Effect of magnesium salt contamination on the microstructures and properties of metakaolinite-based geopolymer: the role of MgCl2 and MgSO4

Ion contamination considerably influences the properties of geopolymers. This study investigated the effects of magnesium chloride (MgCl2) and magnesium sulfate (MgSO4) concentrations on the microstructure and properties of metakaolinite-based geopolymers with Si/Al ratios of 1.3 (adding NaOH) and 1.6 (adding NaOH and Na2SiO3). The chemical structures and compositions of as-obtained geopolymers formed through the alkali acti-vation of kaolinite were characterized through X-ray diffraction, Fourier-transform infrared spectroscopy, and nuclear magnetic resonance (NMR) spectroscopy, while the microstructure was characterised via scanning electron microscopy and mercury intrusion porosimetry. The salt-free geopolymer with a Si/Al ratio of 1.3 exhibited a loose microstructure and hence poor 28-day compressive strength (9.4 MPa). The addition of magnesium salt to the geopolymer did not considerably influence its microstructure and mechanical properties. Comparatively, the geopolymer with a Si/Al ratio of 1.6 exhibited a compact microstructure and high 28-day compressive strength (43.0 MPa). The geopolymer compressive strength gradually decreased with increasing salt concentration. MgCl2 addition induced halite formation, increased pore diameter and pore volume, and reduced Si incorporated into the geopolymer matrix. In contrast, MgSO4 addition resulted in relatively less change in the pore diameter and pore volume and based on NMR result, more Si was incorporated into the geopolymer matrix. Thus, MgCl2 was more detrimental to the mechanical properties of the kaolinite-based geopolymers than MgSO4.