Editorial for Special Issue “Coal Fly Ash as a Resource: Advances in Characterization, Utilization and Sustainable Solutions”

Coal fly ash (CFA), the fine particulate by-product of coal combustion in thermal power plants, has traditionally been regarded as an environmental liability. Its large volumes, heterogeneous composition, and potential for trace metal release have posed persistent challenges related to disposal, land use, and long-term environmental management. Increasingly, however, this perception is shifting. Rather than being viewed solely as waste, CFA is now recognized as a secondary resource rich in reactive mineral phases and valuable elements suitable for industrial and environmental applications. This evolving perspective aligns closely with the principle of the circular economy, in which materials once destined for disposal are redirected into productive value chains, thereby reducing environmental burdens while conserving primary raw materials. Within this context, the present Special Issue was conceived to consolidate recent advances that deepen our understanding of CFA properties while showcasing innovative strategies for its beneficiation, functionalization, and utilization across diverse sectors. Emphasis has been placed on rigorous characterization, novel processing routes, and practical demonstrations of application pathways capable of transforming CFA into high-value products. Between 2024 and 2026, five contributions were assembled that collectively fulfill these objectives. Although differing in technical focus and application domain, all studies share a common theme: the systematic integration of detailed physicochemical and mineralogical characterization with targeted performance outcomes. Together, they demonstrate how fundamental understanding enables the rational and sustainable design of value-added uses for CFA.

Building on the broader framework of waste valorization, Moyo et al. (2024) (Contribution 1) demonstrated how CFA can be transformed into a functional engineering material through targeted surface modification. South Africa generates about 35 million tons of CFA annually, yet only a small fraction (about 7%) is reused, with the remainder disposed of in ash dumps, which present risks of leaching and particulate emissions. Recognizing the high combined silica and alumina content (>85% SiO₂ + Al₂O₃), the authors explored its suitability as a reactive precursor and filler for rubber composites. Two physicochemical treatments—ammonium sulfate roasting followed by aqueous leaching, and direct sulfuric acid leaching—modified the surface properties of CFA without altering the particle size distribution. Notably, the BET specific surface area increased nearly an order of magnitude, from 0.99 m²/g for untreated CFA to 7.97 and 11.02 m²/g after the two treatments, respectively, thereby enhancing interfacial contact potential. The roasting–leaching process produced rougher surfaces and reduced agglomeration, features that promote mechanical interlocking and stronger filler–matrix adhesion in composites. Subsequent silane grafting imparted pronounced hydrophobicity, as confirmed by large contact angles, which would improve compatibility with non-polar rubber matrices. When incorporated into cis-1,4-polyisoprene, treated ashes outperformed untreated material, with the roasting–leaching route showing the greatest benefit. Importantly, the treatments were compatible with upstream aluminum recovery processes, suggesting an integrated pathway in which metal extraction and residue upgrading occur sequentially, enabling near-zero-waste utilization. These results highlight the promise of modified CFA as an effective, low-cost filler in polymer composites.

Complementing this application-driven approach, Berti et al. (2025) (Contribution 2) focused on the mineralogical hosts of critical elements within CFA, with particular emphasis on zircon (Zr) and associated Zr-bearing phases as potential reservoirs of rare earth elements (REEs) and yttrium (Y). While CFA is often evaluated in terms of bulk chemistry, this study underscores the importance of resolving element partitioning at the mineral scale to inform beneficiation strategies. Beneficiated ash from an eastern Kentucky coal blend was examined using complementary techniques, including scanning electron microscopy with energy-dispersive spectroscopy (SEM–EDS), focused ion beam (FIB) preparation, and high-resolution transmission electron microscopy (TEM) with electron diffraction (ED) and electron energy-loss spectroscopy (EELS). In addition to REE phosphates, the authors identified zircon (ZrSiO₄), baddeleyite, fergusonite, yttrialite, and xenotime, confirming that multiple Y- and REE-bearing phases remain thermally stable during combustion and contribute to the critical element inventory. Many zircon grains exhibited complex internal textures, with dense cores surrounded by porous rims enriched in Y and heavy REEs. These rims likely formed through combustion-induced heating of metamict, radiation-damaged minerals, producing porous structures that host fine intergrowths of REE-rich phases. Although some fragile rims may detach during boiler turbulence, the resulting fragments remain within the ash and thus remain accessible for recovery. By elucidating these microtextures and formation pathways, the study provides essential guidance for the development of selective beneficiation and extraction processes.

Suárez-Navarro et al. (2025) (Contribution 3) addressed an equally important, though often underexplored, dimension of CFA utilization: natural radioactivity in CFA. Because coal combustion can concentrate naturally occurring radionuclides, understanding their distribution and partitioning is critical for safe reuse. Ten CFA samples and one bottom ash (BA) sample from Spanish power plants were characterized chemically and radiologically. Through the combined application of X-ray fluorescence (XRF) and high-resolution gamma spectrometry, the study identifies clear geochemical controls on radionuclide distribution, supported by multivariate statistics and neural network modeling. Principal component analysis (PCA) explains 95.7% of the variance, revealing strong associations between thorium-series radionuclides (²²⁸Ac, ²¹²Pb) and Al₂O₃–TiO₂-rich phases, while uranium-series radionuclides show more complex and less direct relationships with oxide composition. Garson-based sensitivity analysis demonstrates that minor oxides—particularly P₂O₅, followed by K₂O and Na₂O—exert a greater influence on radionuclide activity concentrations than the major oxides Fe₂O₃ and Al₂O₃, with strong correlations such as ²³⁴Th–P₂O₅ (r = 0.85). The study also distinguishes compositional and radiological differences between CFA and BA, with the latter exhibiting higher radionuclide activity, except for ²¹⁰Pb and ⁴⁰K. Despite measurable enrichment, the calculated hazard indices and radiological dose assessments remained within the regulatory safety thresholds established in Spain for public exposure, confirming the suitability of these materials for most industrial and construction applications. These findings support the continued beneficial use of CFA, provided that appropriate monitoring is maintained, and they reinforce the need for site-specific evaluation given variability among fuels and combustion systems.

Golboylu et al. (2025) (Contribution 4) advanced the field of value-added conversion by demonstrating a template-free route for the selective synthesis of zeolites from Class C CFA. The heterogeneous composition and low silicon-to-aluminum ratio (Si/Al) of such ashes typically limit the formation of high-value zeolite phases. To address this, the authors implemented acid-leaching pretreatment to remove calcium-, sulfur-, and iron-bearing impurities and increase the Si/Al ratio, thereby creating a more favorable precursor. Subsequent alkaline fusion–hydrothermal synthesis enabled controlled crystallization of FAU-, CHA-, GIS-, and MER-type frameworks, with phase selectivity governed by alkali composition, temperature, and crystallization time. Structural differences were confirmed by XRD and SEM, and functional performance was assessed through water sorption measurements. FAU and CHA frameworks exhibited substantially higher adsorption capacities than GIS and MER, reflecting their larger pore systems and improved accessibility. This work establishes practical synthesis windows for producing targeted zeolite structures and demonstrates that impurity-rich CFA can be reproducibly converted into functional porous materials through scalable, template-free methods.

Finally, Chrysakopoulou et al. (2026) (Contribution 5) provided a comprehensive mineralogical, chemical, and petrographic evaluation of CFA and BA from the Agios Dimitrios power plant in northern Greece, assessing their suitability as supplementary cementitious materials or fillers in concrete. Integrated analyses using XRD, XRF, SEM–EDS, and magnetic separation revealed the dominance of silicate and aluminosilicate phases, including quartz, plagioclase, gehlenite, and amorphous glass, consistent with Class C-type behavior. CFA particles were predominantly spherical, including cenospheres and glassy microspheres favorable for cementitious performance, whereas BA exhibited coarser, angular morphologies. Iron-rich fractions, including ferrospheres, were identified and selectively removed. Both ash types met several criteria relevant to use as mineral additives or fillers, particularly after appropriate processing such as grinding or magnetic removal of undesirable fractions. The presence of reactive glassy material and fine particle size in CFA was identified as favorable for incorporation into blended cements, while BA may require additional treatment to optimize performance. The study demonstrates that these combustion by-products constitute viable secondary raw materials and reinforces their role in reducing reliance on virgin resources in the construction sector.

Collectively, the contributions in this Special Issue illustrate the multifaceted potential of CFA, spanning materials engineering, critical element recovery, and sustainable construction. By coupling detailed characterization with purposeful processing and application design, these studies attempt to move beyond simple reuse toward true resource valorization. Together, they reinforce the view that CFA may be a versatile feedstock capable of supporting circular and sustainable material systems.