Marine mollusk shells are a promising renewable source of calcium-based materials, offering a sustainable alternative for their synthesis. In this study, five types of marine shells—Chamelea gallina, Mya arenaria, Rapana venosa, Mytilus edulis, and Pecten maximus—were calcined at 900 °C for 2 h. The resulting powders were characterized by XRD, FTIR, SEM, PSD, and zeta potential analyses. XRD confirmed the dominant presence of CaO, with residual calcite and portlandite. FTIR spectra supported these findings, indicating the decomposition of carbonate phases and the formation of Ca–O bonds. SEM imaging revealed species-specific microstructures ranging from lamellar and wrinkled textures to compact aggregates, while particle size distributions varied from 15 to 37 μm. Thermogravimetric analysis revealed a two-step decomposition process for all samples, with significant species-dependent differences in mass loss and conversion efficiency, highlighting the influence of biogenic origin on the thermal stability and CaO yield of the resulting powders. Zeta potential measurements showed low colloidal stability, with the best performance found in Rapana venosa and Pecten maximus calcinated samples. Antibacterial activity was evaluated using a direct contact method against Escherichia coli and Enterococcus faecalis. All samples exhibited complete inactivation of E. coli, regardless of exposure time, while E. faecalis required prolonged contact (3.3 h) for full inhibition. The results highlight the potential of biogenic CaCO₃ and CaO powders as functional, antimicrobial materials suitable for environmental and biomedical applications. This study underscores the viability of marine shell waste valorization within a circular economy framework.

Metakaolin-based geopolymers modified with ZnO and surface-functionalized ZnO were developed and investigated in terms of structure, morphology, textural properties, and photocatalytic performance. ZnO, ZnO@AS(zinc oxide functionalized with steric acid), and ZnO@PEG(zinc oxide functionalized with polyethylene glycol) were incorporated into the geopolymer matrix and characterized by XRD(X-ray diffraction), ATR–FTIR(Fourier transform infrared spectroscopy in attenuated total reflectance, SEM–EDS (scanning electron microscopy coupled with spectroscopy with energy-dispersive X-ray spectroscopy), and BET(Brunauer–Emmett–Teller) analysis. The results showed that ZnO incorporation did not significantly modify the amorphous aluminosilicate network but affected the morphology and porosity depending on the functionalization method. ZnO@AS induced matrix densification and reduced accessible porosity, while ZnO@PEG improved particle dispersion and preserved the porous structure. Among the ZnO-modified metakaolin-based geopolymers, GP/ZnO@PEG(geopolymer with ZnO@PEG particles) exhibited the highest photocatalytic performance, characterized by a BET surface area of 17.22 m²/g, an apparent kinetic constant of 0.01668 min⁻¹, and a half-life of approximately 41 min, achieving approximately 90% methylene blue removal after 120 min of UV-A irradiation. The study demonstrates that ZnO surface functionalization controls the interfacial interaction with the geopolymer matrix and plays a key role in the performance of geopolymer-based photocatalytic materials.

The sustainable valorization of marine shell waste as functional additives for geopolymer materials represents a promising strategy for developing multifunctional materials for environmental remediation. In this study, metakaolin-based geopolymers were modified with calcium-rich particles obtained by calcination of five marine shell species (Chamelea gallina, Mya arenaria, Mytilus edulis, Pecten maximus, and Rapana venosa) under identical synthesis conditions to evaluate the influence of shell mineralogy on the structural, textural, adsorption, and antibacterial properties of the resulting composites. The materials were comprehensively characterized by Fourier transform infrared spectroscopy in attenuated total reflectance (ATR-FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), and nitrogen adsorption–desorption (BET/BJH) analyses. Functional performance was assessed through methylene blue (MB) adsorption experiments, adsorption kinetic modeling, and antibacterial tests against Escherichia coli (E. coli). ATR-FTIR and XRD analyses confirmed the formation of a stable amorphous geopolymer network containing residual crystalline phases together with shell-derived calcium carbonate, predominantly as calcite or aragonite depending on shell origin. The incorporation of shell-derived particles modified the pore architecture of the geopolymers. GP-SJ exhibited the highest BET specific surface area (94.30 m² g⁻¹) and the most developed mesoporous structure. Among the investigated formulations, GP-RP showed the most favorable overall combination of functional properties under the investigated conditions, exhibiting the highest methylene blue removal efficiency (63.97%) and experimental adsorption capacity at 160 min (9.60 mg g⁻¹), together with a comparatively high reduction in recoverable E. coli colonies during preliminary antibacterial screening. The combined structural and functional analyses demonstrate that the environmental performance of shell-modified geopolymers cannot be predicted from a single parameter such as BET surface area or calcium content alone, but results from the synergistic interaction between mineralogical composition, particle dispersion, pore accessibility, and matrix compactness. Under the investigated conditions, these findings provide evidence for proposed structure–property correlations under the investigated conditions and suggests that shell-derived calcium particles act as microstructural regulators of geopolymer matrices, providing a basis for the further development of sustainable multifunctional materials for simultaneous dye removal and bacterial reduction in wastewater treatment.

Hen eggshell is an abundant, calcite-rich food waste that is usually valorised by calcination at 800–1000 °C, an energy-intensive step that releases its carbonate CO₂ and destroys its biogenic microstructure. Low-temperature routes that keep the calcite intact have received little attention, and their effect on the structure and functions of geopolymer gels is unknown. This work examined whether eggshell conditioned at only 250 °C (ES-250) can replace 10 wt% of the metakaolin in a geopolymer gel, and what it changes. ES-250 remained pure biogenic calcite, with intracrystalline vesicles and residual organic matter, and the route needs ≈3–6 times less heat than calcination, including drying. In the hardened binder, ES-250 persisted as discrete 2–5 µm calcite particles without detectable Ca diffusion into the aluminosilicate gel. The Si–O–T bonding, the total pore volume and the N₂-accessible porosity (≈41%) were preserved, and only the BET surface area decreased (36.2 → 31.6 m² g⁻¹). Dye uptake in the dark was maintained, but the strong antibacterial effect of the plain geopolymer, which left no culturable Escherichia coli after 4 h, was largely lost (22–34% reduction). Low-temperature-conditioned eggshell is therefore a decarbonation-free, structurally inert filler for geopolymer gels, but not a functional additive.