Water adsorption and diffusion in phosphoric acid-based geopolymer using molecular modeling

Phosphoric acid-based geopolymer (PABG) is an environmentally friendly cementitious material. The adsorption and diffusion of water in PABG are investigated at the atomic scale. The adsorption isotherms are fitted by the Dubinin-Astakhov (DA) equation, and the adsorption isotherm is Type-V, qualitatively consistent with porous materials' adsorption characteristics. Adsorption isotherms and isosteric heats indicate that high temperatures weaken the attraction between water molecules and PABG, decreasing the adsorption amount with temperature. Water MSD suggests that the diffusion type is subdiffusion (one type of anomalous diffusion), verified by monitoring water molecules' diffusion trajectories, where the water molecule mainly fluctuates inside the pore and rarely jumps between pores. Effects of temperature and concentration on diffusion are analyzed. The anomalous diffusion exponent alpha, which reflects the diffusion rate, rises with temperature. Higher temperature enhances water molecule movement, and diffusion is accelerated. Higher water loading reduces alpha due to steric hindrance.