© 2021 Elsevier Inc.In this study, ‘‘One-pot fusion assisted hydrothermal synthesis’’ approach was employed to understand the nucleation and crystallization behaviour of the fusion product of Class F fly ash for zeolite 4A synthesis. Two sets of experiments were conducted by tailoring the control parameter of H2O/Na2O molar ratio (L/S) to investigate the nucleation and crystallization behaviour of zeolite 4A. The molar ratios of SiO2/Al2O3 and Na2O/SiO2 were tuned before alkali fusion by adding different amounts of Al(OH)3 to the fly ash along with the alkali agent, Na2CO3. Under the conditions of 12h aging at 47 °C and 7h crystallization at 90 °C, the sample with high L/S ratio (3.1 Na2O: 2.2 SiO2: 1.0 Al2O3: 79.2 H2O) yielded pure phase of Zeolite 4A with 94% relative crystallinity, while the sample with low L/S ratio (3.1 Na2O: 2.2 SiO2: 1.0 Al2O3: 42.2 H2O) resulted in the highest sodalite impurity. In general, the alkalinity level was observed to be the most dominant factor among all synthesis parameters. Nevertheless, low L/S ratio led to rapid crystallization in just 2.5h at 75 °C. Because of the dependency on the alkalinity level, the dissolution of low-carnegieite (LC) was determined as the decisive factor in the formation of zeolite 4A from the fusion product. The total time of hydrothermal treatment and aging temperature were the other main factors. The crystal growth of zeolite 4A was more sensitive to changes induced by crystallization time and temperature at high L/S ratio. Relative crystallinity of zeolite 4A varied in an expanded range in terms of crystallization time (36%–86%) and temperature (25%–73%) at high L/S ratio.