Integrating Fe2O3-rich soda residue into alkali-activated materials: Evaluation of mechanical and structural properties

Soda residue, a by-product of the Na₂CO₃ industry, is recognized for its high alkalinity, which can lead to corrosion and the deposition of iron oxide (Fe₂O₃) from industrial procedures and lab infrastructure. This study delves into the effects of incorporating Fe₂O₃-rich soda residue (FSR) into alkali-activated materials (AAMs), specifically alkali-activated fly ash (FA) and/or slag powder (SP), on the fresh and hardened properties of the resulting materials. The methodology encompassed an evaluation of the materials' workability through electrical conductivity (EC) and fluidity measurements. The recordings of drying shrinkage, flexural, and compressive strengths over time were conducted to assess the long-term hardened performance of the AAMs. To dissect the underlying mechanisms, a suite of analytical techniques was employed. X-ray diffraction (XRD) was utilized to delineate the mineral phases present, while Fourier-transform infrared spectroscopy (FTIR) elucidated the chemical bonding within the materials. Scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS) provided detailed insights into the morphological and elemental composition. Thermogravimetric-differential scanning calorimetry (TG-DSC) further characterized the thermal decomposition behavior of the constituents. Results indicate that the incorporation of 20 % FSR into the alkali-activated FA-SP matrix resulted in a significant enhancement in flexural and compressive strengths (higher than those of alkali-activated FA-only or SP-only matrix), reaching 3.3 MPa and 24.3 MPa, respectively, after a 360-day curing period. Additionally, a modest reduction in drying shrinkage and a pronounced decrease in EC and fluidity were observed. The release of Mg²⁺ (from SP) and Ca²⁺ (from SP and FSR) cations was identified as a key factor in the polymerization of Si–O–Al chains, leading to the formation of amorphous aluminosilicate structures. The physical presence of Fe₂O₃ and Cr₂O₃, due to its low solubility, functioned as a filler, improving the mechanical properties of the AAMs. Therefore, the strategic integration (active vs. inert) of FSR into FA-SP systems can significantly influence the formation of aluminosilicate structures, offering a promising avenue for the valorization of industrial by-products in the realm of sustainable AAMs.