Reaction Mechanism of Active Al2O3 Groups in Geopolymers: A DFT Study

Abstract

Although the concept of geopolymers was proposed over 40 years ago, there still remains a lack of clarity regarding their atomic-level structure and formation. In this study, the Dmol3 quantum chemistry calculation program, based on density functional theory (DFT), was used to determine a range of electronic structural properties associated with the initial, intermediate (IM), transition (TS), and final states of Al2O3/[Al(OH)4]− conversion reactions in the alkali-activator. The properties analyzed included total energy, Gibbs free energy, electrostatic potential (ESP), Fukui functions, and frontier orbitals, comprising the highest-occupied molecular orbital (HOMO) and the lowest-unoccupied molecular orbital (LUMO). The simulation results indicated that electrons were transferred from the HOMO of O in H2O and OH− to the LUMO of Al in (AlO2)− or (AlO)+, leading to an increase in the LUMO energy level of Al and a reduction in the chemical reactivity of the newly formed Al monomers. The transformation processes from Al2O3 to [Al(OH)4]− involved varying numbers of steps, energy release, and energy barriers. Notably, during the transition state conversion process, the breaking and reformation of O─ H bonds often occurred as necessary conditions for the formation of transition states. These findings have significant implications for the advancement of new technologies based on geopolymer conversion processes.