Study of metakaolins with different amorphities and particle sizes activated by KOH and K2SiO3

The study of alkaline activated materials is very promising regarding the production of materials with high durability. The particle size and the content of reactive silica e alumina (amorphous) influence the performance of the geopolymer as well as the type of activator. In this study, geopolymers are produced using metakaolin precursors with similar chemical composition and different amorphities and particle sizes. KOH and K2SiO3 are used as activators, therefore, potassium solutions provide a lower viscosity of the pastes, better workability with a lower liquid to solids ratio and, consequently, better mechanical strength, denser and exhibited low porosity than sodium solutions. Produced precursors and pastes were characterized in the solid state by fluorescence X-ray spectroscopy, X-ray diffraction, infrared spectroscopy, nuclear magnetic resonance single pulse and nuclear magnetic resonance cross-polarization. The pastes were also evaluated for compressive strength, bulk density and water absorption. The results indicate that geopolymers can be obtained from metakaolins of incomplete calcination and with average grain diameter of 23.5 µm and 12.0 µm. Despite having similar bulk density and a slight difference in water absorption, the mechanical strength of the paste produced with coarser and more amorphous metakaolin at 28 days is 50 MPa, while the other paste reaches 22 MPa. The difference in mechanical strength is likely due to the greater extent of geopolymerization indicated by the NMR. The difference in the precursors and consequently in the pastes impacts on the expected durability.