Microstructural and morphological evolution of fly ash based geopolymers

The microstructural and morphological evolution of fly ash geopolymer have been studied in relation to synthesis condition. The variable parameters for synthesis were NaOH concentration (6, 8 and 10M) and curing temperature (27, 45 and 60 °C). The effect of these parameters on the early geopolymerization was elucidated using Isothermal conduction calorimeter (ICC) whereas on the final reaction product through Thermo gravimetric analysis (TGA). ICC results revealed that the main reaction peak corresponding to geopolymerization increases linearly with alkali concentration. The maximum weight loss in samples cured at 45 °C as shown by TGA was due to formation of N-A-S-H (NNa2O, AAl2O3, SSiO2, HH2O) gel with more OH− molecules. Scanning electron microscopy, X-ray diffractometer and Fourier transform infrared spectroscopy have been used for structural characterization and indexing of morphological features. The dependency of structural re-organization on alkali concentration and curing temperature was evidenced by the change in Si/Al ratio in the reaction products, shifting of spectrum corresponding to SiOSi and AlOSi, and formation of new phases. The increase in Si/Al ratio with the higher molarity and decrease with elevating temperature shows that dissolution of silica was more influenced by alkali concentration whereas dissolution of alumina was more influenced by curing temperature.