Abstract
Geopolymers are produced by mixing aluminate and silicate containing minerals or aluminosilicates (e.g. fly ash or clays) with an activating highly alkaline solution. As a
result a three dimensional network of silica and alumina tetrahedra sharing oxide bonds is formed. The mechanism of reactions is still disputed. It should be noted that a true
geopolymer should be low in calcium in order to avoid formation of discrete compounds thereof.
The main reasons for the on-going research on geopolymers are partly environmental in order to reduced CO2 emissions relative to Portland cement, and partly economical in
particular when unexploited fly ash can be used as aluminosilicate source.
Good early strength can be achieved, in particular when heat cured, but the strength level is very sensitive to the balance between reactive Si/Al and the amount of alkalis. In
terms of durability, geopolymers seems to have better heat and acid resistance than ordinary Portland cement.
Even though geopolymers appear to be one of the promising building materials for the future, there is still range of concerns that need to be addressed. Inherent variations in
the chemical and physical composition of the raw materials can affect the final properties of the geopolymer (e.g. fly ash, calcined clay). When cured at ambient temperature problematic drying shrinkage has been observed. Chemical admixtures designed for Portland cement e.g. plasticizing admixtures do not necessarily work for geopolymers. The high alkaline solution complicates handling of the geopolymers and can also lead to extensive efflorescence on the surface leading to less aesthetic appearance.