Sustainable Alkali-Activated Binders from Iron Ore Tailings

In a recent article in the journal Mining, researchers explored the feasibility of employing iron ore tailings as primary raw materials in alkali-activated systems, complemented by industrial wastes such as carbide lime and activated using sodium silicate solutions. The overarching goal is to develop a sustainable, low-cost construction material that valorizes mining waste and mitigates environmental impact, aligning with circular economy principles and responsible resource management.

Background

Mining tailings are typically stored in large basins, where their slurry-like consistency leads to low mechanical stability and considerable environmental hazards if not properly managed. These residues often contain heavy metals and other hazardous constituents that pose risks of soil and water contamination. The traditional stabilization techniques, often relying on Portland cement, are not always efficient in stabilizing tailings and are environmentally costly. The production of Portland cement involves significant energy consumption and carbon dioxide emissions, further exacerbating the environmental footprint.

Alkali-activation emerges as an environmentally favorable alternative, offering the ability to stabilize such residues effectively. The process involves a chemical reaction between aluminosilicate-rich precursors and alkaline solutions, forming cementitious gels such as N-A-S-H (sodium aluminosilicate hydrate) and C-A-S-H (calcium aluminosilicate hydrate). These gels contribute to the development of mechanical strength and durability in the resulting material. Importantly, the nature of the precursors—ranging from calcined clays to industrial byproducts, including fly ash, slag, and in this case, iron ore tailings—plays a vital role in reactivity and final properties.

The Study

The research employed a systematic experimental approach to evaluate the potential of iron ore tailings as a precursor for alkali-activated binders. Mixtures were prepared with varying proportions of sodium silicate solution, carbide lime, and tailings to identify optimal formulations. Sodium silicate concentrations ranged from 10% to 30%, while carbide lime content varied between 5% and 10%. These ratios were established based on prior knowledge of alkali-activation chemistry and preliminary tests to optimize mechanical performance. The materials were mixed thoroughly to ensure homogeneity, cast into molds, and then cured at a controlled temperature of approximately 23 °C for seven days.

A comprehensive suite of analytical techniques was used to evaluate the physical and chemical behavior of the developed binders. Unconfined compressive strength (UCS) tests were conducted to measure mechanical performance, a critical parameter for construction applications. Mineralogical characterization was performed using X-ray diffraction (XRD) to identify the formation of cementitious gels such as N-A-S-H and C-A-S-H, which indicate successful activation reactions. Fourier-transform infrared spectroscopy (FTIR) complemented these analyses by providing insights into the molecular bonding and gel formation mechanisms.

Results and Discussion

The experimental findings demonstrated that the developed alkali-activated binders achieved promising mechanical performance, with the highest compressive strength recorded at approximately 0.33 MPa after seven days of curing. Notably, formulations containing 20% sodium silicate and 10% carbide lime delivered the optimal balance of strength and environmental safety. The enhancement in mechanical properties was linked to the formation of typical geopolymer gels—mainly N-A-S-H and C-A-S-H—as confirmed by XRD and FTIR analyses. These gels contribute to a cohesive and dense microstructure, which improves load-bearing capacity.

The study revealed a sensitive dependence on mixture ratios, particularly the concentration of sodium silicate. Excessive silicate levels (beyond 20%) were observed to diminish strength, likely due to the dissolution of silica from the tailings and subsequent destabilization of the gel network. Conversely, silicate levels that were too low resulted in insufficient activation, underscoring the importance of optimizing constituent proportions for maximum reactivity and strength.

Leaching tests indicated that the binders effectively encapsulated metals, with most released concentrations falling below regulatory thresholds. Specifically, metals such as lead, chromium, and zinc showed immobilization within the gel matrix, thereby reducing environmental risks. Although some elements like barium demonstrated increased mobility under high alkalinity because of their amphoteric nature, overall, the binder’s sealing ability was deemed sufficient to meet environmental safety standards.

Conclusion

This research substantiates the feasibility of utilizing iron ore tailings as a raw precursor in alkali-activated binders. The developed mixtures, especially those with 20% sodium silicate and 10% carbide lime, demonstrated adequate mechanical strength after curing, along with low levels of metal leaching, satisfying environmental safety standards. The formation of geopolymer gels such as N-A-S-H and C-A-S-H was confirmed as central to the material’s structural integrity, underpinning its potential for application in construction.

The environmental advantages are significant; the approach promotes the recycling of mining waste, reduces dependence on Portland cement, and contributes to lower greenhouse gas emissions. Such sustainable materials align with global efforts to foster circular economy practices in the construction sector. Despite current limitations in strength, the findings open avenues for further optimization, including long-term durability studies and process scaling.