Relations between structural characteristics and compressive strength in volcanic ash based one-part geopolymer systems

Relations between structural characteristics and compressive strength in volcanic ash based one-part geopolymer systems are investigated employing XRD, FTIR spectroscopy, SEM/EDS and compressive strength measurements. Volcanic ash based one-part geopolymers are synthesized at ambient conditions with varying molar Si/Al and Na/Al ratios ranging between regions of 2.5-4.5 and 1.1-5.1, respectively. Volcanic ash is used without any thermal activation procedure. Geopolymer structures consist of x-ray amorphous glassy networks together with crystalline components (aluminosilicate and iron oxide phases) embedded in the amorphous matrices. The position of the main absorption band in the FTIR spectra indicating asymmetric stretching vibrations of Si-O-T bonds (T: Si, Al) in the geopolymeric framework constantly shifts to lower frequencies with increasing molar Si/Al and Na/Al ratios. These changes are coupled with systematic decreases in the intensities of the FTIR bands corresponding to Si-OH and carbonates. Concomitantly, the corresponding compressive strength values of one-part geopolymers ascend from 13.89 MPa to 19.6 MPa as the SEM morphologies of the specimens display denser, smoother and more homogenized matrices.