capping systems.

Physical properties exhibited by geopolymeric materials include high early strength, fast setting and ease of application in the plastic state. For example, fly ash-based geopolymers can attain a compressive and flexural strength, in 24 hours, of 50 MPa and 4 MPa respectively, without the inclusion of fibres or reinforcing in the material. The 7-day compressive and flexural strengths of the materials are 95 MPa and 10 MPa respectively. Proven methods of application include pumping, spraying, moulding and most other methods available to the cement and concrete industry.

Applications

The following industrial sectors have been identified where fly ash based geopolymer technology can be applied

1. Building & Construction 2. Road Building 3. Mining 4. Environmental 5. Refractory 6. Ceramic 7. Adhesives 8. Architecture

For the Building and Construction industrial sector, the following is a list of typical products that can be manufactured using the fly ash based geopolymer technology:

• Solid prefabricated walls • Sandwich panels • Hollow core and/or foamed building panels • Pre-cast fencing panels • Bricks (e.g. besser bricks and hebel bricks) • Internal fire resistant partitioning walls • Fire barriers/walls • Pre-cast sewage pipes, box culvets. • Acid resistant pipes • Pipe linings for the reduction of steel corrosion • Chemical resistant floors & tiles

It is important to note that geopolymer technology allows the utilisation of large quantities of by-products, such as fly ash, to form mass produced products which are of commercial value.