Metakaolin-based geopolymers filled with volcanic fly ashes: FT-IR, thermal characterization, and antibacterial property

In countries where volcanic activity is widespread, fly ash (FA) formation can represent both a waste to be disposed of and a resource of inorganic substances that can be utilized. Among the technologies able to incorporate FA, geopolymers (GP) or inorganic aluminosilicate amorphous materials are very suitable for this purpose. In this study, GP are realized using metakaolin (MK), sodium hydroxide solution (NaOH 8M), sodium silicate solution (Na2SiO3), and FA as filler (20 wt% with respect to MK content). The samples were cured at 25 or 40°C for 24 h and the physicochemical, thermal, and antibacterial properties of this material through the integrity test, weight loss test, Fourier-transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), and Kirby-Bauer assay were assessed. Integrity and weight loss tests indirectly revealed the stability of the macroscopic 3D networks and that the curing at 40°C led to more stable GP. The shift of the Si–O–T absorption band (from 1,090 cm−1 of the MK to 1,017–1,012 cm−1 of the specimens with and without FA) in FT-IR spectra suggested the occurrence of the geopolymerizazion reactions, while TGA study confirmed the higher stability of samples cured at 40°C (with a mass loss equal to 7–13% at 800°C under nitrogen atmosphere). Finally, the antimicrobial activity shed light on the ability of the synthesized GP with the filler and treated at 40°C to have a great effect against Escherichia coli and Pseudomonas aeruginosa.