Molecular simulations of the structure-property relationships of N-A-S-H gels

Abstract

A good understanding of the relationship between the micro- and macroscopic properties of N-A-S-H gel is crucial for designing and developing geopolymer binders. This paper develops molecular models for N-A-S-H gels based on aluminosilicate oligomers with different Si/Al ratios and configurations. Our models mimic the geopolymerization process through a poly-condensation reaction between hydroxyls, generating water and forming the aluminosilicate network. The simulation results indicate that more than 50% of the hydroxyls in oligomers are reacted, reflecting the geopolymerization degree. The oligomers with a higher Si/Al ratio and linear configuration exhibit a higher reaction degree, while those with cyclic configurations exhibit a higher complexity for network structure. Decreasing the Si/Al ratio narrows the bond angle distribution and reduces the skeleton stability of N-A-S-H gel structure. This also decreases the dynamic water mobility due to the higher restriction of hydrogen atoms. Chains’ breakage and the associated hydrolysis reaction at their ends are observed with the consumption of almost 2.5% of the generated water molecules. Furthermore, the enhancement in tensile strength is mainly attributed to increased bridge oxygen numbers, especially for those in Si-O-Si bonds. The elastic modulus is mainly influenced by the amount of hydroxyl and network complexity. In addition, increasing the Si/Al ratio increases the number of Si-O-Si bonds and decreases the hydroxyls, enhancing the tensile strength and elastic modulus of N-A-S-H gels. N-A-S-H gels constructed with cyclic oligomers show around 20% higher tensile strength than those with linear oligomers. Thus, this study will provide valuable insights on the molecular structure understanding and design optimization of geopolymer materials.