Precursors and Additives for Geopolymer Synthesis

Abstract

The raw materials typically used in geopolymer manufacturing are described. The use of metakaolin, coal derived fly ash and slag as aluminosilicate sources is presented. The use of readily available coal fly ash in geopolymer products offers sustainability and technical benefits, especially in niche applications such as fire resistant products. There are national standards for the use of fly ash in concrete and these have some applicability to geopolymer production. For instance, beneficiation of fly ash by screening and milling can significantly improve physical properties of resultant geopolymer products. Activation of high calcium ground blast furnace slag and other slags result in more complex microstructures and are not as suitable for fire resistant products. Metakaolin is a purer source of aluminosilicate than fly ash. It is capable of ambient cures but has the penalty of high water demand which influences property development. The investigation of volcanic ashes as aluminosilicate sources has grown in importance recently. Alkaline hydroxides and silicates are used to activate the aluminosilicate sources and the sodium and potassium derivatives are commercially available. The speciation of alkali metal silicates is briefly outlined together with the influence of the alkali metal cation on this speciation. The use of sodium aluminate derivatives is examined, in particular the use of Bayer liquor, a by-product of alumina extraction from bauxite. The structure of sodium aluminate solutions is also reviewed. A final section describes the admixtures, typically superplasticisers and fibres, and fillers employed to modify the plastic and cured properties of geopolymers with emphasis on optimisation of fire resistant properties.