Molecular dynamics study of the dissolution mechanism of kaolinite basal surfaces in alkali media

ZN Khorshidi and XL Tan and Q Liu and P Choi, APPLIED CLAY SCIENCE, 152, 29-37 (2018).

DOI: 10.1016/j.clay.2017.10.025

The geopolymerization process involves the dissolution of aluminosilicates, in an alkali solution followed by the polymerization of the dissolved aluminate and silicate oligomers to form an amorphous geopolymer. It is generally accepted that the dissolution determines the nature of the oligomers formed, thereby the properties of the resultant geopolymer. Accordingly, in this work, a series of molecular dynamics (MD) simulations were carried out in the isothermal-isobaric (NPT) ensemble at 298 K and 1 atm to study the initial stage of dissolution process that takes place at the two basal surfaces (partially deprotonated octahedral and tetrahedral) of kaolinite in alkali media. Three different alkali media containing sodium (Na+), potassium (K+) and a mixture of 50/50 Na+/K+ cations were modeled at low (1 M) and medium (5 M) concentrations. The MD results showed that cations migrated to the vicinity of the deprotonated sites, trigging the dissociation of the nearby surface hydroxyl groups and increasing the interatomic distances between the aluminum atom that were adjacent to the deprotonated site and those attached such aluminum atom, thereby dissolving them in the form of aluminates into the alkali solution. Radial distribution functions and structural analyses indicated that the crystallinity of the model kaolinite decreased as simulation time and alkali solution concentration increased. The distortion occurred mostly on the octahedral surface that was attributed to the change of the coordination of aluminum from 6-fold octahedral to 5- and 4-fold tetrahedral. However, Na+ and K+ exhibited different dissolution mechanisms. In particular, Na+ with a higher charge density induced more dissociation of the surface hydroxyl groups, whereas K+ with a lower charge density resulted in more dissociation of aluminum atoms in the form of aluminates. In the case of the solution containing 50/50 Na+/K+, there was a synergistic effect of both cations that the solution led to more surface structure distortion as quantified by the number of 5- and 4-fold aluminum atoms.

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