Heating Mechanism of High Aluminum Fly Ash Activated by Na₂CO₃ in Microwave Field

The microwave-absorption capabilities of high-alumina fly ash (HAFA), Na₂CO₃ and HAFA–Na₂CO₂ mixtures were investigated by determining the dielectric properties [real part (ε′), imaginary part (ε″) and dielectric loss tangent (tan δ)] from 25°C to 800°C, using the microwave resonant cavity technique at a microwave frequency of 2450 MHz. The mass ratio of the HAFA–Na₂CO₃ mixtures used in all experiments was 1:1. The study found that the microwave absorption capabilities of HAFA alone do not change significantly within the range of temperatures examined. For both Na₂CO₃ and HAFA–Na₂CO₃ mixtures, however, a sharp increase in absorption capability was observed above 400°C. The microwave heating behavior of the HAFA–Na₂CO₃ mixtures can be divided into two stages: in the first stage (< 350°C), heating rates were found to be lower due to the weaker microwave absorption capability of the HAFA and Na₂CO₃; however, in the second stage (≥ 350°C), a sharp increase in heating rate was observed, linked to the strengthened absorption capability of the Na₂CO₃. The study found that HAFA–Na₂CO₃ mixtures have a good microwave-absorption ability and can be heated rapidly by microwaves; therefore, it is feasible to use microwave heating for the activation of high-alumina fly ash with Na₂CO₃.