Physico-mechanical and microstructural investigation of a fly ash-based geopolymer and an inorganic waste blend-reinforced fly ash-based geopolymer pastes: A comparative study

The high carbon emissions associated with ordinary Portland cement production have intensified the search for sustainable and low-impact alternatives in the construction sector. Geopolymers, synthesized from aluminosilicate-rich industrial by-products, present an effective low-carbon binder system. However, optimizing their mechanical and structural performance when combining several waste materials remains a challenge due to complex chemical interactions. This research investigates the effects of incorporating ceramic waste powder (CWP), bottom ash powder (BAP), and marble waste powder (MWP), individually, in binary, and in ternary combinations, on the properties of fly ash-based geopolymers. The study evaluated the mechanical, physical, structural, microstructural, and thermal behavior of these systems to determine their suitability for sustainable construction applications. Mechanical testing showed that geopolymer pastes reinforced with low or no MWP content, particularly the ternary combination with CWP and BAP, exhibited significantly improved strength and physical performance compared to the unreinforced reference mix. In contrast, high MWP content negatively affected the matrix integrity. Structural analyses (XRD, FTIR) confirmed that the type and proportion of reinforcement impacted the formation of amorphous geopolymer gels, with optimal gel formation observed in samples with balanced ternary reinforcement. Microstructural characterization (SEM/EDX, TEM) revealed that well-performing geopolymers developed denser, more homogeneous gels with fewer unreacted particles and less porosity. Thermal analysis (TGA-DTA-DSC) further highlighted differences in water loss and carbonate decomposition across the samples, with higher thermal stability observed in the denser geopolymer matrices. The results confirm that combining industrial wastes in suitable proportions enhances the mechanical, structural, and thermal performance of fly ash-based geopolymers. This study provides new experimental evidence supporting the design of high-performance, multi-waste geopolymers and promotes sustainable resource utilization within the circular economy framework.