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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. [ABSTRACT FROM AUTHOR]

Point defects, particularly vacancies, exist abundantly in the structure of a variety of clay minerals (e.g., kaolinite). Such defects significantly alter the physical/chemical characteristics of the minerals. Dissolution of kaolinite in an alkali medium is a critical step in the geopolymerization process and it determines the properties of the final product. 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 influence of structural vacancies (Al and Si vacancies in particular) on the interaction/dissolution of the two basal surfaces of kaolinite (partially deprotonated octahedral and tetrahedral surfaces) exposed to alkali media. Two different alkali media were used. One contained sodium cation (Na + ) only while the other potassium cation (K + ) and their concentrations were 3 M and 5 M. The MD results showed that regardless of the type of cation and cation concentration, Al vacancies on the octahedral surface promoted the dissolution of Al into the solution compared to the surface without Al vacancies. However, there existed a vacancy concentration (2 Al vacancies per 576 Al atoms) at which the dissolution amount was the maximum and above which the dissolution decreased with increasing Al vacancy concentration. The presence of Si vacancies in the tetrahedral surface did not show significant effect on the dissociation of Si. Radial distribution function (RDF) analyses indicated that the degree of crystallinity of the octahedral surface decreased as the number of Al vacancies increased but this was not the case for the tetrahedral surface. [ABSTRACT FROM AUTHOR]