Modeling the Polymerization Process for Geopolymer Synthesis through Reactive Molecular Dynamics Simulations

Geopolymers are a class of inorganic aluminosilicate polymers composed of silicate and aluminate tetrahedrons that are linked by sharing oxygen atoms. A reactive molecular dynamics (MD) simulation approach was used to model the polymerization process and molecular structure of geopolymer gels. Reactive silicate and aluminate monomers were first optimized with density functional theory simulations and "polymerized" subsequently in MD models with a reactive Feuston and Garofalini potential. MD models with Si/Al molar ratios of 2 and 3 were simulated at temperatures ranging from 650 to 1800 K to investigate the effect of Si/Al ratio and temperature on the polymerization process and the properties of computationally synthesized "geopolymer gels". Geopolymer gels close to those produced experimentally were computationally "synthesized" for the first time. The distribution of Si4(mAl) and radial distribution functions of the modeled geopolymer gels showed good agreement with the respective experimental results of geopolymers in the literature. After a three-stage polymerization process, involving oligomerization, aggregation, and condensation, the molecular structure of geopolymer gels with the bulk density was obtained. A higher temperature enhanced the rate and degree of condensation and decreased the bulk density of final geopolymer gel structures, whereas a lower Si/Al ratio resulted in a more compact geopolymeric network.