Structural characteristics of metakaolin-red mud and C-type fly ash based geopolymer systems

Geopolymers are inorganic polymers that are alternatives to cementitious materials for sustainable construction and building products. In this thesis, metakaolin-red mud- and C type fly ash- (CFA) based geopolymers are synthesized and characterized in detail mainly by Fourier Transform Infrared Spectroscopy (FTIR), X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) techniques. In the first part of this thesis, the structure and mechanical performance of red mud (RM)–metakaolin (MK) based geopolymers with varying RM contents (0–40 wt.% RM in RM–MK raw material mixture) are studied by XRD, FTIR, SEM and compressive strength measurements. Systematic addition of RM into geopolymer system resulted in constant loss of intensity and increased broadening of the main features in XRD and FTIR spectra. RM incorporation in this system resulted in a decreasing trend in the compressive strength correlated with the structural changes. The second part of this thesis focused on the structural characteristics of the CFA-based geopolymers. A wide variety of geopolymer compositions were prepared with molar Si/Al ratios between 2 to 10, and these samples were characterized by XRD, FTIR spectroscopy, SEM and thermogravimetric analysis (TGA). XRD and FTIR spectroscopy results indicate that geopolymers with 2 ≤ Si/Al ≤ 6 display amorphous geopolymeric structures involving Ca including components namely calcium silicate hydrate and calcite phases. On the other hand, geopolymers with 7 ≤ Si/Al ≤ 10 display a disordered geopolymer matrix possibly coexisting with sodium aluminosilicate hydrate gel together with sodium aluminum hydride phase. These phases along with crystalline phases observed in the XRD analysis are also verified by the SEM results. SEM results show that the microstructures of geopolymers evolve into a more homogeneous but highly crystalline structure as the molar Si/Al ratio is raised to a value of 10. TGA conducted on CFA-based geopolymer samples revealed that increasing Si/Al molar ratio caused higher weight losses. Most of the weight losses are related to the evaporation of structural water and decomposition of carbonate materials.