Effects of kaolin characteristics on the mechanical properties of alkali-activated binders

The present study examines the potential use of six Kaolin samples collected from different deposits to produce calcined kaolin-based alkali-activated binders. The samples were initially characterized to assess their chemical compositions and reactivity through x-ray fluorescence (XRF), x-ray diffraction (XRD), scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS), and Chapelle-test. Mortar specimens were prepared for measuring the hydraulic activity as well as the compressive strength. The chemical analysis of the Kaolin showed a wide range of Al2O3 content (7.4–28.9 wt%), whereas the difference in the SiO2 was minor. Chapelle test results reflected the quality of the Kaolin used in this investigation, with values ranging from 350 to 700 mg/g. All Kaolin samples exhibited clay-based crystalline compounds that disappeared after calcination and converted to amorphous phases. A wide range of compressive strength values was obtained for the resultant alkali-activated binder (15–40 MPa) at 90 days. Direct correlations were found between calcined kaolin characteristics and the 90-days compressive strength of the alkali-activated binder. Kaolin samples with high pozzolanicity and higher Si/Al ratio were found to be more pronounced to produce alkali-activated binder with higher compressive strength values. Higher Fe and Al contents in the calcined Kaolin were found to lower the compressive strength of the alkali-activated binder.