Molecular Insight into the Formation and Fracture Process of Sodium Aluminosilicate Hydrate Gels

Geopolymer is an environmentally friendly cement made by reacting an aluminosilicate material in an alkaline solution. This innovative material has emerged as a promising alternative to traditional Portland cement. Sodium aluminosilicate hydrate (NASH) gels, the main hydration product of the aluminosilicate-rich geopolymer, determine the service life of the geopolymer. However, the formation mechanism, the evolution of amorphous products, and the relationship between the microstructure and macroscopic properties of NASH gels remain poorly understood. In this study, molecular reaction dynamics were used to investigate the formation process, molecular structure, and fracture process of NASH with varying Si/Al ratios. The model simulates the geopolymerization process, which occurs through a condensation reaction between hydroxyl groups that produces bridging oxygen atoms and generates water. The monomer gradually polymerizes to form a three-dimensional network structure. As the proportion of Al in the system increases, tetrahedral aluminum transforms to pentahedral and hexahedral structures, providing more hydroxyl binding sites for the main dehydration polycondensation reaction and intensifying the occurrence of the polycondensation reaction. Moreover, with the increase of the proportion of Al, the distribution range of bond angles becomes narrower, which reduces the distance between the silicon–oxygen tetrahedron and the aluminum–oxygen tetrahedron and increases the overall degree of polymerization, but reduces the skeleton stability of the NASH gel structure. The mechanical properties of NASH gel were analyzed by a uniaxial tensile simulation process, during which the aluminosilicate mesh structure depolymerizes into branched or chainlike structures to assume the role of resisting tensile loads. The improvement of the tensile strength depends on the amount of bridging oxygen in the system, particularly in Si–O–Si, and the modulus of elasticity is mainly affected by the quantity of the hydroxyl group. As the Si/Al ratio increases, the Si–O–Si in the system increases significantly, the quantity of hydroxyl groups decreases, and the tensile strength and elastic modulus of the NASH gel increase. This study provides valuable insights into the mechanism of the polymerization reaction and the mechanical properties of geopolymers.