Characteristics of Australian brown coal fly ash blended geopolymers

• Usage of Australian brown coal fly ash as geopolymer source material.
• Blending brown coal fly ash with slag and class F fly ash enhances strength.
• Sulphate present in brown coal fly ash may adversely affect geopolymers.

The present study evaluates the potential re-use options for a suite of low rank brown coal fly ashes (ASTM class C fly ash) as feedstock for geopolymer binder synthesis. Ash suitability was evaluated using a number of conventional characterisation tools including XRF, calorimetric and microscopy to analyse solid phase distribution in relation to compositional differences derived from ternary diagram analogues. Blends of brown coal ash with selected black coal fly ash (class F) and blast furnace slag were developed for compressive strength and durability tests. The study show that brown coal fly ash with higher Al content improved the rate of reaction of blended binders and that observed high contents of sulphate and magnesium oxides available in some brown coal fly ashes induced accelerated decomposition in high moist environments. Overall, it was evident from both microscopy and mineralogical analysis of binder mixtures that brown coal fly ash participate in geopolymer network formation rather than act as filler material. Factors influencing brown coal ash reactivity and the potential for incorporating this class of ash in developing stable and usable geopolymer binder systems are discussed.