Synthesis and Reaction Mechanism of Geopolymer Gels with Increasing Calcium Content: From Experiments to Molecular Dynamics Simulation

This work aims to investigate the effects of calcium on the performances of metakaolin-based geopolymer with the aid of a combination of the experiment study and molecular dynamics simulation. These impacts were comprehensively characterized by fresh properties through setting time and consistency of paste with calcium oxide (CaO) partial substitution, while the hardened properties were conducted via compressive strength and elasticity modulus tests. The microstructural characteristics of reaction products were analyzed by X-ray diffraction (XRD), thermogravimetric differential thermal analysis (TG-DTG), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM-EDS), and Brunauer-Emmett-Teller (BET) method. Moreover, various structures of sodium aluminate silicate hydrate (N─ A─ S─ H), calcium aluminate silicate hydrate (C─ A─ S─ H), and five types of geopolymer gels models with increasing calcium content were established and optimized by molecular dynamics simulation. The results showed that the workability of paste decreased with the increase in calcium content, while there was a threshold for mechanical properties at all ages. The products of geopolymer with calcium composite incorporation inclusion were mainly amorphous phases of N(C)─ A─ S─ H containing large amounts of calcite, while even Ca(OH)2 and unreacted CaO could be found in a high-calcium system with higher crystallinity. The coexistence of gels increased compactness and also formed a relatively denser network structure with many mesopores, but poor pore structure caused by unacceptable polymerization degree and bound water consumption of hydration products occurred when the CaO replacement ratio reached 20%. Acceptable agreement between the simulations and experimental results was obtained with a significant decrease in the bond lengths of Si─ O and Ca─ O. Overall, the reaction mechanism of calcium in the system was innovatively revealed through the establishment of a macroscopic properties–microstructures–atomic model (multiscale) relationship.