Effect of Inorganic Salt Contaminants on the Dissolution of Kaolinite Basal Surfaces in Alkali Media: A Molecular Dynamics Study

Owing to the increasing popularity of using waste disposal as a source material, inorganic salts such as CaCl₂ and MgCl₂ are 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 CaCl₂ and MgCl₂ contaminants (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.