Structure - property relationship in volcanic ASH based one part geopolymers and perlite-metakaolin based two part geopolymers

Abstract

In this thesis, structure-property relations in volcanic ash and perlite-metakaolin based geopolymer systems are investigated. In the first part of the thesis, volcanic ash is used as raw material to synthesize one-part (just add water) geopolymers using anhydrous sodium metasilicate particles as the sole alkaline activator. Volcanic ash based one-part geopolymers are synthesized at ambient temperature with varying molar Si/Al (2.5-4.5) and Na/Al (1.1-5.1) ratios with a constant water/solid (wt/wt) ratio of 0.3. The geopolymers are comprised of X-ray amorphous phases and some undissolved crystalline phases (aluminosilicate and iron oxide phases) embedded in the amorphous matrices. The main absorption band in the FTIR spectra assigned to the asymmetric stretching vibrations of SiO-T (T: Si, Al) shifts to lower frequencies with increasing Si/Al molar ratio, which is a significant indication of the formation of amorphous aluminosilicate gel phase. The corresponding compressive strength values of one–part geopolymers ascend from 13.89 MPa to 19.60 MPa as the SEM morphologies of the specimens display denser, smoother and more homogenized glassy matrices. In the second part of the thesis, perlite-metakaolin based foamed geopolymers are synthesized with varying metakaolin contents of 0, 10, 20, 30, 40, 50, 100 wt% in raw material. The specimens are foamed by means of 3 wt% hydrogen peroxide. XRD patterns depict that the halo peak centered at around 23°shifts to the higher angles (~27-29° 2θ) as Si/Al molar ratio decreases, which is indication of polymerization. Zeolite crystalline phases are remarkable for geopolymers including 20 wt% and higher proportions of metakaolin, consistent with the corresponding SEM micrographs. As water/solid ratio increases, total pore volume of geopolymers increases leading to a decreasing trend in density of the geopolymers. The lowest density achieved in this study is 1.74 g/cm3. Thermal gravimetric analysis indicates that the lowest thermal stability is attained for sample with the highest total pore volume.