Relations between the structural characteristics and compressive strength in metakaolin based geopolymers with different molar Si/Al ratios

X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM) and compressive strength measurements were performed to fully characterize metakaolin-based geopolymers with molar Si/Al ratios of 1.12, 1.77 and 2.20 in order to establish structure performance relationships in this system. XRD results showed that the geopolymer samples with Si/Al ratio of 1.12 contained crystalline components, namely zeolite A and/or sodalite phases, whereas geopolymers with Si/Al ratio of 1.77 and 2.20 were `X-ray amorphous' with minor amount of quartz content. The microstructure as revealed from SEM verified that geopolymer with Si/Al ratio of 1.12 consisted of crystalline components whereas samples with Si/Al ratio of 1.77 and 2.20 included amorphous microstructures. The position and width of the main band in FTIR spectra is found to systematically increase with increasing molar Si/Al ratio. The intensity of this band decreased with increasing Si content in the system indicating increased geopolymerization. This increase was related with formation of a three dimensional structure with stronger bonding environment with increasing SiO2 incorporation in geopolymer matrix. Compressive strength of these samples were also found to increase systematically with increasing molar Si/Al ratio. The evolution of the structural characteristics obtained from spectroscopy, diffraction and microscopy are shown to be directly correlated with the results of compressive strength measurements performed on these geopolymers. (C) 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.