Researchers have studied the potential of iron-rich metallurgical slag as a sustainable precursor in alkali-activated materials for radioactive waste immobilization in alkali-activated materials (AAMs). The research was published in Minerals.
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Moving Away From Portland Cement
The study situates itself within a growing body of research that prioritizes the valorization of industrial by-products, aligning with global efforts to minimize waste and reduce reliance on traditional Portland cement, which is an energy-intensive material.
Iron-rich slags, especially those derived from steelmaking, have distinctive mineralogies characterized by high Fe2O3 content, residual metallic phases, and crystalline oxides, notably magnetite and hematite. These phases influence the reactivity and microstructural development of geopolymer matrices.
Prior research shows mixed outcomes regarding the effect of iron oxides on the setting behavior, microstructure, and long-term durability of alkali-activated systems.
Some studies suggest that iron can participate in gel formation or act as a network modifier, while others warn of potential microcracking due to oxidation or residual metallic phases.
The overarching challenge is to understand how high-Fe slags interact within the geopolymer network, particularly in terms of reaction kinetics, pore structure development, and mechanical stability - factors crucial for ensuring the long-term containment of radioactive contaminants.
The Study
The study adopts a multi-technical experimental approach to comprehensively characterize the performance of iron-rich slags incorporated into alkali-activated matrices.
Two types of materials are synthesized: a reference blast furnace slag geopolymer (Ref GP) and an experimental mix labeled Aachen GP, which comprises an equal weight (50 %) blend of blast furnace slag and iron-rich slag with approximately 24.6 % Fe2O3 content.
To understand the reaction mechanisms, the researchers employ isothermal calorimetry, measuring heat flow during the early stages to evaluate the influence of iron-rich slag on activation kinetics.
Fourier Transform Infrared Spectroscopy (FTIR) provides insights into silicate gel formation, while X-ray diffraction (XRD), combined with Rietveld analysis, enables the identification and quantification of crystalline and amorphous phases in both raw and cured samples at different curing ages (7 and 28 days). Thermogravimetric analysis (TGA) examines thermal stability and decomposition processes of the gel phases.
Porosity and pore structure are characterized using the Brunauer–Emmett–Teller (BET) surface area analysis and mercury intrusion porosimetry, assessing how the addition of iron affects microstructural densification.
Scanning Electron Microscopy coupled with Energy Dispersive X-ray spectroscopy (SEM-EDX) is used to observe microstructural features and elemental distribution, with a particular focus on unreacted phases and the presence of metallic Fe and crystalline oxides.
Mechanical performance testing involves determining compressive and flexural strength at 7 and 28 days, following standard protocols. Permeability tests measure the water diffusivity of the materials, which directly impacts their capacity to contain radionuclides.
Results and Discussion
The results reveal a significant influence of the iron-rich slag on the geopolymerization process. Calorimetric data show a delayed reaction onset in the Aachen GP compared to the reference system, indicative of retarded geopolymer gel formation. The cumulative heat evolved is considerably lower in the mixed slag sample, reflecting a reduced degree of reaction or slower kinetics, likely attributable to the high Fe2O3 content and associated crystalline phases that are less reactive under the curing conditions.
FTIR spectra confirm the formation of silicate gels in both systems, but with noticeable differences. Specifically, the Aachen GP exhibits a less pronounced silica polymerization, corroborated by XRD analysis, which detects unreacted crystalline phases, including residual crystalline oxides and some metallic iron particles.
From a microstructural perspective, porosity measurements indicate that the iron-rich slag system exhibits a higher porosity (approximately 38.4%) compared to the reference, characterized by larger pore sizes and an increased interconnected pore network.
BET surface area analyses support this, showing the mixed slag material has approximately three times the surface area of the pure BFS geopolymer, which influences permeability.
Correspondingly, permeability tests indicate higher water diffusivity in the Aachen GP, correlating with its microstructural features. These results collectively suggest that the presence of non-reactive crystalline phases and metallic iron particles hampers densification, leading to increased porosity.
Mechanically, the mixed slag geopolymer displays reduced compressive strength at 28 days (~14.4 MPa) compared to the BFS-only system (~43 MPa). Despite this reduction, the strength remains above the minimum threshold (8 MPa) required for low- to intermediate-level waste forms in certain regulatory contexts. The reduced strength and increased porosity raise concerns about long-term durability, particularly regarding leaching resistance and structural integrity under thermal and radiation stress.
Conclusion
The study demonstrates that metallurgical iron-rich slag can be effectively incorporated into alkali-activated matrices as part of a sustainable approach to radioactive waste containment.
The study advocates continued research into the mineralogical control of geopolymer matrices, aiming to produce durable, microstructurally stable waste forms that can withstand the harsh conditions of long-term geological repositories. Ultimately, integrating metallurgical slag processing with waste management practices can foster more sustainable mining and industrial practices, reduce the environmental footprint of both sectors, and promote resource efficiency within the mineral economy.
Journal Reference
Ali S.A.F., Frederickx L., et al (2025). Iron-Rich Slag-Based Alkali-Activated Materials for Radioactive Waste Management: Characterization and Performance. Minerals 15(12):1229. DOI: 10.3390/min15121229, https://www.mdpi.com/2075-163X/15/12/1229