Atomic-scale control of stability and mechanical performance in hydrated products during polycondensation process of low-carbon geopolymers

• Reactive molecular dynamics simulation on nanostructure evolution of Phosphoric Acid Geopolymers (PAG). • Aluminum transitions from tetrahedral to stable octahedral coordination. • Q⁴ polymerization increases network cross-linking via P–O–Al/Si bonds. • Q⁴(3AlVI–O–P) identified as the lowest-energy stable phase. • Mechanical performance strongly correlates with AlVI and Q⁴ content. • PAG exhibits 56% higher stiffness than NaOH-activated systems. This study investigates the atomic-scale mechanisms governing the stability and mechanical performance of phosphoric-acid-activated geopolymers (PAG), aiming to resolve existing uncertainties regarding their nanostructure and structure–property relationships. In this work, a novel atomistic investigation of phosphate acid geopolymer nanostructure is carried out using reactive molecular dynamics simulations to link chemical environment, structural stability and macroscopic properties in these low-carbon binders during polycondensation process. Structural validation was performed via simulated X-ray diffraction and vibrational density of states, compared with experimental XRD and FTIR data. Results confirm the formation of an amorphous alumino-phospho-silicate network, where silicon remains tetrahedrally coordinated across all environments, while aluminum undergoes a progressive transition from tetrahedral to predominantly octahedral coordination stabilized by phosphate groups. The evolution of P–O bonding indicates a gradual conversion of terminal P=O bonds into bridging P–O–Al and P–O–Si linkages, enhancing network cross-linking. The Q⁴(3Al VI –O–P) phase is identified as the most stable configuration, exhibiting superior mechanical properties. Finally, PAG shows a 56% higher stiffness for aluminum 6-coordinated and highly Q⁴ polymerized environments compared to conventional alkali-activated geopolymers. These findings demonstrate that controlling aluminum coordination and silicate–phosphate polymerization is key to optimizing performance, providing a fundamental basis for the rational design of next-generation low-carbon geopolymer materials with enhanced mechanical strength and durability.