Metakaolin as a model system for understanding geopolymers

Abstract

Geopolymers are a class of amorphous aluminosilicate materials that exhibit a range of properties depending on synthesis parameters. Determining the molecular interactions responsible for the different characteristics experimentally is hindered by the compositional variation of the source materials. Computational methods are thus used to provide atom level insights, with metakaolin used as a model system to represent the Al/Si geopolymer matrix. The formation of metakaolin through the thermal de-hydroxylation of kaolinite was simulated with molecular dynamics using an interatomic potential model identi_ed through testing of several models from the literature. The simulated metakaolin exhibited a 1:1 Al/Si ordering with a loss in periodicity due to the migration of aluminium ions through the structure. The change in the aluminium coordination as a function of de-hydroxylation results in a final structure composed of primarily 4-fold Al with up to 20% of the Al in a 5-fold coordination.A complex cavity network was identified and characterised in metakaolin and provided sites for the inclusion of sodium, potassium and calcium interstitial defect ions. The results showed that whilst ionic size controlled the final locations of the defect ions, ionic charge influenced the degree of interaction with the surrounding oxygen atoms and resulted in greater variations in the final defect site characteristics. Introducing hydroxyl groups into the structure caused the interactions of the defects with the aluminium to increase compared to silicon, demonstrating that the degree of source material hydration is as important as the type of metal cations present in the geopolymerisation reaction.A procedure for the generation of stable, partially hydrated metakaolin surfaces was developed and the resulting surfaces had a high degree of roughness that increased in the presence of water. The Al-terminated surfaces in metakaolin demonstrated the greatest level of interaction with water compared to Si, causing a surface puckering effect that resulted in a widening of the surface layers. The results indicate that water plays an important role, as the presence of water in the reaction mixture combined with high levels of structural disorder in the source materials increase their susceptibility to the caustic attack involved in geopolymerisation.