•Fly ash containing large amounts of unburned carbon was sintered by applying separated electrical field of microwaves.•The fly ash exhibited anomalous heating behavior at 450 °C.•At 900–1200 °C, the amount of chlorine and carbon in the fly ash decreased as the temperature increased.•The chlorine concentration in the sample decreased as a result of the heating, leading to the addition of carbon. Recently, the application of microwave heating to high-temperature processing has received increased attention. In this study, fly ash containing large amounts of unburnt carbon and sodium chloride was sintered by applying microwaves as part of an investigation of the application of microwave heating to high-temperature processes. Microwave fields were separated by cavities, and pure electrical fields were employed as a heat source. The heating rate and maximum temperature increased with the microwave power, while the fly ash exhibited anomalous heating behavior at 450 °C. In this case, unburnt carbon in the fly ash plays an important role in the microwave absorption properties. At 900–1200 °C, no change was observed in the hardness or composition even when the heating duration was increased. In addition, it was found that the amount of chlorine and carbon in the fly ash decreased as the temperature increased. The chlorine concentration in the sample decreased due to the addition of carbon.