Molecular insights into the microstructure, dynamics, wettability and mechanical properties of sodium aluminum silicate hydrate with saturated nanopores

Abstract

Sodium aluminosilicate hydrate (NASH) serves as the primary binding phase governing the physical, mechanical, and durability characteristics of geopolymer binders. However, the intricate interactions at the water-nanopore interface and the inherent heterogeneity of geopolymer binders present formidable challenges in exploring the intrinsic microstructure and micromechanical properties of this pivotal binding phase. In response to these challenges, we conducted reliable molecular dynamics simulations aimed at unraveling the intricate molecular-level features of NASH within saturated nanopores. Using the state-of-the-art simulations, results reveal that the structural properties of NASH remain unaffected by variations in nanopore size. Nevertheless, a critical nanopore size is identified, impacting the distribution of water molecules within the nanopore, as evidenced by the convergence of the water molecule number density. In terms of dynamics, diffusion of water molecules within the nanopore exhibits quasi-two-dimensionality, while in NASH demonstrates characteristics of supercooled liquids due to constraints imposed by the aluminosilicate network. Notably, the mechanical properties of NASH within saturated nanopores exhibit a significant decline as nanopore size increases, with uniaxial tension tests indicating the absence of strain softening. Additionally, the presence of hydroxyl groups on the NASH surface influences the adhesion of water molecules through hydrogen bonding, resulting in a pronounced hydrophilicity with a contact angle of 12.2°. The findings offer valuable insights into the behavior of NASH with saturated nanopores, shedding light on the molecular-level details of its structure, dynamics, mechanical properties, and wettability.