The improvement of strength and microstructural properties of fly ash-based geopolymer by adding elemental aluminum powder

In this study, 1, 2, and 3% micronized aluminum powder were used by replacing with fly ash to improve the properties of F class fly ash-based geopolymer mortar samples. The produced geopolymer mortar samples were heat cured at 60, 70, and 80 °C temperatures for 24 and 48 h. Following heat curing, the unit weights, flexural, and compressive strengths of the hardened geopolymer mortar samples were determined. In addition, XRD, FESEM, EDX, and isothermal calorimetry experiments were carried out to examine the changes in the microstructure with the addition of aluminum in detail. Based on the results obtained, it was observed that the substitution of micronized aluminum powder slightly decreased the workability of the geopolymer mortar samples. Moreover, it increased the flexural and compressive strength significantly (more than double) for 24 h heat curing. The XRD, FESEM, EDX and isothermal calorimetry experiments showed that the substitution of aluminum powder in the fly ash-based geopolymer system increased the geopolymerization reactions and, in comparison to the reference sample, created sodalite minerals in the microstructure. Therefore, it was concluded that the aluminum powder substitution transforms the microstructure into a stronger structure, thus improving the mechanical properties. Graphical abstract