Utilizing basic oxygen furnace slag and fly ash to stabilize wet flue gas desulfurization gypsum for construction applications

Gypsum production in South Africa has surged due to the country’s dependence on coal-fired power plants, resulting in increased generation of fly ash (FA), wet flue gas desulfurization gypsum (WFGDG), and basic oxygen furnace slag (BOFS). This study proposes a sustainable solution for repurposing these industrial by-products into eco-friendly construction materials. A novel binder was developed by blending varying proportions of WFGDG, FA, and BOFS. In these composite blocks, up to 50 wt% of WFGDG was substituted with FA and BOFS. The blends were evaluated for compressive strength, durability, resistance to wet-dry cycles, and environmental impact. The optimum mix-containing 10 wt% FA and 40 wt% BOFS - achieved a compressive strength of 4.4 MPa after 90 days of ambient curing at 40 °C, exceeding the SANS 10145 requirement for Class III mortar. Compaction tests showed that increasing FA content reduced reactivity, with the best performance observed at the mentioned ratio. Microstructural analysis using Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD) revealed the development of new hydration products and confirmed crystallinity. Despite FA's pozzolanic potential, it does not self-activate due to the absence of alkalis and sulfates. Environmental compliance was verified through the Toxicity Characteristic Leaching Procedure (TCLP), meeting the safety criteria of SANS 227:2007 and ASTM C34–13. The study highlights the potential of these waste-derived composite bricks for load-bearing applications, offering an environmentally and economically sustainable alternative for the construction sector.