Interfacial properties and interaction of calcium-based geopolymer gels-SiO2 aggregate based on molecular dynamics simulations

Abstract

This study utilizes the molecular dynamics simulation to explore the bonding modes, structural characteristics, diffusion behaviour, and mechanical properties at the interface between calcium-based geopolymer gels and SiO₂ aggregates. The findings reveal that the interface is predominantly interconnected through Al-O-Si covalent bonds, Na/Ca-O ionic bonds, and hydrogen bonds, with the hydrogen bonds playing a pivotal role. A low-density region, referred to the interfacial transition zone (ITZ), is established at the juncture between the geopolymer gels and SiO₂ aggregates. The diffusion rates of various components exhibit distinct patterns, with H₂O molecules demonstrating the highest diffusion rate, followed by Ca²⁺ and Na⁺ ions, while the diffusion rate of aluminosilicates is comparatively slower. Notably, the diffusion rates of components at the interface, excluding H₂O molecules, are lower than those within the gel matrix. The adhesion performance at the interface is inferior to that observed within the gel. During uniaxial tensile simulations, cracks and voids initially manifest at the interface and progressively expand with increasing stress, ultimately resulting in model fracture. The tensile strength at the interface is measured at 1.40 GPa, and the Young's modulus is determined to be 37.77 GPa. This study provides a theoretical foundation for optimizing the interface design of geopolymer (recycled) concrete.