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Owing to the increasing popularity of using waste disposal as a source material, inorganic salts such as CaCl2and MgCl2are frequently present in geopolymerization typically taking place in alkali media. Such contaminants influence the dissolution of the aluminosilicate source materials and consequently the properties of the geopolymers made. This work is particularly aimed at elucidating the dissolution mechanism of a well-known clay, namely, kaolinite, in alkali media with the presence of two aforementioned aqueous medium contaminants using molecular dynamics (MD) simulation. A series of MD simulations was carried out on model kaolinite, with its tetrahedral and deprotonated octahedral surfaces exposed to the alkali solutions containing neat Na+or neat K+cations at two concentrations, 3 and 5 M. Different concentrations of CaCl2and MgCl2contaminants (i.e., 0.1, 0.3, and 0.5 M) were added to such alkali solutions. Atomic density profiles show that all cations, including those from the contaminants adsorbed on the two basal surfaces, intensify the dissociation of the aluminate groups from the deprotonated octahedral surface. The dissociation mechanism is somewhat similar to that of the alkali media without contaminants, in which cations weaken the interaction between the aluminum and bridging oxygen atoms. The number of the aluminate groups dissociated decreased with increasing contaminant concentration. In fact, at the highest contaminant concentration used (i.e., 0.5 M), the number of dissociated aluminate groups was even lower than that of the system without contaminants. This observation was attributed to the fact that most of chloride anions remained in the bulk solution at 0.1 M but an increasing amount of chloride ions started to cluster around the cations at 0.5 M, thereby screening the interaction between cations and the deprotonated octahedral surface. As a result, the screening effect reduced the number of aluminate groups dissociated from the surface. Structural analyses of the deprotonated octahedral surface indicated that the crystallinity of the surface decreased with increasing simulation time and alkali solution concentration. No dissolution of the tetrahedral surface was observed for all systems studied.

Molecular dynamics simulation was used to study the role of water in the intercalation of CO2with a model Mg–Al–Cl-hydrotalcite mineral at ambient pressure and temperature. The ClayFF force field was used along with a model Mg–Al–Cl-hydrotalcite containing different amounts of water (H2O) and carbon dioxide (CO2) molecules in its interlayer spacing. It was observed that high CO2content, say 3.85 mmol g–1, could be achieved at low water concentrations or even without the presence of water. However, high water concentrations (e.g., 2 H2O molecules/hydrotalcite unit cell, the maximum allowed water concentration observed experimentally) could also yield similar CO2content, but in this case, the presence of water led to a significant interlayer spacing expansion (from 23.0 Å (no water) to 28.5 Å). The expansion was likely due to the change in the orientation distribution of the CO2molecules. Analyzing the orientation of CO2molecules revealed that they preferred to orientate parallel to the mineral surface at low water concentrations. However, as water concentration increased, CO2molecules exhibited a wider range of orientations with a significant fraction of them orienting more or less perpendicular to the mineral surface, especially at high CO2contents. The observed change in the orientation of CO2was attributed to the dipole interaction between H2O and CO2molecules and the reduced interaction between CO2and the hydroxyl groups on hydrotalcite. Also, it was observed that water molecules formed extensive hydrogen bond networks. All of the above findings seem to explain the contradicting results reported in the literature that water is needed under certain conditions to increase the amount of CO2captured by hydrotalcites. Here, we showed that high amounts of CO2can be intercalated with the presence of water.