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

Geopolymer is an environmentally friendly cement madeby reactingan aluminosilicate material in an alkaline solution. This innovativematerial has emerged as a promising alternative to traditional Portlandcement. Sodium aluminosilicate hydrate (NASH) gels, the main hydrationproduct of the aluminosilicate-rich geopolymer, determine the servicelife of the geopolymer. However, the formation mechanism, the evolutionof amorphous products, and the relationship between the microstructureand macroscopic properties of NASH gels remain poorly understood.In this study, molecular reaction dynamics were used to investigatethe formation process, molecular structure, and fracture process ofNASH with varying Si/Al ratios. The model simulates the geopolymerizationprocess, which occurs through a condensation reaction between hydroxylgroups that produces bridging oxygen atoms and generates water. Themonomer gradually polymerizes to form a three-dimensional networkstructure. As the proportion of Al in the system increases, tetrahedralaluminum transforms to pentahedral and hexahedral structures, providingmore hydroxyl binding sites for the main dehydration polycondensationreaction and intensifying the occurrence of the polycondensation reaction.Moreover, with the increase of the proportion of Al, the distributionrange of bond angles becomes narrower, which reduces the distancebetween the silicon-oxygen tetrahedron and the aluminum-oxygentetrahedron and increases the overall degree of polymerization, butreduces the skeleton stability of the NASH gel structure. The mechanicalproperties of NASH gel were analyzed by a uniaxial tensile simulationprocess, during which the aluminosilicate mesh structure depolymerizesinto branched or chainlike structures to assume the role of resistingtensile loads. The improvement of the tensile strength depends onthe amount of bridging oxygen in the system, particularly in Si-O-Si,and the modulus of elasticity is mainly affected by the quantity ofthe hydroxyl group. As the Si/Al ratio increases, the Si-O-Siin the system increases significantly, the quantity of hydroxyl groupsdecreases, and the tensile strength and elastic modulus of the NASHgel increase. This study provides valuable insights into the mechanismof the polymerization reaction and the mechanical properties of geopolymers.