Valorisation of fly ashes by geopolymerisation

Coal fly ashes and metallurgical slags are currently widely used as supplementary
cementitious materials in production of Portland cement-based concretes. However, this
application makes very poor use of the intrinsic reactivity of the glassy phases present in the
waste materials, and can hardly therefore be considered ‘valorisation’ in the true sense of the
word. Addition of these materials to Portland cements can also cause difficulties in early
strength development, limiting their use to certain applications. Geopolymerisation, on the
other hand, makes full use of the glassy ash and slag materials by using them as the key
reactants in synthesis of aluminosilicate gel binders for waste-based concrete production. The
activation of the glassy phases by alkaline solutions provides the opportunity to greatly reduce
the Portland cement content of a concrete, but requires a sound understanding of the ash
chemistry and its effects on workability, water demand and setting time if it is to be
implemented successfully on a commercial scale. In this paper, various aspects of fly ash
valorisation via geopolymerisation are discussed, including in particular the determination of
ash reactivity by a recently-developed technique utilising dilatometric data. The correlations
between ash reactivity as measured by dilatometry and geopolymer mechanical strength are
discussed in detail, and comparisons with other measures of ash reactivity presented. Some
commercial examples of geopolymer concrete in-place are also discussed to highlight these
differences in real world usage.