Microstructural and 29Si and 27Al MAS NMR spectroscopic evaluations of alkali cation and curing effects on Class C fly ash-based geopolymer

Abstract

The present paper investigates the effect of the alkali cation and curing conditions on the microstructure of Class C fly ash-based geopolymer using 27 Al and 29 Si MAS NMR spectroscopy. 27 Al and 29 Si MAS NMR spectroscopy has been utilized to characterize the geopolymer with a different alkaline activator (Na and/or K) and curing at different condition temperatures (25, 40 and 70°C) and oven curing time (8, 12 and 24h). Also, X-ray fluorescence spectrometer (XRF), X-ray diffraction (XRD), Fourier Transform Infrared (FT–IR) spectroscopy and Scanning electron microscope (SEM) were applied for the characterization of raw materials and samples. 29 Si MAS NMR chemical shift of different silicon sites revealed that the alkali cation, the oven curing time and temperature affect the microstructure of the formed material. The results showed that in KNa-based gels; the silica content in the gel is higher and the width of the 29Si MAS-NMR component line is reduced moderately which indicates that a more ordered phase has been formed.