Analisi XAS multi-elementare sulla struttura dei geopolimeri a base di fosfato

Phosphate-based geopolymers (PBGs) offer a sustainable alternative to Portland cement, characterized by lower CO₂ emissions, enhanced durability, and superior thermal and mechanical properties.[1] One way to synthesize PBGs is through the room-temperature reaction of metakaolin with diluted phosphoric acid, which provides an amorphous phosphoaluminate matrix. This study explores chemical kinetics and amorphous nature of PBGs while it is being formed from MK, which is an aluminosilicate precursor.[2],[3] While conventional spectroscopic techniques like infrared and Solid-State Nuclear Magnetic Resonance (SS-NMR) provide valuable insights into PBGs' structure, they struggle with signal overlap and limited depth analysis. Our research addresses these limitations using X-ray Absorption Spectroscopy (XAS) to probe the local structural changes of phosphorus, aluminum, and silicon during PBG formation. Conducted at the Solaris synchrotron in Poland, our study used tender X-rays to collect K-edge spectra from PBG samples. XAS analysis revealed that as geopolymerization advances, the structure transitions from resembling metakaolin to resembling silicon dioxide nanoparticles (SiO₂). Phosphorus K-edge spectra showed that variscite mineral-like features are consistent across all samples. However, challenges such as low penetration depth of tender X-rays, self-absorption effects, and overlapping spectral features of P, Al, and Si K-edges complicate accurate data interpretation. To overcome these challenges, complementary techniques like SS-NMR, SEM/EDS, and XRD were employed, confirming the coexistence of two amorphous phases and unreacted metakaolin. Our results indicate that while local structures are consistent, microscale variations in synthesis conditions significantly affect properties like microporosity and density. Optimizing the final properties of PBGs involves balancing the reactivity of metakaolin (with an ideal Al/P ratio close to 1.0) and minimizing water content during synthesis. This study advances our understanding of PBGs' molecular framework and provides insights into their potential applications as sustainable building materials and encapsulation matrices for waste residues.