Synthesis and characterization of metakaolin, fly ash and red mud based geopolymers

Abstract

Geopolymers stand as potential future alternatives to ordinary cement offering to reduce global CO2 emissions with added advantages such as better mechanical strength and increased design lives. In this thesis, metakaolin, fly ash (C type), and fly ash-red mud based geopolymers were synthesized, mechanically tested and characterized in detail by spectroscopy, diffraction and microscopy techniques. In the first part of this thesis, metakaolin-based geopolymers with molar Si/Al ratios of 1.12, 1.77 and 2.20 were investigated. 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 mainly ‘x-ray amorphous’. The position and width of the main band in FTIR spectra was 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. Compressive strength of these samples were also found to increase systematically with increasing molar Si/Al ratio. The second part of this thesis focused on fly ash and fly ash-red mud based geopolymers. The FTIR spectra of fly-ash based geopolymers with molar Si/Al ratio of 1.95 exhibited the presence of mainly two bands positioned at 941 cm-1 and 1099 cm-1. These bands possibly indicated i) a CASH and/or CSH gel and ii) NASH gel, respectively. These gel phases are also observed in SEM images of these specimens. Increasing Si/Al ratio up until 2.40 results in the merging of these bands together. The corresponding compressive strength values seem to increase with increasing Si/Al ratio. On the other hand, fly ash-red mud based geopolymers with low red mud contents reveal similar phase separated structure as evident from FTIR spectroscopy. However, as red mud content increases in the system, two bands corrresponding to CASH and/or CSH gel and NASH gel merge into a broader single feature. Transition from a two-phase structure into a relatively more uniform one should be responsible in corresponding increases in the measured compressive strength values.