Effect of Ca content on the synthesis and properties of FA-GGBFS geopolymer: combining experiments and molecular dynamics simulation

The elemental components in the precursor of geopolymers are different, and their mechanical properties are also different. As one of the major influencing factors, calcium content is worth further research and exploration. This paper aims to explore the impact of calcium content on fly ash (FA)-ground granulated blast furnace slag (GGBFS) geopolymer in a comprehensive and multi-scale manner through various research methods such as indoor experiments, microscopic experiments, and molecular dynamics simulations. Using GGBFS as the main source of calcium content in geopolymers, different calcium content samples were prepared by adjusting the GGBFS content. The consistency, setting time, compressive strength, flexural strength, elastic modulus, self-shrinkage value, and dry-shrinkage value of the test blocks were tested at corresponding ages. Analyze its microscopic characteristics through scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM-EDS). Finally, the structural models of six geopolymer gels with different Ca contents were constructed using Materials Studio software, and the mechanism of calcium influence was further characterized from the atomic level. The results indicate that an increase in calcium content is beneficial for the rapid hardening of geopolymer mortar, with varying degrees of increase in compressive strength, flexural strength, and elastic modulus. However, with the increase in calcium content, shrinkage is greater, which may lead to durability drawbacks such as cracking in practical engineering. Therefore, the calcium content should be appropriately controlled. The microscopic morphology showed that the polymer became denser with increasing calcium content, and the Ca/Si ratio in EDS also increases accordingly. Molecular dynamics simulation results also show that Ca can form Ca-O bonds with distant oxygen atoms, making the gel structure more stable.