Geopolymers

Geopolymer is a term covering a class of synthetic aluminosilicate materials with potential use in a number of areas, essentially as a replacement for Portland cement and for advanced high-tech composites, ceramic applications or as a form of cast stone. The name Geopolymer was first applied to these materials by Joseph Davidovits in the 1970s, although similar materials had been developed in the former Soviet Union since the 1950s, originally under the name "soil cements". However, this name never found widespread usage in the English language, as it is more often applied to the description of soils which are consolidated with a small amount of Portland cement to enhance strength and stability. Geopolymer cements are an example of the broader class of alkali-activated binders, which also includes alkali-activated metallurgical slags and other related materials.

Research

Much of the drive behind research carried out in academic institutions involves the development of geopolymer cements as a potential large-scale replacement for concrete produced from Portland cement. This is due to geopolymers’ alleged lower carbon dioxide production emissions, greater chemical and thermal resistance and better mechanical properties at both ambient and extreme conditions. On the other side, industry has implemented geopolymer binders in advanced high-tech composites and ceramics for heat- and fire-resistant applications, up to 1200 °C. There is some debate as to whether geopolymer cement has lower CO₂ emissions compared to Portland cement. Calcination of limestone in production of Portland cement is responsible for CO₂ emissions (one ton of cement produced releases one ton of CO₂), while some processes of formation of lyme also release CO₂. Mainly it is the ratio of CO₂ reduction that is under debate, and it is process-dependent.

Production

Geopolymer binders and geopolymer cements are generally formed by reaction of an aluminosilicate powder with an alkaline silicate solution at roughly ambient conditions. Metakaolin is a commonly used starting material for laboratory synthesis of geopolymers, and is generated by thermal activation of kaolinite clay. Geopolymer cements can also be made from sources of pozzolanic materials, such as lava or fly ash from coal. Most studies on geopolymer cements have been carried out using natural or industrial waste sources of metakaolin and other aluminosilicates. Industrial and high-tech applications rely on more expensive and sophisticated siliceous raw materials.

Theory

The majority of the Earth’s crust is made up of Si-Al compounds. Davidovits proposed in 1978 that a single aluminium and silicon-containing compound, most likely geological in origin, could react in a polymerization process with an alkaline solution. The binders created were termed "geopolymers" but, now, the majority of aluminosilicate sources are by-products from organic combustion, such as fly ash from coal burning. These inorganic polymers have a chemical composition somewhat similar to zeolitic materials but exist as amorphous solids, rather than having a crystalline microstructure. Some have alleged that ancient "Roman cement" is a geopolymer cement, but in reality this material is chemically unlike alkali activated geopolymer cements because it is made using lime and forms calcium-silicate-hydrates, making it much closer to Portland cement from a chemical standpoint.

Structure

The chemical reaction that takes place to form geopolymers follows a multi-step process:

1. Dissolution of Si and Al atoms from the source material due to hydroxide ions in solution,
2. Reorientation of precursor ions in solution, and
3. Setting via polycondensation reactions into an inorganic "polymer" (actually a crystalline-like lattice).

The inorganic polymer network is in general a highly-coordinated 3-dimensional aluminosilicate gel, with the negative charges on tetrahedral Al(III) sites charge-balanced by alkali metal cations.

History

Davidovits has proposed that some of the major pyramids, rather than being blocks of solid limestone hauled into position, are composed of geopolymers, cast in their final positions in the structure. He also considers that Roman cement and the small artifacts, previously thought to be stone, of the Tiwanaku civilisation were made using knowledge of geopolymer techniques. However, because Roman cement forms calcium-silicate-hydrates, and requires calcined limestone as a reactant/precursor, it is more similar to Portland cement than alkali-activated "geopolymer cements" such as Pyrament cement of LoneStar.

External links

• Geopolymer Institute
• Geopolymer Alliance