Low-Temperature-Conditioned Biogenic Calcite from Waste Eggshells in Metakaolin Geopolymer Gels: Structural, Textural and Functional Properties

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.