Editorial for Special Issue “Metallurgy Waste Used for Backfilling Materials”

Solid waste from mining and metallurgical activities—such as mine tailings, waste rock, and slags—arises from processes such as extraction, processing, and smelting. Its accumulation consumes an extensive quantity of land resources and contributes to serious environmental pollution. Effectively managing this waste is a critical challenge for mining enterprises and constitutes a global barrier to the sustainable development of the mineral resources industry. With advancements in industrial technology and increased ecological awareness, mine backfill technology has emerged as a promising solution. It enables efficient resource recovery while facilitating the reuse of solid waste. Such waste can serve as aggregate in backfilling, while materials with latent cementitious properties can be developed into composite binders to replace orthodox cement. Using mining and metallurgical waste for backfill binder production has become a prominent focus in sustainable resource utilization research.

This Special Issue comprises five articles—one review and four research papers—showcasing cutting-edge research into the transformation of metallurgical byproducts into functional backfilling materials for mining and construction applications. Collectively, these contributions address critical challenges in material design, performance optimization, and environmental safety while also advancing innovative strategies for waste valorization and carbon footprint reduction. The authors highlight three key breakthroughs: (i) waste reactivation—mechanical and chemical treatments are employed to activate the latent reactivity of metallurgical waste, thereby reducing dependency on conventional binders; (ii) multi-scale design—hierarchical material engineering (ranging from microscale hydration products (e.g., C-S-H gels) to macroscale fiber-reinforced networks) enhances the overall performance and structural integrity of the backfill; and (iii) environmental synergy—all the proposed approaches emphasize immobilizing heavy metals, reducing carbon emissions, and aligning with circular economy principles.

The authors of the first paper [1] systematically investigated the mechanical activation of lead–zinc tailings (LZTs) to enhance their pozzolanic activity for composite cementitious materials. After 2 h mechanical grinding, the 28-day pozzolanic activity of LZTs increased to 76%, enabling their integration into high-performance backfills. The optimal formulation (60% LZTs, 22% GGBS, 8% SS, and 10% DG) yielded compressive strengths of 13.8 MPa and 15.7 MPa at 7 and 28 days, respectively. XRD and SEM analyses confirmed the critical role of AFt crystals and amorphous C-S-H gels in strength development in addition to their ability to ensure minimal heavy metal leaching.

The second paper [2], a review of ladle furnace slag (LFS), highlights its potential as a supplementary cementitious material (SCM). While LFS shares chemical similarities with clinker, challenges such as γ-C2S, free lime, and free MgO content hinder its reactivity. Rapid air/water quenching emerges as the most effective treatment for producing amorphous LFS with enhanced reactivity, offering a sustainable alternative to ordinary Portland cement (OPC) and alleviating the supply–demand gap regarding blast furnace slag.

The authors of the third paper [3] investigated five tailings-based insulation materials to better describe their thermal and mechanical properties. Proportion E demonstrated the highest compressive strength (0.53 MPa), while Proportion A exhibited the lowest thermal conductivity (0.261 W/(K·m)). Numerical simulations revealed that a 10 cm insulation layer thickness optimized cooling efficiency (0.9 K reduction) and economic feasibility. Adjustments to thermal conductivity and ventilation velocity further amplified the temperature control effects, with a maximum reduction of 1.42 K.

The fourth paper [4] explores the effects of polypropylene fiber (PPF) on the rheological and thixotropic properties of cemented paste backfill (CPB) using the concept of water film thickness (WFT). Wet packing tests proved more accurate for measuring packing density. PPF content and length influenced packing behavior through filling and wedge effects, with 0.2% content and 9 mm length identified as optimal. WFT showed a strong linear correlation with yield stress. These findings support improved CPB mix design, enhancing flowability, stability, and operational efficiency in mining backfill applications.

The fifth and final paper of this Special Issue [5] evaluates the impact of polypropylene and polyacrylonitrile fibers on the mechanical and flow properties of cement-based foam backfill (CFB). Tests revealed that while the addition of fiber reduced fluidity due to entanglement, it significantly improved strength and crack resistance through anchoring and network effects. After 28 days, uniaxial compressive strength increased by over 200% with optimal fiber types and contents. Fiber reinforcement also shifted failure behavior from brittle to ductile, demonstrating its potential to enhance the durability and safety of CFB in underground mining applications.

The articles presented in this Special Issue address critical topics related to the transformative potential of metallurgical waste in backfilling technologies. By integrating insights from materials science, environmental engineering, and industrial ecology, we aim to contribute to the advancement of safer mining practices, the mitigation of resource depletion, and the promotion of a more sustainable built environment. We extend our sincere gratitude to all the contributing authors and reviewers—experts in their respective fields—for their valuable insights, constructive evaluations, and thoughtful recommendations. We would also like to express our deep gratitude to Prof. Dr. Leonid Dubrovinsky, Editor-in-Chief of Minerals, as well as the associate editors, for their indispensable support throughout the rigorous peer-review process. Their technical and procedural guidance has been instrumental in bringing this issue to fruition. Undoubtedly, this Special Issue would not have been possible without the unwavering commitment and contributions of these dedicated individuals.