Geopolymers

Geopolymer is a term covering a class of synthetic aluminosilicate materials with potential use in a number of areas, but predominantly as a replacement for Portland cement. 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 under the name Soil cements [ [http://dx.doi.org/10.1016/j.jhazmat.2006.05.008 Stabilization/solidification of hazardous and radioactive wastes with alkali-activated cements] Science Direct "Journal of Hazardous Materials" 2005-08-13] [ [http://dx.doi.org/10.1007/s10853-006-0637-z Geopolymer technology: the current state of the art] "Journal of Materials Science", 2006-06-04]

Research

Much of the drive behind research is to investigate the development of geopolymers as a potential large-scale replacement for concrete produced from Portland cement. This is due to geopolymers’ lower carbon dioxide emissions, greater chemical and thermal resistance and better mechanical properties at both atmospheric and extreme conditions.

Production

Geopolymers 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. Geopolymers can also be made from natural sources of pozzolanic materials, such as lava or fly ash from coal. Most studies have been carried out using natural or industrial waste sources of metakaolin and other aluminosilicates.

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.

tructure

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

# Dissolution of Si and Al atoms from the source material due to hydroxide ions in solution,
# Reorientation of precursor ions in solution, and
# Setting via polycondensation reactions into an inorganic polymer.

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 combined his expertise in alumino-silicate chemistry with a long-standing interest in archeology, particularly the archeology of ancient Egypt, and his examination of the building blocks of some of the major pyramids have led him to the conclusion that, rather than being blocks of solid limestone hauled into position, they 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 Tiahuanaco civilisation were made using knowledge of geopolymer techniques. [cite book
last = Davidovits
first = Joseph
authorlink = Joseph Davidovits
coauthors = Morris, Margie
title = The pyramids: an enigma solved
publisher = Hippocrene Books
date = 1988
location = New York
pages =
url =
doi =
id =
isbn = 0 87052 559 X] [cite web
last = Davidovits
first = Joseph
authorlink = Joseph Davidovits
coauthors = Aliaga, Francisco
title = Fabrication of stone objects, by geopolymeric synthesis, in the pre-incan Huanka civilization (Peru)
work = Making Cements with Plant Extracts
publisher = Geopolymer Institute
date = 1981
url = http://www.geopolymer.org/index.php?p=55
format =
doi =
accessdate = 2008-01-09]

References

External links

* [http://www.geopolymer.org Geopolymer Institute]

* [http://www.geopolymers.com.au/ Geopolymer Alliance]