Influences of the chemical composition of fly ashes on their reactivity

The use of hard coal fly ash (Class F fly ash) as a main constituent of cementitious systems is becoming increasingly more attractive. The use of fly ash as a supplementary cementitious material saves primary raw materials and fuels and reduces the CO2 emissions of the cement industry. For the production of efficient binder systems containing fly ash, a fundamental understanding of the hydration reactions of fly ashes is essential — not only in cement clinker based matrices but also in other media. Therefore, the research focussed on both the investigation of the corrosion mechanisms of fly ashes in alkaline media and the investigation of «pure» fly ash reaction products. Relations between the chemical composition of fly ashes and possible reactions were accentuated.
Fly ashes react pozzolanic to calcium silicate hydrates (C-S-H) and calcium aluminate hydrates (C-A-H), and due to alkaline activation to aluminosilicate gels. Both the pozzolanic reaction of a fly ash and other reactions, like the formation of zeolites, are influenced by the reactivity and the interaction of different elements such as Si, Al, the alkalis and others. If fly ash reacts in a matrix based on Portland cement clinker, the differentiation of the reaction products can be extremely complicated. An extended knowledge of the composition of these products is essential for a better understanding of fly ash reactivity. Thus eight fly ashes with different contents of Si, Al, Fe, and Ca were selected. These fly ashes were suspended in artificial cement pore solutions. Afterwards the eluate was examined by means of ion chromatography and photometric methods. „Pure” fly ash reaction products were generated by the reaction of different fly ashes with calcium hydroxide in different reaction media. These products were analysed by means of X-ray diffraction and dynamic scanning calorimetry.
As a result conclusions about the relationship between available dissolved elements during the reaction and the hydration products of the fly ash could be drawn. Furthermore, impacts of the chemical composition of the fly ash glass on the conversion of the fly ash will be presented.

Author: S.E. Schulze, J. Rickert

Section: Waste utilization

Keywords: hard coal fly ash, reactivity, chemical composition, conversion, compressive strength