Improved microstructure and free efflorescence geopolymer binders

Two geopolymer binders that did not exhibit efflorescence, even after partial immersion in water for more than 45 days were successfully obtained from a mixture of aluminosilicates (kaolin and bauxite) with talc, and a mixture solution of potassium hydroxide and potassium silicate. Leaching tests in water were not able to quantify the occurrence of free alkali metals in the surface. The identification of structural characteristics was by solid-state nuclear magnetic resonance spectroscopy, X-ray diffraction, fourier transform infrared spectroscopy and scanning electron microscopy. They showed the occurrence of a structural arrangement with K2O–MgO–Al2O3–SiO2 where K2O and MgO are additional components from usual geopolymers. The improved structural characteristics limit drastically the leakage of alkali metals and their diffusion toward the surface. In comparison to usual geopolymers-based solely on aluminosilicates, the linear shrinkage is reduced, and the mechanical strength is similar. The results turned the use of talc and potassium hydroxide and silicate as a promising way of achieving an improved durability that is required for ensuring the properties in use of geopolymers.