Degree of reactivity of two kaolinitic minerals in alkali solution using zeolitic tuff or silica sand filler

Four geopolymers were synthesized by NaOH-activation of a mixture of kaolinite (Jordanian kaolinite or Ukrainian kaolinite) and a filler (zeolitic tuff or silica sand). X-ray powder diffraction (XRD), Fourier-transform infrared spectrometry (FTIR), and solid-state magic angle spinning nuclear magnetic resonance (MAS-NMR) were employed to monitor the extent of reaction and to characterize the phases in the geopolymer. Remaining kaolinite in all produced geopolymer specimens unambiguously indicated an incomplete reaction. The 29Si MAS-NMR spectra of the geopolymers revealed the presence of tetrahedral-SiO4 whereas the 27Al MAS-NMR spectra revealed the presence of both tetrahedral-AlO4 and octahedral-AlO6. The XRD patterns of geopolymers showed the formation of a new feldspar mineral. Replacing silica sand filler by zeolitic tuff enhanced markedly the specific surface area of the corresponding geopolymers.