Iron slag substitution in soil-based geopolymers: a comparative study with coal fly ash for non-load bearing applications

Abstract

The production of traditional Ordinary Portland Cement (OPC) drives significant greenhouse gas emissions, necessitating a shift toward sustainable, low-carbon building materials. This study investigates the development of geopolymerized mud concrete (GMC) by valorizing two industrial by-products—coal fly ash (CFA) and iron slag (IS)—as primary aluminosilicate precursors. The research aimed to establish baseline parameters for a reference CFA-based GMC, optimize an IS-substituted alternative, and evaluate its comparative feasibility. Microstructural characterization confirmed both materials possess essential glass-forming oxides. While CFA relies on abundant silica and alumina to develop robust aluminosilicate networks, IS drives binder formation through complex Fe-A-S-H and C-A-S-H gels. Utilizing an alkaline activator comprising 10M sodium hydroxide and 2% sodium chloride, the methodology was included in two phases. First, optimal thermal curing conditions were established for the 20% CFA reference. Subsequently, to maximize mechanical performance, IS substitution levels (10% to 40%), curing temperatures (ambient to 100 °C), and durations (6 to 24 hours) were systematically optimized through 7-, 14-, and 28-day compressive strength testing. The benchmark CFA mix (cured at 100 °C for 6 hours) achieved a peak 28-day compressive strength of 3.241 MPa. Conversely, the optimized IS matrix (30% IS, cured at 60 °C for 24 hours) attained 1.601 MPa. Although yielding approximately 49% of the CFA baseline capacity, the 1.601 MPa IS-based geopolymer satisfies requirements for non-load-bearing masonry. Ultimately, this study demonstrates that IS is a resilient, low-cost, and eco-friendly precursor alternative that advances circular economy principles, particularly in regions lacking accessible CFA reserves.