Development of green binder systems based on flue gas desulfurization gypsum and fly ash incorporating slag or steel slag powders

Flue gas desulfurization gypsum (FGDG) and fly ash (FA) are by-products from coal-fired power plants. This study focuses on using them as the basic components to develop green binder materials. The effects of mineral additions, including ground-granulated blast-furnace slag (GGBS), steel slag (SS), cement and hydrated lime, and chemical additives, Na2SO4, CaCl2, Al2(SO4)3 and KOH, on the mechanical strengths of the systems were studied.

The results showed that with proper dosages, mineral additions in general have positive effects in improving the compressive strengths of the FGDG-FA system. Based on the results, optimal mixes were selected for the three basic systems, i.e. the FFA system (FGDG-FA-cement-lime), the GFA system (GGBS-FGDG-FA-cement-lime) and the SFA system (SS-FGDG-FA-cement-lime).

The chemical additives showed different effects on the basic systems. The best chemical additives and dosages for the FFA, GFA and SFA basic systems are 2% CaCl2, 4% CaCl2 and 4% Al2(SO4)3, respectively. Basic properties of the selected optimal mixes, including water demand, setting time, soundness and shrinkage, were tested.

It is of interest to mention that the SFA system with Al2(SO4)3 as the additive may be used for shrinkage compensating purposes. Microstructural analyses showed that the main hydration products of the studied systems generally include gypsum, ettringite, C-S-H gels and/or calcium aluminate gels. The results also indicated that the addition of CaCl2 may not induce the formation of the Friedel's salts in the FFA and GFA systems, and the hydration products of the SFA system with Al2(SO4)3 at late ages have some distinct features compared with that of the FFA and GFA systems.